sh

SH(1)                        General Commands Manual                       SH(1)

NAME
     sh — command interpreter (shell)

SYNOPSIS
     sh [-/+abCEefhIimnPpTuVvx] [-/+o longname] [script [arg ...]]
     sh [-/+abCEefhIimnPpTuVvx] [-/+o longname] -c string [name [arg ...]]
     sh [-/+abCEefhIimnPpTuVvx] [-/+o longname] -s [arg ...]

DESCRIPTION
     The sh utility is the standard command interpreter for the system.  The
     current version of sh is close to the IEEE Std 1003.1 (“POSIX.1”)
     specification for the shell.  It only supports features designated by
     POSIX, plus a few Berkeley extensions.  This man page is not intended to be
     a tutorial nor a complete specification of the shell.

   Overview
     The shell is a command that reads lines from either a file or the terminal,
     interprets them, and generally executes other commands.  It is the program
     that is started when a user logs into the system, although a user can
     select a different shell with the chsh(1) command.  The shell implements a
     language that has flow control constructs, a macro facility that provides a
     variety of features in addition to data storage, along with built-in
     history and line editing capabilities.  It incorporates many features to
     aid interactive use and has the advantage that the interpretative language
     is common to both interactive and non-interactive use (shell scripts).
     That is, commands can be typed directly to the running shell or can be put
     into a file, which can be executed directly by the shell.

   Invocation
     If no arguments are present and if the standard input of the shell is
     connected to a terminal (or if the -i option is set), the shell is
     considered an interactive shell.  An interactive shell generally prompts
     before each command and handles programming and command errors differently
     (as described below).  When first starting, the shell inspects argument 0,
     and if it begins with a dash (‘-’), the shell is also considered a login
     shell.  This is normally done automatically by the system when the user
     first logs in.  A login shell first reads commands from the files
     /etc/profile and then .profile in a user's home directory, if they exist.
     If the environment variable ENV is set on entry to a shell, or is set in
     the .profile of a login shell, the shell then subjects its value to
     parameter expansion and arithmetic expansion and reads commands from the
     named file.  Therefore, a user should place commands that are to be
     executed only at login time in the .profile file, and commands that are
     executed for every shell inside the ENV file.  The user can set the ENV
     variable to some file by placing the following line in the file .profile in
     the home directory, substituting for .shrc the filename desired:

           ENV=$HOME/.shrc; export ENV

     The first non-option argument specified on the command line will be treated
     as the name of a file from which to read commands (a shell script), and the
     remaining arguments are set as the positional parameters of the shell ($1,
     $2, etc.).  Otherwise, the shell reads commands from its standard input.

     Unlike older versions of sh the ENV script is only sourced on invocation of
     interactive shells.  This closes a well-known, and sometimes easily
     exploitable security hole related to poorly thought out ENV scripts.

   Argument List Processing
     All of the single letter options to sh have a corresponding long name, with
     the exception of -c and -/+o.  These long names are provided next to the
     single letter options in the descriptions below.  The long name for an
     option may be specified as an argument to the -/+o option of sh.  Once the
     shell is running, the long name for an option may be specified as an
     argument to the -/+o option of the set built-in command (described later in
     the section called Built-in Commands).  Introducing an option with a dash
     (‘-’) enables the option, while using a plus (‘+’) disables the option.  A
     “--” or plain ‘-’ will stop option processing and will force the remaining
     words on the command line to be treated as arguments.  The -/+o and -c
     options do not have long names.  They take arguments and are described
     after the single letter options.

     -a allexport
             Flag variables for export when assignments are made to them.

     -b notify
             Enable asynchronous notification of background job completion.
             (UNIMPLEMENTED)

     -C noclobber
             Do not overwrite existing files with ‘>’.

     -E emacs
             Enable the built-in emacs(1) command line editor (disables the -V
             option if it has been set; set automatically when interactive on
             terminals).

     -e errexit
             Exit immediately if any untested command fails in non-interactive
             mode.  The exit status of a command is considered to be explicitly
             tested if the command is part of the list used to control an if,
             elif, while, or until; if the command is the left hand operand of
             an “&&” or “||” operator; or if the command is a pipeline preceded
             by the ! keyword.  If a shell function is executed and its exit
             status is explicitly tested, all commands of the function are
             considered to be tested as well.

             It is recommended to check for failures explicitly instead of
             relying on -e because it tends to behave in unexpected ways,
             particularly in larger scripts.

     -f noglob
             Disable pathname expansion.

     -h trackall
             A do-nothing option for POSIX compliance.

     -I ignoreeof
             Ignore EOF's from input when in interactive mode.

     -i interactive
             Force the shell to behave interactively.

     -m monitor
             Turn on job control (set automatically when interactive).  A new
             process group is created for each pipeline (called a job).  It is
             possible to suspend jobs or to have them run in the foreground or
             in the background.  In a non-interactive shell, this option can be
             set even if no terminal is available and is useful to place
             processes in separate process groups.

     -n noexec
             If not interactive, read commands but do not execute them.  This is
             useful for checking the syntax of shell scripts.

     -P physical
             Change the default for the cd and pwd commands from -L (logical
             directory layout) to -P (physical directory layout).

     -p privileged
             Turn on privileged mode.  This mode is enabled on startup if either
             the effective user or group ID is not equal to the real user or
             group ID.  Turning this mode off sets the effective user and group
             IDs to the real user and group IDs.  When this mode is enabled for
             interactive shells, the file /etc/suid_profile is sourced instead
             of ~/.profile after /etc/profile is sourced, and the contents of
             the ENV variable are ignored.

     -s stdin
             Read commands from standard input (set automatically if no file
             arguments are present).  This option has no effect when set after
             the shell has already started running (i.e., when set with the set
             command).

     -T trapsasync
             When waiting for a child, execute traps immediately.  If this
             option is not set, traps are executed after the child exits, as
             specified in IEEE Std 1003.2 (“POSIX.2”).  This nonstandard option
             is useful for putting guarding shells around children that block
             signals.  The surrounding shell may kill the child or it may just
             return control to the tty and leave the child alone, like this:

                   sh -T -c "trap 'exit 1' 2 ; some-blocking-program"

     -u nounset
             Write a message to standard error when attempting to expand a
             variable, a positional parameter or the special parameter ! that is
             not set, and if the shell is not interactive, exit immediately.

     -V vi   Enable the built-in vi(1) command line editor (disables -E if it
             has been set).

     -v verbose
             The shell writes its input to standard error as it is read.  Useful
             for debugging.

     -x xtrace
             Write each command (preceded by the value of the PS4 variable
             subjected to parameter expansion and arithmetic expansion) to
             standard error before it is executed.  Useful for debugging.

     nolog   Another do-nothing option for POSIX compliance.  It only has a long
             name.

     The -c option causes the commands to be read from the string operand
     instead of from the standard input.  Keep in mind that this option only
     accepts a single string as its argument, hence multi-word strings must be
     quoted.

     The -/+o option takes as its only argument the long name of an option to be
     enabled or disabled.  For example, the following two invocations of sh both
     enable the built-in emacs(1) command line editor:

           set -E
           set -o emacs

     If used without an argument, the -o option displays the current option
     settings in a human-readable format.  If +o is used without an argument,
     the current option settings are output in a format suitable for re-input
     into the shell.

   Lexical Structure
     The shell reads input in terms of lines from a file and breaks it up into
     words at whitespace (blanks and tabs), and at certain sequences of
     characters called “operators”, which are special to the shell.  There are
     two types of operators: control operators and redirection operators (their
     meaning is discussed later).  The following is a list of valid operators:

     Control operators:
                   &     &&    (     )     \n
                   ;;    ;&    ;     |     ||

     Redirection operators:
                   <     >     <<    >>    <>
                   <&    >&    <<-   >|

     The character ‘#’ introduces a comment if used at the beginning of a word.
     The word starting with ‘#’ and the rest of the line are ignored.

     ASCII NUL characters (character code 0) are not allowed in shell input.

   Quoting
     Quoting is used to remove the special meaning of certain characters or
     words to the shell, such as operators, whitespace, keywords, or alias
     names.

     There are four types of quoting: matched single quotes, dollar-single
     quotes, matched double quotes, and backslash.

     Single Quotes
             Enclosing characters in single quotes preserves the literal meaning
             of all the characters (except single quotes, making it impossible
             to put single-quotes in a single-quoted string).

     Dollar-Single Quotes
             Enclosing characters between $' and ' preserves the literal meaning
             of all characters except backslashes and single quotes.  A
             backslash introduces a C-style escape sequence:

             \a          Alert (ring the terminal bell)

             \b          Backspace

             \cc         The control character denoted by ^c in stty(1).  If c
                         is a backslash, it must be doubled.

             \e          The ESC character (ASCII 0x1b)

             \f          Formfeed

             \n          Newline

             \r          Carriage return

             \t          Horizontal tab

             \v          Vertical tab

             \\          Literal backslash

             \'          Literal single-quote

             \"          Literal double-quote

             \nnn        The byte whose octal value is nnn (one to three digits)

             \xnn        The byte whose hexadecimal value is nn (one or more
                         digits only the last two of which are used)

             \unnnn      The Unicode code point nnnn (four hexadecimal digits)

             \Unnnnnnnn  The Unicode code point nnnnnnnn (eight hexadecimal
                         digits)

             The sequences for Unicode code points are currently only useful
             with UTF-8 locales.  They reject code point 0 and UTF-16
             surrogates.

             If an escape sequence would produce a byte with value 0, that byte
             and the rest of the string until the matching single-quote are
             ignored.

             Any other string starting with a backslash is an error.

     Double Quotes
             Enclosing characters within double quotes preserves the literal
             meaning of all characters except dollar sign (‘$’), backquote
             (‘`’), and backslash (‘\’).  The backslash inside double quotes is
             historically weird.  It remains literal unless it precedes the
             following characters, which it serves to quote:

                   $     `     "     \     \n

     Backslash
             A backslash preserves the literal meaning of the following
             character, with the exception of the newline character (‘\n’).  A
             backslash preceding a newline is treated as a line continuation.

   Keywords
     Keywords or reserved words are words that have special meaning to the shell
     and are recognized at the beginning of a line and after a control operator.
     The following are keywords:

           !        {        }        case      do
           done     elif     else     esac      fi
           for      if       then     until     while

   Aliases
     An alias is a name and corresponding value set using the alias built-in
     command.  Wherever the command word of a simple command may occur, and
     after checking for keywords if a keyword may occur, the shell checks the
     word to see if it matches an alias.  If it does, it replaces it in the
     input stream with its value.  For example, if there is an alias called “lf”
     with the value “ls -F”, then the input

           lf foobar

     would become

           ls -F foobar

     Aliases are also recognized after an alias whose value ends with a space or
     tab.  For example, if there is also an alias called “nohup” with the value
     “nohup ”, then the input

           nohup lf foobar

     would become

           nohup ls -F foobar

     Aliases provide a convenient way for naive users to create shorthands for
     commands without having to learn how to create functions with arguments.
     Using aliases in scripts is discouraged because the command that defines
     them must be executed before the code that uses them is parsed.  This is
     fragile and not portable.

     An alias name may be escaped in a command line, so that it is not replaced
     by its alias value, by using quoting characters within or adjacent to the
     alias name.  This is most often done by prefixing an alias name with a
     backslash to execute a function, built-in, or normal program with the same
     name.  See the Quoting subsection.

   Commands
     The shell interprets the words it reads according to a language, the
     specification of which is outside the scope of this man page (refer to the
     BNF in the IEEE Std 1003.2 (“POSIX.2”) document).  Essentially though, a
     line is read and if the first word of the line (or after a control
     operator) is not a keyword, then the shell has recognized a simple command.
     Otherwise, a complex command or some other special construct may have been
     recognized.

   Simple Commands
     If a simple command has been recognized, the shell performs the following
     actions:

     1.   Leading words of the form “name=value” are stripped off and assigned
          to the environment of the simple command (they do not affect
          expansions).  Redirection operators and their arguments (as described
          below) are stripped off and saved for processing.

     2.   The remaining words are expanded as described in the section called
          Word Expansions, and the first remaining word is considered the
          command name and the command is located.  The remaining words are
          considered the arguments of the command.  If no command name resulted,
          then the “name=value” variable assignments recognized in 1) affect the
          current shell.

     3.   Redirections are performed as described in the next section.

   Redirections
     Redirections are used to change where a command reads its input or sends
     its output.  In general, redirections open, close, or duplicate an existing
     reference to a file.  The overall format used for redirection is:

           [n] redir-op file

     The redir-op is one of the redirection operators mentioned previously.  The
     following gives some examples of how these operators can be used.  Note
     that stdin and stdout are commonly used abbreviations for standard input
     and standard output respectively.

           [n]> file     redirect stdout (or file descriptor n) to file

           [n]>| file    same as above, but override the -C option

           [n]>> file    append stdout (or file descriptor n) to file

           [n]< file     redirect stdin (or file descriptor n) from file

           [n]<> file    redirect stdin (or file descriptor n) to and from file

           [n1]<&n2      duplicate stdin (or file descriptor n1) from file
                         descriptor n2

           [n]<&-        close stdin (or file descriptor n)

           [n1]>&n2      duplicate stdout (or file descriptor n1) to file
                         descriptor n2

           [n]>&-        close stdout (or file descriptor n)

     The following redirection is often called a “here-document”.

           [n]<< delimiter
           here-doc-text
           ...
           delimiter

     All the text on successive lines up to the delimiter is saved away and made
     available to the command on standard input, or file descriptor n if it is
     specified.  If the delimiter as specified on the initial line is quoted,
     then the here-doc-text is treated literally, otherwise the text is
     subjected to parameter expansion, command substitution, and arithmetic
     expansion (as described in the section on Word Expansions).  If the
     operator is “<<-” instead of “<<”, then leading tabs in the here-doc-text
     are stripped.

   Search and Execution
     There are three types of commands: shell functions, built-in commands, and
     normal programs.  The command is searched for (by name) in that order.  The
     three types of commands are all executed in a different way.

     When a shell function is executed, all of the shell positional parameters
     (except $0, which remains unchanged) are set to the arguments of the shell
     function.  The variables which are explicitly placed in the environment of
     the command (by placing assignments to them before the function name) are
     made local to the function and are set to the values given.  Then the
     command given in the function definition is executed.  The positional
     parameters are restored to their original values when the command
     completes.  This all occurs within the current shell.

     Shell built-in commands are executed internally to the shell, without
     spawning a new process.  There are two kinds of built-in commands: regular
     and special.  Assignments before special builtins persist after they finish
     executing and assignment errors, redirection errors and certain operand
     errors cause a script to be aborted.  Special builtins cannot be overridden
     with a function.  Both regular and special builtins can affect the shell in
     ways normal programs cannot.

     Otherwise, if the command name does not match a function or built-in
     command, the command is searched for as a normal program in the file system
     (as described in the next section).  When a normal program is executed, the
     shell runs the program, passing the arguments and the environment to the
     program.  If the program is not a normal executable file (i.e., if it does
     not begin with the “magic number” whose ASCII representation is “#!”,
     resulting in an ENOEXEC return value from execve(2)) but appears to be a
     text file, the shell will run a new instance of sh to interpret it.

     Note that previous versions of this document and the source code itself
     misleadingly and sporadically refer to a shell script without a magic
     number as a “shell procedure”.

   Path Search
     When locating a command, the shell first looks to see if it has a shell
     function by that name.  Then it looks for a built-in command by that name.
     If a built-in command is not found, one of two things happen:

     1.   Command names containing a slash are simply executed without
          performing any searches.

     2.   The shell searches each entry in the PATH variable in turn for the
          command.  The value of the PATH variable should be a series of entries
          separated by colons.  Each entry consists of a directory name.  The
          current directory may be indicated implicitly by an empty directory
          name, or explicitly by a single period.

   Command Exit Status
     Each command has an exit status that can influence the behavior of other
     shell commands.  The paradigm is that a command exits with zero for normal
     or success, and non-zero for failure, error, or a false indication.  The
     man page for each command should indicate the various exit codes and what
     they mean.  Additionally, the built-in commands return exit codes, as does
     an executed shell function.

     If a command is terminated by a signal, its exit status is greater than
     128.  The signal name can be found by passing the exit status to kill -l.

     If there is no command word, the exit status is the exit status of the last
     command substitution executed, or zero if the command does not contain any
     command substitutions.

   Complex Commands
     Complex commands are combinations of simple commands with control operators
     or keywords, together creating a larger complex command.  More generally, a
     command is one of the following:

           simple command

           pipeline

           list or compound-list

           compound command

           function definition

     Unless otherwise stated, the exit status of a command is that of the last
     simple command executed by the command, or zero if no simple command was
     executed.

   Pipelines
     A pipeline is a sequence of one or more commands separated by the control
     operator ‘|’.  The standard output of all but the last command is connected
     to the standard input of the next command.  The standard output of the last
     command is inherited from the shell, as usual.

     The format for a pipeline is:

           [!] command1 [| command2 ...]

     The standard output of command1 is connected to the standard input of
     command2.  The standard input, standard output, or both of a command is
     considered to be assigned by the pipeline before any redirection specified
     by redirection operators that are part of the command.

     Note that unlike some other shells, sh executes each process in a pipeline
     with more than one command in a subshell environment and as a child of the
     sh process.

     If the pipeline is not in the background (discussed later), the shell waits
     for all commands to complete.

     If the keyword ! does not precede the pipeline, the exit status is the exit
     status of the last command specified in the pipeline.  Otherwise, the exit
     status is the logical NOT of the exit status of the last command.  That is,
     if the last command returns zero, the exit status is 1; if the last command
     returns greater than zero, the exit status is zero.

     Because pipeline assignment of standard input or standard output or both
     takes place before redirection, it can be modified by redirection.  For
     example:

           command1 2>&1 | command2

     sends both the standard output and standard error of command1 to the
     standard input of command2.

     A ‘;’ or newline terminator causes the preceding AND-OR-list (described
     below in the section called Short-Circuit List Operators) to be executed
     sequentially; an ‘&’ causes asynchronous execution of the preceding AND-OR-
     list.

   Background Commands (&)
     If a command is terminated by the control operator ampersand (‘&’), the
     shell executes the command in a subshell environment (see Grouping Commands
     Together below) and asynchronously; the shell does not wait for the command
     to finish before executing the next command.

     The format for running a command in background is:

           command1 & [command2 & ...]

     If the shell is not interactive, the standard input of an asynchronous
     command is set to /dev/null.

     The exit status is zero.

   Lists (Generally Speaking)
     A list is a sequence of zero or more commands separated by newlines,
     semicolons, or ampersands, and optionally terminated by one of these three
     characters.  The commands in a list are executed in the order they are
     written.  If command is followed by an ampersand, the shell starts the
     command and immediately proceeds onto the next command; otherwise it waits
     for the command to terminate before proceeding to the next one.

   Short-Circuit List Operators
     “&&” and “||” are AND-OR list operators.  “&&” executes the first command,
     and then executes the second command if the exit status of the first
     command is zero.  “||” is similar, but executes the second command if the
     exit status of the first command is nonzero.  “&&” and “||” both have the
     same priority.

   Flow-Control Constructs (if, while, for, case)
     The syntax of the if command is:
           if list
           then list
           [elif list
           then list] ...
           [else list]
           fi

     The exit status is that of selected then or else list, or zero if no list
     was selected.

     The syntax of the while command is:
           while list
           do list
           done

     The two lists are executed repeatedly while the exit status of the first
     list is zero.  The until command is similar, but has the word until in
     place of while, which causes it to repeat until the exit status of the
     first list is zero.

     The exit status is that of the last execution of the second list, or zero
     if it was never executed.

     The syntax of the for command is:
           for variable [in word ...]
           do list
           done

     If in and the following words are omitted, in "$@" is used instead.  The
     words are expanded, and then the list is executed repeatedly with the
     variable set to each word in turn.  The do and done commands may be
     replaced with ‘{’ and ‘}’.

     The syntax of the break and continue commands is:
           break [num]
           continue [num]

     The break command terminates the num innermost for or while loops.  The
     continue command continues with the next iteration of the innermost loop.
     These are implemented as special built-in commands.

     The syntax of the case command is:
           case word in
           pattern) list ;;
           ...
           esac

     The pattern can actually be one or more patterns (see Shell Patterns
     described later), separated by ‘|’ characters.  Tilde expansion, parameter
     expansion, command substitution, arithmetic expansion and quote removal are
     applied to the word.  Then, each pattern is expanded in turn using tilde
     expansion, parameter expansion, command substitution and arithmetic
     expansion and the expanded form of the word is checked against it.  If a
     match is found, the corresponding list is executed.  If the selected list
     is terminated by the control operator ‘;&’ instead of ‘;;’, execution
     continues with the next list, continuing until a list terminated with ‘;;’
     or the end of the case command.

   Grouping Commands Together
     Commands may be grouped by writing either

           (list)

     or

           { list; }

     The first form executes the commands in a subshell environment.  A subshell
     environment has its own copy of:

     1.   The current working directory as set by cd.

     2.   The file creation mask as set by umask.

     3.   Resource limits as set by ulimit.

     4.   References to open files.

     5.   Traps as set by trap.

     6.   Known jobs.

     7.   Positional parameters and variables.

     8.   Shell options.

     9.   Shell functions.

     10.  Shell aliases.

     These are copied from the parent shell environment, except that trapped
     (but not ignored) signals are reset to the default action and known jobs
     are cleared.  Any changes do not affect the parent shell environment.

     A subshell environment may be implemented as a child process or
     differently.  If job control is enabled in an interactive shell, commands
     grouped in parentheses can be suspended and continued as a unit.

     For compatibility with other shells, two open parentheses in sequence
     should be separated by whitespace.

     The second form never forks another shell, so it is slightly more
     efficient.  Grouping commands together this way allows the user to redirect
     their output as though they were one program:

           { echo -n "hello"; echo " world"; } > greeting

   Functions
     The syntax of a function definition is

           name ( ) command

     A function definition is an executable statement; when executed it installs
     a function named name and returns an exit status of zero.  The command is
     normally a list enclosed between ‘{’ and ‘}’.

     Variables may be declared to be local to a function by using the local
     command.  This should appear as the first statement of a function, and the
     syntax is:

           local [variable ...] [-]

     The local command is implemented as a built-in command.  The exit status is
     zero unless the command is not in a function or a variable name is invalid.

     When a variable is made local, it inherits the initial value and exported
     and readonly flags from the variable with the same name in the surrounding
     scope, if there is one.  Otherwise, the variable is initially unset.  The
     shell uses dynamic scoping, so that if the variable x is made local to
     function f, which then calls function g, references to the variable x made
     inside g will refer to the variable x declared inside f, not to the global
     variable named x.

     The only special parameter that can be made local is ‘-’.  Making ‘-’ local
     causes any shell options (including those that only have long names) that
     are changed via the set command inside the function to be restored to their
     original values when the function returns.

     The syntax of the return command is

           return [exitstatus]

     It terminates the current executional scope, returning from the closest
     nested function or sourced script; if no function or sourced script is
     being executed, it exits the shell instance.  The return command is
     implemented as a special built-in command.

   Variables and Parameters
     The shell maintains a set of parameters.  A parameter denoted by a name
     (consisting solely of alphabetics, numerics, and underscores, and starting
     with an alphabetic or an underscore) is called a variable.  When starting
     up, the shell turns all environment variables with valid names into shell
     variables.  New variables can be set using the form

           name=value

     A parameter can also be denoted by a number or a special character as
     explained below.

     Assignments are expanded differently from other words: tilde expansion is
     also performed after the equals sign and after any colon and usernames are
     also terminated by colons, and field splitting and pathname expansion are
     not performed.

     This special expansion applies not only to assignments that form a simple
     command by themselves or precede a command word, but also to words passed
     to the export, local or readonly built-in commands that have this form.
     For this, the builtin's name must be literal (not the result of an
     expansion) and may optionally be preceded by one or more literal instances
     of command without options.

   Positional Parameters
     A positional parameter is a parameter denoted by a number greater than
     zero.  The shell sets these initially to the values of its command line
     arguments that follow the name of the shell script.  The set built-in
     command can also be used to set or reset them.

   Special Parameters
     Special parameters are parameters denoted by a single special character or
     the digit zero.  They are shown in the following list, exactly as they
     would appear in input typed by the user or in the source of a shell script.

     $*      Expands to the positional parameters, starting from one.  When the
             expansion occurs within a double-quoted string it expands to a
             single field with the value of each parameter separated by the
             first character of the IFS variable, or by a space if IFS is unset.

     $@      Expands to the positional parameters, starting from one.  When the
             expansion occurs within double-quotes, each positional parameter
             expands as a separate argument.  If there are no positional
             parameters, the expansion of @ generates zero arguments, even when
             @ is double-quoted.  What this basically means, for example, is if
             $1 is “abc” and $2 is “def ghi”, then "$@" expands to the two
             arguments:

                   "abc"   "def ghi"

     $#      Expands to the number of positional parameters.

     $?      Expands to the exit status of the most recent pipeline.

     $-      (hyphen) Expands to the current option flags (the single-letter
             option names concatenated into a string) as specified on
             invocation, by the set built-in command, or implicitly by the
             shell.

     $$      Expands to the process ID of the invoked shell.  A subshell retains
             the same value of $ as its parent.

     $!      Expands to the process ID of the most recent background command
             executed from the current shell.  For a pipeline, the process ID is
             that of the last command in the pipeline.  If this parameter is
             referenced, the shell will remember the process ID and its exit
             status until the wait built-in command reports completion of the
             process.

     $0      (zero) Expands to the name of the shell script if passed on the
             command line, the name operand if given (with -c) or otherwise
             argument 0 passed to the shell.

   Special Variables
     The following variables are set by the shell or have special meaning to it:

     CDPATH    The search path used with the cd built-in.

     EDITOR    The fallback editor used with the fc built-in.  If not set, the
               default editor is ed(1).

     FCEDIT    The default editor used with the fc built-in.

     HISTSIZE  The number of previous commands that are accessible.

     HOME      The user's home directory, used in tilde expansion and as a
               default directory for the cd built-in.

     IFS       Input Field Separators.  The default value is ⟨space⟩, ⟨tab⟩, and
               ⟨newline⟩ in that order.  This default also applies if IFS is
               unset, but not if it is set to the empty string.  See the White
               Space Splitting section for more details.

     LINENO    The current line number in the script or function.

     MAIL      The name of a mail file, that will be checked for the arrival of
               new mail.  Overridden by MAILPATH.

     MAILPATH  A colon (‘:’) separated list of file names, for the shell to
               check for incoming mail.  This variable overrides the MAIL
               setting.  There is a maximum of 10 mailboxes that can be
               monitored at once.

     OPTIND    The index of the next argument to be processed by getopts.  This
               is initialized to 1 at startup.

     PATH      The default search path for executables.  See the Path Search
               section for details.

     PPID      The parent process ID of the invoked shell.  This is set at
               startup unless this variable is in the environment.  A later
               change of parent process ID is not reflected.  A subshell retains
               the same value of PPID.

     PS1       The primary prompt string, which defaults to “$ ”, unless you are
               the superuser, in which case it defaults to “# ”.  PS1 may
               include any of the following formatting sequences, which are
               replaced by the given information:

               \H      This system's fully-qualified hostname (FQDN).

               \h      This system's hostname.

               \W      The final component of the current working directory.

               \w      The entire path of the current working directory.

               \$      Superuser status.  “$ ” for normal users and “# ” for
                       superusers.

               \\      A literal backslash.

     PS2       The secondary prompt string, which defaults to “> ”.  PS2 may
               include any of the formatting sequences from PS1.

     PS4       The prefix for the trace output (if -x is active).  The default
               is “+ ”.

   Word Expansions
     This clause describes the various expansions that are performed on words.
     Not all expansions are performed on every word, as explained later.

     Tilde expansions, parameter expansions, command substitutions, arithmetic
     expansions, and quote removals that occur within a single word expand to a
     single field.  It is only field splitting or pathname expansion that can
     create multiple fields from a single word.  The single exception to this
     rule is the expansion of the special parameter @ within double-quotes, as
     was described above.

     The order of word expansion is:

     1.   Tilde Expansion, Parameter Expansion, Command Substitution, Arithmetic
          Expansion (these all occur at the same time).

     2.   Field Splitting is performed on fields generated by step (1) unless
          the IFS variable is null.

     3.   Pathname Expansion (unless the -f option is in effect).

     4.   Quote Removal.

     The ‘$’ character is used to introduce parameter expansion, command
     substitution, or arithmetic expansion.

   Tilde Expansion (substituting a user's home directory)
     A word beginning with an unquoted tilde character (‘~’) is subjected to
     tilde expansion.  All the characters up to a slash (‘/’) or the end of the
     word are treated as a username and are replaced with the user's home
     directory.  If the username is missing (as in ~/foobar), the tilde is
     replaced with the value of the HOME variable (the current user's home
     directory).

   Parameter Expansion
     The format for parameter expansion is as follows:

           ${expression}

     where expression consists of all characters until the matching ‘}’.  Any
     ‘}’ escaped by a backslash or within a single-quoted or double-quoted
     string, and characters in embedded arithmetic expansions, command
     substitutions, and variable expansions, are not examined in determining the
     matching ‘}’.  If the variants with ‘+’, ‘-’, ‘=’ or ‘?’ occur within a
     double-quoted string, as an extension there may be unquoted parts (via
     double-quotes inside the expansion); ‘}’ within such parts are also not
     examined in determining the matching ‘}’.

     The simplest form for parameter expansion is:

           ${parameter}

     The value, if any, of parameter is substituted.

     The parameter name or symbol can be enclosed in braces, which are optional
     except for positional parameters with more than one digit or when parameter
     is followed by a character that could be interpreted as part of the name.
     If a parameter expansion occurs inside double-quotes:

     1.   Field splitting is not performed on the results of the expansion, with
          the exception of the special parameter @.

     2.   Pathname expansion is not performed on the results of the expansion.

     In addition, a parameter expansion can be modified by using one of the
     following formats.

     ${parameter:-word}
             Use Default Values.  If parameter is unset or null, the expansion
             of word is substituted; otherwise, the value of parameter is
             substituted.

     ${parameter:=word}
             Assign Default Values.  If parameter is unset or null, the
             expansion of word is assigned to parameter.  In all cases, the
             final value of parameter is substituted.  Quoting inside word does
             not prevent field splitting or pathname expansion.  Only variables,
             not positional parameters or special parameters, can be assigned in
             this way.

     ${parameter:?[word]}
             Indicate Error if Null or Unset.  If parameter is unset or null,
             the expansion of word (or a message indicating it is unset if word
             is omitted) is written to standard error and the shell exits with a
             nonzero exit status.  Otherwise, the value of parameter is
             substituted.  An interactive shell need not exit.

     ${parameter:+word}
             Use Alternate Value.  If parameter is unset or null, null is
             substituted; otherwise, the expansion of word is substituted.

     In the parameter expansions shown previously, use of the colon in the
     format results in a test for a parameter that is unset or null; omission of
     the colon results in a test for a parameter that is only unset.

     The word inherits the type of quoting (unquoted, double-quoted or here-
     document) from the surroundings, with the exception that a backslash that
     quotes a closing brace is removed during quote removal.

     ${#parameter}
             String Length.  The length in characters of the value of parameter.

     The following four varieties of parameter expansion provide for substring
     processing.  In each case, pattern matching notation (see Shell Patterns),
     rather than regular expression notation, is used to evaluate the patterns.
     If parameter is one of the special parameters * or @, the result of the
     expansion is unspecified.  Enclosing the full parameter expansion string in
     double-quotes does not cause the following four varieties of pattern
     characters to be quoted, whereas quoting characters within the braces has
     this effect.

     ${parameter%word}
             Remove Smallest Suffix Pattern.  The word is expanded to produce a
             pattern.  The parameter expansion then results in parameter, with
             the smallest portion of the suffix matched by the pattern deleted.

     ${parameter%%word}
             Remove Largest Suffix Pattern.  The word is expanded to produce a
             pattern.  The parameter expansion then results in parameter, with
             the largest portion of the suffix matched by the pattern deleted.

     ${parameter#word}
             Remove Smallest Prefix Pattern.  The word is expanded to produce a
             pattern.  The parameter expansion then results in parameter, with
             the smallest portion of the prefix matched by the pattern deleted.

     ${parameter##word}
             Remove Largest Prefix Pattern.  The word is expanded to produce a
             pattern.  The parameter expansion then results in parameter, with
             the largest portion of the prefix matched by the pattern deleted.

   Command Substitution
     Command substitution allows the output of a command to be substituted in
     place of the command name itself.  Command substitution occurs when the
     command is enclosed as follows:

           $(command)

     or the backquoted version:

           `command`

     The shell expands the command substitution by executing command and
     replacing the command substitution with the standard output of the command,
     removing sequences of one or more newlines at the end of the substitution.
     Embedded newlines before the end of the output are not removed; however,
     during field splitting, they may be translated into spaces depending on the
     value of IFS and the quoting that is in effect.  The command is executed in
     a subshell environment, except that the built-in commands jobid, jobs, and
     trap return information about the parent shell environment and times
     returns information about the same process if they are the only command in
     a command substitution.

     If a command substitution of the $( form begins with a subshell, the $( and
     ( must be separated by whitespace to avoid ambiguity with arithmetic
     expansion.

   Arithmetic Expansion
     Arithmetic expansion provides a mechanism for evaluating an arithmetic
     expression and substituting its value.  The format for arithmetic expansion
     is as follows:

           $((expression))

     The expression is treated as if it were in double-quotes, except that a
     double-quote inside the expression is not treated specially.  The shell
     expands all tokens in the expression for parameter expansion, command
     substitution, arithmetic expansion and quote removal.

     The allowed expressions are a subset of C expressions, summarized below.

           Values     All values are of type intmax_t.

           Constants  Decimal, octal (starting with 0) and hexadecimal (starting
                      with 0x) integer constants.

           Variables  Shell variables can be read and written and contain
                      integer constants.

           Unary operators
                      ! ~ + -

           Binary operators
                      * / % + - << >> < <= > >= == != & ^ | && ||

           Assignment operators
                      = += -= *= /= %= <<= >>= &= ^= |=

           Conditional operator
                      ? :

     The result of the expression is substituted in decimal.

   White Space Splitting (Field Splitting)
     In certain contexts, after parameter expansion, command substitution, and
     arithmetic expansion the shell scans the results of expansions and
     substitutions that did not occur in double-quotes for field splitting and
     multiple fields can result.

     Characters in IFS that are whitespace (⟨space⟩, ⟨tab⟩, and ⟨newline⟩) are
     treated differently from other characters in IFS.

     Whitespace in IFS at the beginning or end of a word is discarded.

     Subsequently, a field is delimited by either

     1.   a non-whitespace character in IFS with any whitespace in IFS
          surrounding it, or

     2.   one or more whitespace characters in IFS.

     If a word ends with a non-whitespace character in IFS, there is no empty
     field after this character.

     If no field is delimited, the word is discarded.  In particular, if a word
     consists solely of an unquoted substitution and the result of the
     substitution is null, it is removed by field splitting even if IFS is null.

   Pathname Expansion (File Name Generation)
     Unless the -f option is set, file name generation is performed after word
     splitting is complete.  Each word is viewed as a series of patterns,
     separated by slashes.  The process of expansion replaces the word with the
     names of all existing files whose names can be formed by replacing each
     pattern with a string that matches the specified pattern.  There are two
     restrictions on this: first, a pattern cannot match a string containing a
     slash, and second, a pattern cannot match a string starting with a period
     unless the first character of the pattern is a period.  The next section
     describes the patterns used for Pathname Expansion, the four varieties of
     parameter expansion for substring processing and the case command.

   Shell Patterns
     A pattern consists of normal characters, which match themselves, and meta-
     characters.  The meta-characters are ‘*’, ‘?’, and ‘[’.  These characters
     lose their special meanings if they are quoted.  When command or variable
     substitution is performed and the dollar sign or back quotes are not
     double-quoted, the value of the variable or the output of the command is
     scanned for these characters and they are turned into meta-characters.

     An asterisk (‘*’) matches any string of characters.  A question mark (‘?’)
     matches any single character.  A left bracket (‘[’) introduces a character
     class.  The end of the character class is indicated by a ‘]’; if the ‘]’ is
     missing then the ‘[’ matches a ‘[’ rather than introducing a character
     class.  A character class matches any of the characters between the square
     brackets.  A locale-dependent range of characters may be specified using a
     minus sign.  A named class of characters (see wctype(3)) may be specified
     by surrounding the name with ‘[:’ and ‘:]’.  For example, ‘[[:alpha:]]’ is
     a shell pattern that matches a single letter.  The character class may be
     complemented by making an exclamation point (‘!’) the first character of
     the character class.  A caret (‘^’) has the same effect but is non-
     standard.

     To include a ‘]’ in a character class, make it the first character listed
     (after the ‘!’ or ‘^’, if any).  To include a ‘-’, make it the first or
     last character listed.

   Built-in Commands
     This section lists the built-in commands.

     :       A null command that returns a 0 (true) exit value.

     . file  The commands in the specified file are read and executed by the
             shell.  The return command may be used to return to the . command's
             caller.  If file contains any ‘/’ characters, it is used as is.
             Otherwise, the shell searches the PATH for the file.  If it is not
             found in the PATH, it is sought in the current working directory.

     [       A built-in equivalent of test(1).

     alias [name[=string] ...]
             If name=string is specified, the shell defines the alias name with
             value string.  If just name is specified, the value of the alias
             name is printed.  With no arguments, the alias built-in command
             prints the names and values of all defined aliases (see unalias).
             Alias values are written with appropriate quoting so that they are
             suitable for re-input to the shell.  Also see the Aliases
             subsection.

     bg [job ...]
             Continue the specified jobs (or the current job if no jobs are
             given) in the background.

     bind [-aeklrsv] [key [command]]
             List or alter key bindings for the line editor.  This command is
             documented in editrc(5).

     break [num]
             See the Flow-Control Constructs subsection.

     builtin cmd [arg ...]
             Execute the specified built-in command, cmd.  This is useful when
             the user wishes to override a shell function with the same name as
             a built-in command.

     cd [-L | -P] [-e] [directory]

     cd -    Switch to the specified directory, to the directory specified in
             the HOME environment variable if no directory is specified or to
             the directory specified in the OLDPWD environment variable if
             directory is -.  If directory does not begin with /, ., or .., then
             the directories listed in the CDPATH variable will be searched for
             the specified directory.  If CDPATH is unset, the current directory
             is searched.  The format of CDPATH is the same as that of PATH.  In
             an interactive shell, the cd command will print out the name of the
             directory that it actually switched to if the CDPATH mechanism was
             used or if directory was -.

             If the -P option is specified, .. is handled physically and
             symbolic links are resolved before .. components are processed.  If
             the -L option is specified, .. is handled logically.  This is the
             default.

             The -e option causes cd to return exit status 1 if the full
             pathname of the new directory cannot be determined reliably or at
             all.  Normally this is not considered an error, although a warning
             is printed.

     chdir   A synonym for the cd built-in command.

     command [-p] [utility [argument ...]]

     command [-p] -v utility

     command [-p] -V utility
             The first form of invocation executes the specified utility,
             ignoring shell functions in the search.  If utility is a special
             builtin, it is executed as if it were a regular builtin.

             If the -p option is specified, the command search is performed
             using a default value of PATH that is guaranteed to find all of the
             standard utilities.

             If the -v option is specified, utility is not executed but a
             description of its interpretation by the shell is printed.  For
             ordinary commands the output is the path name; for shell built-in
             commands, shell functions and keywords only the name is written.
             Aliases are printed as “alias name=value”.

             The -V option is identical to -v except for the output.  It prints
             “utility is description” where description is either the path name
             to utility, a special shell builtin, a shell builtin, a shell
             function, a shell keyword or an alias for value.

     continue [num]
             See the Flow-Control Constructs subsection.

     echo [-e | -n] [string ...]
             Print a space-separated list of the arguments to the standard
             output and append a newline character.

             -n      Suppress the output of the trailing newline.

             -e      Process C-style backslash escape sequences.  The echo
                     command understands the following character escapes:

                     \a      Alert (ring the terminal bell)

                     \b      Backspace

                     \c      Suppress the trailing newline (this has the side-
                             effect of truncating the line if it is not the last
                             character)

                     \e      The ESC character (ASCII 0x1b)

                     \f      Formfeed

                     \n      Newline

                     \r      Carriage return

                     \t      Horizontal tab

                     \v      Vertical tab

                     \\      Literal backslash

                     \0nnn   (Zero) The character whose octal value is nnn

                     If string is not enclosed in quotes then the backslash
                     itself must be escaped with a backslash to protect it from
                     the shell.  For example

                           $ echo -e "a\vb"
                           a
                            b
                           $ echo -e a\\vb
                           a
                            b
                           $ echo -e "a\\b"
                           a\b
                           $ echo -e a\\\\b
                           a\b

             Only one of the -e and -n options may be specified.

     eval string ...
             Concatenate all the arguments with spaces.  Then re-parse and
             execute the command.

     exec [command [arg ...]]
             Unless command is omitted, the shell process is replaced with the
             specified program (which must be a real program, not a shell built-
             in command or function).  Any redirections on the exec command are
             marked as permanent, so that they are not undone when the exec
             command finishes.

     exit [exitstatus]
             Terminate the shell process.  If exitstatus is given it is used as
             the exit status of the shell.  Otherwise, if the shell is executing
             an EXIT trap, the exit status of the last command before the trap
             is used; if the shell is executing a trap for a signal, the shell
             exits by resending the signal to itself.  Otherwise, the exit
             status of the preceding command is used.  The exit status should be
             an integer between 0 and 255.

     export name ...

     export [-p]
             The specified names are exported so that they will appear in the
             environment of subsequent commands.  The only way to un-export a
             variable is to unset it.  The shell allows the value of a variable
             to be set at the same time as it is exported by writing

                   export name=value

             With no arguments the export command lists the names of all
             exported variables.  If the -p option is specified, the exported
             variables are printed as “export name=value” lines, suitable for
             re-input to the shell.

     false   A null command that returns a non-zero (false) exit value.

     fc [-e editor] [first [last]]

     fc -l [-nr] [first [last]]

     fc -s [old=new] [first]
             The fc built-in command lists, or edits and re-executes, commands
             previously entered to an interactive shell.

             -e editor
                     Use the editor named by editor to edit the commands.  The
                     editor string is a command name, subject to search via the
                     PATH variable.  The value in the FCEDIT variable is used as
                     a default when -e is not specified.  If FCEDIT is null or
                     unset, the value of the EDITOR variable is used.  If EDITOR
                     is null or unset, ed(1) is used as the editor.

             -l (ell)
                     List the commands rather than invoking an editor on them.
                     The commands are written in the sequence indicated by the
                     first and last operands, as affected by -r, with each
                     command preceded by the command number.

             -n      Suppress command numbers when listing with -l.

             -r      Reverse the order of the commands listed (with -l) or
                     edited (with neither -l nor -s).

             -s      Re-execute the command without invoking an editor.

             first

             last    Select the commands to list or edit.  The number of
                     previous commands that can be accessed are determined by
                     the value of the HISTSIZE variable.  The value of first or
                     last or both are one of the following:

                     [+]num  A positive number representing a command number;
                             command numbers can be displayed with the -l
                             option.

                     -num    A negative decimal number representing the command
                             that was executed num of commands previously.  For
                             example, -1 is the immediately previous command.

                     string  A string indicating the most recently entered
                             command that begins with that string.  If the
                             old=new operand is not also specified with -s, the
                             string form of the first operand cannot contain an
                             embedded equal sign.

             The following variables affect the execution of fc:

             FCEDIT    Name of the editor to use for history editing.

             HISTSIZE  The number of previous commands that are accessible.

     fg [job]
             Move the specified job or the current job to the foreground.

     getopts optstring var
             The POSIX getopts command.  The getopts command deprecates the
             older getopt(1) command.  The first argument should be a series of
             letters, each possibly followed by a colon which indicates that the
             option takes an argument.  The specified variable is set to the
             parsed option.  The index of the next argument is placed into the
             shell variable OPTIND.  If an option takes an argument, it is
             placed into the shell variable OPTARG.  If an invalid option is
             encountered, var is set to ‘?’.  It returns a false value (1) when
             it encounters the end of the options.  A new set of arguments may
             be parsed by assigning OPTIND=1.

     hash [-rv] [command ...]
             The shell maintains a hash table which remembers the locations of
             commands.  With no arguments whatsoever, the hash command prints
             out the contents of this table.

             With arguments, the hash command removes each specified command
             from the hash table (unless they are functions) and then locates
             it.  With the -v option, hash prints the locations of the commands
             as it finds them.  The -r option causes the hash command to delete
             all the entries in the hash table except for functions.

     jobid [job]
             Print the process IDs of the processes in the specified job.  If
             the job argument is omitted, use the current job.

     jobs [-lps] [job ...]
             Print information about the specified jobs, or all jobs if no job
             argument is given.  The information printed includes job ID, status
             and command name.

             If the -l option is specified, the PID of each job is also printed.
             If the -p option is specified, only the process IDs for the process
             group leaders are printed, one per line.  If the -s option is
             specified, only the PIDs of the job commands are printed, one per
             line.

     kill    A built-in equivalent of kill(1) that additionally supports sending
             signals to jobs.

     local [variable ...] [-]
             See the Functions subsection.

     printf  A built-in equivalent of printf(1).

     pwd [-L | -P]
             Print the path of the current directory.  The built-in command may
             differ from the program of the same name because the built-in
             command remembers what the current directory is rather than
             recomputing it each time.  This makes it faster.  However, if the
             current directory is renamed, the built-in version of pwd(1) will
             continue to print the old name for the directory.

             If the -P option is specified, symbolic links are resolved.  If the
             -L option is specified, the shell's notion of the current directory
             is printed (symbolic links are not resolved).  This is the default.

     read [-p prompt] [-t timeout] [-er] variable ...
             The prompt is printed if the -p option is specified and the
             standard input is a terminal.  Then a line is read from the
             standard input.  The trailing newline is deleted from the line and
             the line is split as described in the section on White Space
             Splitting (Field Splitting) above, and the pieces are assigned to
             the variables in order.  If there are more pieces than variables,
             the remaining pieces (along with the characters in IFS that
             separated them) are assigned to the last variable.  If there are
             more variables than pieces, the remaining variables are assigned
             the null string.

             Backslashes are treated specially, unless the -r option is
             specified.  If a backslash is followed by a newline, the backslash
             and the newline will be deleted.  If a backslash is followed by any
             other character, the backslash will be deleted and the following
             character will be treated as though it were not in IFS, even if it
             is.

             If the -t option is specified and the timeout elapses before a
             complete line of input is supplied, the read command will return an
             exit status as if terminated by SIGALRM without assigning any
             values.  The timeout value may optionally be followed by one of
             ‘s’, ‘m’ or ‘h’ to explicitly specify seconds, minutes or hours.
             If none is supplied, ‘s’ is assumed.

             The -e option exists only for backward compatibility with older
             scripts.

             The exit status is 0 on success, 1 on end of file, between 2 and
             128 if an error occurs and greater than 128 if a trapped signal
             interrupts read.

     readonly [-p] [name ...]
             Each specified name is marked as read only, so that it cannot be
             subsequently modified or unset.  The shell allows the value of a
             variable to be set at the same time as it is marked read only by
             using the following form:

                   readonly name=value

             With no arguments the readonly command lists the names of all read
             only variables.  If the -p option is specified, the read-only
             variables are printed as “readonly name=value” lines, suitable for
             re-input to the shell.

     return [exitstatus]
             See the Functions subsection.

     set [-/+abCEefIimnpTuVvx] [-/+o longname] [-c string] [-- arg ...]
             The set command performs three different functions:

             With no arguments, it lists the values of all shell variables.

             If options are given, either in short form or using the long “-/+o
             longname” form, it sets or clears the specified options as
             described in the section called Argument List Processing.

             If the “--” option is specified, set will replace the shell's
             positional parameters with the subsequent arguments.  If no
             arguments follow the “--” option, all the positional parameters
             will be cleared, which is equivalent to executing the command
             “shift $#”.  The “--” flag may be omitted when specifying arguments
             to be used as positional replacement parameters.  This is not
             recommended, because the first argument may begin with a dash (‘-’)
             or a plus (‘+’), which the set command will interpret as a request
             to enable or disable options.

     setvar variable value
             Assigns the specified value to the specified variable.  The setvar
             command is intended to be used in functions that assign values to
             variables whose names are passed as parameters.  In general it is
             better to write “variable=value” rather than using setvar.

     shift [n]
             Shift the positional parameters n times, or once if n is not
             specified.  A shift sets the value of $1 to the value of $2, the
             value of $2 to the value of $3, and so on, decreasing the value of
             $# by one.  For portability, shifting if there are zero positional
             parameters should be avoided, since the shell may abort.

     test    A built-in equivalent of test(1).

     times   Print the amount of time spent executing the shell process and its
             children.  The first output line shows the user and system times
             for the shell process itself, the second one contains the user and
             system times for the children.

     trap [action] signal ...

     trap -l
             Cause the shell to parse and execute action when any specified
             signal is received.  The signals are specified by name or number.
             In addition, the pseudo-signal EXIT may be used to specify an
             action that is performed when the shell terminates.  The action may
             be an empty string or a dash (‘-’); the former causes the specified
             signal to be ignored and the latter causes the default action to be
             taken.  Omitting the action and using only signal numbers is
             another way to request the default action.  In a subshell or
             utility environment, the shell resets trapped (but not ignored)
             signals to the default action.  The trap command has no effect on
             signals that were ignored on entry to the shell.

             Option -l causes the trap command to display a list of valid signal
             names.

     true    A null command that returns a 0 (true) exit value.

     type [name ...]
             Interpret each name as a command and print the resolution of the
             command search.  Possible resolutions are: shell keyword, alias,
             special shell builtin, shell builtin, command, tracked alias and
             not found.  For aliases the alias expansion is printed; for
             commands and tracked aliases the complete pathname of the command
             is printed.

     ulimit [-HSabcdfklmnopstuvw] [limit]
             Set or display resource limits (see getrlimit(2)).  If limit is
             specified, the named resource will be set; otherwise the current
             resource value will be displayed.

             If -H is specified, the hard limits will be set or displayed.
             While everybody is allowed to reduce a hard limit, only the
             superuser can increase it.  The -S option specifies the soft limits
             instead.  When displaying limits, only one of -S or -H can be
             given.  The default is to display the soft limits, and to set both
             the hard and the soft limits.

             Option -a causes the ulimit command to display all resources.  The
             parameter limit is not acceptable in this mode.

             The remaining options specify which resource value is to be
             displayed or modified.  They are mutually exclusive.

             -b sbsize
                     The maximum size of socket buffer usage, in bytes.

             -c coredumpsize
                     The maximal size of core dump files, in 512-byte blocks.

             -d datasize
                     The maximal size of the data segment of a process, in
                     kilobytes.

             -f filesize
                     The maximal size of a file, in 512-byte blocks.

             -k kqueues
                     The maximal number of kqueues (see kqueue(2)) for this user
                     ID.

             -l lockedmem
                     The maximal size of memory that can be locked by a process,
                     in kilobytes.

             -m memoryuse
                     The maximal resident set size of a process, in kilobytes.

             -n nofiles
                     The maximal number of descriptors that could be opened by a
                     process.

             -o umtxp
                     The maximal number of process-shared locks (see pthread(3))
                     for this user ID.

             -p pseudoterminals
                     The maximal number of pseudo-terminals for this user ID.

             -s stacksize
                     The maximal size of the stack segment, in kilobytes.

             -t time
                     The maximal amount of CPU time to be used by each process,
                     in seconds.

             -u userproc
                     The maximal number of simultaneous processes for this user
                     ID.

             -v virtualmem
                     The maximal virtual size of a process, in kilobytes.

             -w swapuse
                     The maximum amount of swap space reserved or used for this
                     user ID, in kilobytes.

     umask [-S] [mask]
             Set the file creation mask (see umask(2)) to the octal or symbolic
             (see chmod(1)) value specified by mask.  If the argument is
             omitted, the current mask value is printed.  If the -S option is
             specified, the output is symbolic, otherwise the output is octal.

     unalias [-a] [name ...]
             The specified alias names are removed.  If -a is specified, all
             aliases are removed.

     unset [-fv] name ...
             The specified variables or functions are unset and unexported.  If
             the -v option is specified or no options are given, the name
             arguments are treated as variable names.  If the -f option is
             specified, the name arguments are treated as function names.

     wait [job ...]
             Wait for each specified job to complete and return the exit status
             of the last process in the last specified job.  If any job
             specified is unknown to the shell, it is treated as if it were a
             known job that exited with exit status 127.  If no operands are
             given, wait for all jobs to complete and return an exit status of
             zero.

   Commandline Editing
     When sh is being used interactively from a terminal, the current command
     and the command history (see fc in Built-in Commands) can be edited using
     vi-mode command line editing.  This mode uses commands similar to a subset
     of those described in the vi(1) man page.  The command “set -o vi” (or “set
     -V”) enables vi-mode editing and places sh into vi insert mode.  With
     vi-mode enabled, sh can be switched between insert mode and command mode by
     typing ⟨ESC⟩.  Hitting ⟨return⟩ while in command mode will pass the line to
     the shell.

     Similarly, the “set -o emacs” (or “set -E”) command can be used to enable a
     subset of emacs-style command line editing features.

ENVIRONMENT
     The following environment variables affect the execution of sh:

     ENV         Initialization file for interactive shells.

     LANG, LC_*  Locale settings.  These are inherited by children of the shell,
                 and is used in a limited manner by the shell itself.

     OLDPWD      The previous current directory.  This is used and updated by
                 cd.

     PWD         An absolute pathname for the current directory, possibly
                 containing symbolic links.  This is used and updated by the
                 shell.

     TERM        The default terminal setting for the shell.  This is inherited
                 by children of the shell, and is used in the history editing
                 modes.

     Additionally, environment variables are turned into shell variables at
     startup, which may affect the shell as described under Special Variables.

FILES
     ~/.profile           User's login profile.
     /etc/profile         System login profile.
     /etc/shells          Shell database.
     /etc/suid_profile    Privileged shell profile.

EXIT STATUS
     Errors that are detected by the shell, such as a syntax error, will cause
     the shell to exit with a non-zero exit status.  If the shell is not an
     interactive shell, the execution of the shell file will be aborted.
     Otherwise the shell will return the exit status of the last command
     executed, or if the exit builtin is used with a numeric argument, it will
     return the argument.

SEE ALSO
     builtin(1), chsh(1), echo(1), ed(1), emacs(1), kill(1), printf(1), pwd(1),
     test(1), vi(1), execve(2), getrlimit(2), umask(2), wctype(3), editrc(5),
     shells(5)

HISTORY
     A sh command, the Thompson shell, appeared in Version 1 AT&T UNIX.  It was
     superseded in Version 7 AT&T UNIX by the Bourne shell, which inherited the
     name sh.

     This version of sh was rewritten in 1989 under the BSD license after the
     Bourne shell from AT&T System V Release 4 UNIX.

AUTHORS
     This version of sh was originally written by Kenneth Almquist.

BUGS
     The sh utility does not recognize multibyte characters other than UTF-8.
     Splitting using IFS does not recognize multibyte characters.

                                  May 30, 2016