bsh

SH(1)                       General Commands Manual                      SH(1)

NAME
     sh - command interpreter (shell)

SYNOPSIS
     sh [-/+aCefnuvxIimqsVEbc] [-o longname] [target ...]

DESCRIPTION
     Sh is the standard command interpreter for the system. The current
     version of sh is in the process of being changed to conform with the
     POSIX 1003.2 and 1003.2a specifications for the shell.  This version has
     many features which make it appear similar in some respects to the Korn
     shell, but it is not a Korn shell clone (see ksh(1)).  Only features
     designated by POSIX, plus a few Berkeley extensions, are being
     incorporated into this shell.  We expect POSIX conformance by the time
     4.4 BSD is released. This man page is not intended to be a tutorial or 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 running 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.  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 and the file can be
     executed directly by the shell.

   Invocation
     If no args are present and if the standard input of the shell is
     connected to a terminal (or if the -i flag is set), and the -c option is
     not present, 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 .profile 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 next reads commands
     from the file named in ENV.  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.  To set the ENV
     variable to some file, place the following line in your .profile of your
     home directory

           ENV=$HOME/.shinit; export ENV

     substituting for “.shinit” any filename you wish. Since the ENV file is
     read for every invocation of the shell, including shell scripts and non-
     interactive shells, the following paradigm is useful for restricting
     commands in the ENV file to interactive invocations. Place commands
     within the “case” and “esac” below (these commands are described later):

           case $- in *i*)
                 # commands for interactive use only
                 ...
           esac

     If command line arguments besides the options have been specified, then
     the shell treats the first argument 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.

   Argument List Processing
     All of the single letter options have a corresponding name that can be
     used as an argument to the -o option. The set -o name is provided next to
     the single letter option in the description below. Specifying a dash “-”
     turns the option on, while using a plus “+” disables the option.  The
     following options can be set from the command line or with the set(1)
     builtin (described later).

           -a allexport     Export all variables assigned to. (UNIMPLEMENTED
                            for 4.4alpha)

           -c               Read commands from the command line.  No commands
                            will be read from the standard input.

           -C noclobber     Don't overwrite existing files with “>”.
                            (UNIMPLEMENTED for 4.4alpha)

           -e errexit       If not interactive, exit immediately if any
                            untested command fails.  The exit status of a
                            command is considered to be explicitly tested if
                            the command is used to control an if, elif, while,
                            or until; or if the command is the left hand
                            operand of an “&&” or “||” operator.

           -f noglob        Disable pathname expansion.

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

           -u nounset       Write a message to standard error when attempting
                            to expand a variable that is not set, and if the
                            shell is not interactive, exit immediately.
                            (UNIMPLEMENTED for 4.4alpha)

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

           -x xtrace        Write each command to standard error (preceded by
                            a ‘+ ’) before it is executed.  Useful for
                            debugging.

           -q quietprofile  If the -v or -x options have been set, do not
                            apply them when reading initialization files,
                            these being /etc/profile, .profile, and the file
                            specified by the ENV environment variable.

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

           -i interactive   Force the shell to behave interactively.

           -m monitor       Turn on job control (set automatically when
                            interactive).

           -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. with set(1)).

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

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

           -b notify        Enable asynchronous notification of background job
                            completion.  (UNIMPLEMENTED for 4.4alpha)

   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 that are special to the shell called “operators”.  There are
     two types of operators: control operators and redirection operators
     (their meaning is discussed later). Following is a list of operators:

           Control operators:
                 & && (); ;; | || <newline>

           Redirection operator:
                 < > >| << >> <& >& <<- <>

   Quoting
     Quoting is used to remove the special meaning of certain characters or
     words to the shell, such as operators, whitespace, or keywords.  There
     are three types of quoting: matched single quotes, matched double quotes,
     and backslash.

   Backslash
     A backslash preserves the literal meaning of the following character,
     with the exception of ⟨newline⟩.  A backslash preceding a ⟨newline⟩ is
     treated as a line continuation.

   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).

   Double Quotes
     Enclosing characters within double quotes preserves the literal meaning
     of all characters except dollarsign ($), backquote (`), and backslash
     (\).  The backslash inside double quotes is historically weird, and
     serves to quote only the following characters:
           $ ` " \ <newline>.
     Otherwise it remains literal.

   Reserved Words
     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 reserved words:

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

     Their meaning is discussed later.

   Aliases
     An alias is a name and corresponding value set using the alias(1) builtin
     command.  Whenever a reserved word may occur (see above), and after
     checking for reserved words, 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 <return>

     would become

           ls -F foobar <return>

     Aliases provide a convenient way for naive users to create shorthands for
     commands without having to learn how to create functions with arguments.
     They can also be used to create lexically obscure code.  This use is
     discouraged.

   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 POSIX 1003.2 document).  Essentially though, a line is
     read and if the first word of the line (or after a control operator) is
     not a reserved word, 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.
                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 “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 item 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

     where redir-op is one of the redirection operators mentioned previously.
     Following is a list of the possible redirections. The [n] is an optional
     number, as in ‘3’ (not ‘[3]’, that refers to a file descriptor.

           [n]> file   Redirect standard output (or n) to file.

           [n]>| file  Same, but override the -C option.

           [n]>> file  Append standard output (or n) to file.

           [n]< file   Redirect standard input (or n) from file.

           [n1]<&n2    Duplicate standard input (or n1) from file descriptor
                       n2.

           [n]<&-      Close standard input (or n).

           [n1]>&n2    Duplicate standard output (or n1) from n2.

           [n]>&-      Close standard output (or n).

           [n]<> file  Open file for reading and writing on standard input (or
                       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 “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, builtin commands, and
     normal programs -- and the command is searched for (by name) in that
     order.  They each are 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 builtins are executed internally to the shell, without spawning a
     new process.

     Otherwise, if the command name doesn't match a function or builtin, the
     command is searched for as a normal program in the filesystem (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 "#!", so
     execve(2) returns ENOEXEC then) the shell will interpret the program in a
     subshell.  The child shell will reinitialize itself in this case, so that
     the effect will be as if a new shell had been invoked to handle the ad-
     hoc shell script, except that the location of hashed commands located in
     the parent shell will be remembered by the child.

     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 builtin command by that name.
     If a builtin 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 PATH 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 builtin commands return
     exit codes, as does an executed shell function.

   Complex Commands
     Complex commands are combinations of simple commands with control
     operators or reserved words, 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.

   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.

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

     If the reserved word ! 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
     next) to be executed sequentially; a & causes asynchronous execution of
     the preceding AND-OR-list.

     Note that unlike some other shells, each process in the pipeline is a
     child of the invoking shell (unless it is a shell builtin, in which case
     it executes in the current shell -- but any effect it has on the
     environment is wiped).

   Background Commands -- &
     If a command is terminated by the control operator ampersand (&), the
     shell executes the command asynchronously -- that is, 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.

   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 proceed 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 iff the exit status of the
     first command is zero.  “||” is similar, but executes the second command
     iff 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 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 syntax of the for command is

           for variable in word...
           do   list
           done

     The words are expanded, and then the list is executed repeatedly with the
     variable set to each word in turn.  do and done may be replaced with “{”
     and “}”.

     The syntax of the break and continue command is

           break [ num ]
           continue [ num ]

     Break terminates the num innermost for or while loops.  Continue
     continues with the next iteration of the innermost loop.  These are
     implemented as builtin 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.

   Grouping Commands Together
     Commands may be grouped by writing either

           (list)

     or

           { list; }

     The first of these executes the commands in a subshell. Builtin commands
     grouped into a (list) will not affect the current shell. The second form
     does not fork another shell so is slightly more efficient. Grouping
     commands together this way allows you to redirect their output as though
     they were one program:

           { printf " hello " ; printf " world\n" ; } > 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 a local
     command.  This should appear as the first statement of a function, and
     the syntax is

           local [variable | -] ...

     Local is implemented as a builtin command.

     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 you make the variable
     x 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 than can be made local is “-”.  Making “-”
     local any shell options 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 currently executing function.  Return is implemented as
     a builtin command.

   Variables and Parameters
     The shell maintains a set of parameters.  A parameter denoted by a name
     is called a variable. When starting up, the shell turns all the
     environment variables into shell variables.  New variables can be set
     using the form

           name=value

     Variables set by the user must have a name consisting solely of
     alphabetics, numerics, and underscores - the first of which must not be
     numeric.  A parameter can also be denoted by a number or a special
     character as explained below.

   Positional Parameters
     A positional parameter is a parameter denoted by a number (n > 0). The
     shell sets these initially to the values of its command line arguments
     that follow the name of the shell script. The set(1) builtin can also be
     used to set or reset them.

   Special Parameters
     A special parameter is a parameter denoted by one of the following
     special characters.  The value of the parameter is listed next to its
     character.

     *            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 builtin 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.

     0 (Zero.)    Expands to the name of the shell or shell script.

   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 set -f is in effect).

     4.   Quote Removal.

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

   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 quoted string, and characters in
     embedded arithmetic expansions, command substitutions, and variable
     expansions, are 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.   Pathname expansion is not performed on the results of the expansion.

     2.   Field splitting is not performed on the results of the expansion,
          with the exception of @.

     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.  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 Alternative 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.

     ${#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 * 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 (“backquoted” version):

           `command`

     The shell expands the command substitution by executing command in a
     subshell environment and replacing the command substitution with the
     standard output of the command, removing sequences of one or more
     <newline>s at the end of the substitution.  (Embedded <newline>s before
     the end of the output are not removed; however, during field splitting,
     they may be translated into <space>s, depending on the value of IFS and
     quoting that is in effect.)

   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, and quote removal.

     Next, the shell treats this as an arithmetic expression and substitutes
     the value of the expression.

   White Space Splitting (Field Splitting)
     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.

     The shell treats each character of the IFS as a delimiter and use the
     delimiters to split the results of parameter expansion and command
     substitution into fields.

   Pathname Expansion (File Name Generation)
     Unless the -f flag 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 both Pathname Expansion and
     the case(1) 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 range of characters may be specified using a
     minus sign. The character class may be complemented by making an
     exclamation point the first character of the character class.

     To include a “]” in a character class, make it the first character listed
     (after the “!”, if any).  To include a minus sign, make it the first or
     last character listed

   Builtins
     This section lists the builtin commands which are builtin because they
     need to perform some operation that can't be performed by a separate
     process. In addition to these, there are several other commands that may
     be builtin for efficiency (e.g.  printf(1), echo(1), test(1), etc).

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

     . file
            The commands in the specified file are read and executed by the
            shell.

     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 builtin prints the
            names and values of all defined aliases (see unalias).

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

     command command arg...
            Execute the specified builtin command.  (This is useful when you
            have a shell function with the same name as a builtin command.)

     cd [directory]
            Switch to the specified directory (default $HOME).  If an entry
            for CDPATH appears in the environment of the cd command or the
            shell variable CDPATH is set and the directory name does not begin
            with a slash, then the directories listed in CDPATH will be
            searched for the specified directory.  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 this is different from the name that the user gave.  These
            may be different either because the CDPATH mechanism was used or
            because a symbolic link was crossed.

     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
            builtin 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 the exit status of the
            preceding command is used.

     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 it is exported by writing

                  export name=value

            With no arguments the export command lists the names of all
            exported variables.  With the -p option specified the output will
            be formatted suitably for non-interactive use.

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

     getopts optstring var
            The POSIX getopts command, not to be confused with the Bell Labs
            -derived getopt(1).

            The first argument should be a series of letters, each of which
            may be optionally followed by a colon to indicate that the option
            requires an argument.  The variable specified is set to the parsed
            option.

            The getopts command deprecates the older getopt(1) utility due to
            its handling of arguments containing whitespace.

            The getopts builtin may be used to obtain options and their
            arguments from a list of parameters.  When invoked, getopts places
            the value of the next option from the option string in the list in
            the shell variable specified by var and it's index in the shell
            variable OPTIND.  When the shell is invoked, OPTIND is initialized
            to 1.  For each option that requires an argument, the getopts
            builtin will place it in the shell variable OPTARG.  If an option
            is not allowed for in the optstring, then OPTARG will be unset.

            optstring is a string of recognized option letters (see
            getopt(3)).  If a letter is followed by a colon, the option is
            expected to have an argument which may or may not be separated
            from it by white space.  If an option character is not found where
            expected, getopts will set the variable var to a “?”; getopts will
            then unset OPTARG and write output to standard error.  By
            specifying a colon as the first character of optstring all errors
            will be ignored.

            A nonzero value is returned when the last option is reached.  If
            there are no remaining arguments, getopts will set var to the
            special option, “--”, otherwise, it will set var to “?”.

            The following code fragment shows how one might process the
            arguments for a command that can take the options [a] and [b], and
            the option [c], which requires an argument.

                  while getopts abc: f
                  do
                          case $f in
                          a | b)  flag=$f;;
                          c)      carg=$OPTARG;;
                          \?)     echo $USAGE; exit 1;;
                          esac
                  done
                  shift `expr $OPTIND - 1`

            This code will accept any of the following as equivalent:

                  cmd -acarg file file
                  cmd -a -c arg file file
                  cmd -carg -a file file
                  cmd -a -carg -- file file

     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.  Entries which have not been
            looked at since the last cd command are marked with an asterisk;
            it is possible for these entries to be invalid.

            With arguments, the hash command removes the specified commands
            from the hash table (unless they are functions) and then locates
            them.  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.

     jobs   This command lists out all the background processes which are
            children of the current shell process.

     pwd    Print the current directory.  The builtin command may differ from
            the program of the same name because the builtin 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 builtin version of pwd will continue to print the old
            name for the directory.

     read [-p prompt] [-r] 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 word splitting
            above, and the pieces are assigned to the variables in order.  At
            least one variable must be specified.  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. The read builtin will indicate success
            unless EOF is encountered on input, in which case failure is
            returned.

            By default, unless the -r option is specified, the backslash “\”
            acts as an escape character, causing the following character to be
            treated literally.  If a backslash is followed by a newline, the
            backslash and the newline will be deleted.

     readonly name...

     readonly -p
            The specified names are marked as read only, so that they cannot
            be subsequently modified or unset.  The shell allows the value of
            a variable to be set at the same time it is marked read only by
            writing

                  readonly name=value

            With no arguments the readonly command lists the names of all read
            only variables.  With the -p option specified the output will be
            formatted suitably for non-interactive use.

     set [{ -options | +options | -- }] arg...
            The set command performs three different functions.

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

            If options are given, it sets the specified option flags, or
            clears them as described in the section called Argument List
            Processing.

            The third use of the set command is to set the values of the
            shell's positional parameters to the specified args.  To change
            the positional parameters without changing any options, use “--”
            as the first argument to set.  If no args are present, the set
            command will clear all the positional parameters (equivalent to
            executing “shift $#”.)

     setvar variable value
            Assigns value to variable. (In general it is better to write
            variable=value rather than using setvar.  setvar is intended to be
            used in functions that assign values to variables whose names are
            passed as parameters.)

     shift [n]
            Shift the positional parameters n times.  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. If n is greater than the
            number of positional parameters, shift will issue an error
            message, and exit with return status 2.

     times  Print the accumulated user and system times for the shell and for
            processes run from the shell.  The return status is 0.

     trap action signal...
            Cause the shell to parse and execute action when any of the
            specified signals are received. The signals are specified by
            signal number. If signal is 0, the action is executed when the
            shell exits.  action may be null or “-”; the former causes the
            specified signal to be ignored and the latter causes the default
            action to be taken. When the shell forks off a subshell, it 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.

     type [name ...]
            Interpret each name as a command and print the resolution of the
            command search. Possible resolutions are: shell keyword, alias,
            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 [-H | -S] [-a | -tfdscmlpn [value]]
            Inquire about or set the hard or soft limits on processes or set
            new limits. The choice between hard limit (which no process is
            allowed to violate, and which may not be raised once it has been
            lowered) and soft limit (which causes processes to be signaled but
            not necessarily killed, and which may be raised) is made with
            these flags:

            -H          set or inquire about hard limits

            -S          set or inquire about soft limits. If neither -H nor -S
                        is specified, the soft limit is displayed or both
                        limits are set. If both are specified, the last one
                        wins.

            The limit to be interrogated or set, then, is chosen by specifying
            any one of these flags:

            -a          show all the current limits

            -t          show or set the limit on CPU time (in seconds)

            -f          show or set the limit on the largest file that can be
                        created (in 512-byte blocks)

            -d          show or set the limit on the data segment size of a
                        process (in kilobytes)

            -s          show or set the limit on the stack size of a process
                        (in kilobytes)

            -c          show or set the limit on the largest core dump size
                        that can be produced (in 512-byte blocks)

            -m          show or set the limit on the total physical memory
                        that can be in use by a process (in kilobytes)

            -l          show or set the limit on how much memory a process can
                        lock with mlock(2) (in kilobytes)

            -p          show or set the limit on the number of processes this
                        user can have at one time

            -n          show or set the limit on the number files a process
                        can have open at once

            If none of these is specified, it is the limit on file size that
            is shown or set. If value is specified, the limit is set to that
            number; otherwise the current limit is displayed.

            Limits of an arbitrary process can be displayed or set using the
            sysctl(8) utility.

     umask [mask]
            Set the value of umask (see umask(2)) to the specified octal
            value. If the argument is omitted, the umask value is printed.

     unalias [-a] [name]
            If name is specified, the shell removes that alias. If -a is
            specified, all aliases are removed.

     unset name...
            The specified variables and functions are unset and unexported. If
            a given name corresponds to both a variable and a function, both
            the variable and the function are unset.

     wait [job]
            Wait for the specified job to complete and return the exit status
            of the last process in the job. If the argument is omitted, wait
            for all jobs to complete and the return an exit status of zero.

ENVIRONMENT
     HOME       Set automaticly by login(1) from the user's login directory in
                the password file (passwd(4)).  This environment variable also
                functions as the default argument for the cd builtin.

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

     CDPATH     The search path used with the cd builtin.

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

     MAILCHECK  The frequency in seconds that the shell checks for the arrival
                of mail in the files specified by the MAILPATH or the MAIL
                file.  If set to 0, the check will occur at each prompt.

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

     PS1        The primary prompt string, which defaults to “$  ”, unless you
                are the superuser, in which case it defaults to “#  ”.

     PS2        The secondary prompt string, which defaults to “>  ”.

     IFS        Input Field Separators.  This is normally set to <space> <tab>
                and <newline>.  See the White Space Splitting section for more
                details.

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

     HISTSIZE   The number of lines in the history buffer for the shell.

FILES
     $HOME/.profile

     /etc/profile

SEE ALSO
     csh(1), getopt(1), ksh(1), login(1), test(1), getopt(3), passwd(4),
     profile(4), environ(5) sysctl(8)

HISTORY
     A sh command appeared in Version 1 AT&T UNIX.  It was, however,
     unmaintainable so we wrote this one.

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.

BUGS
     Setuid shell scripts should be avoided at all costs, as they are a
     significant security risk.

                                January 9, 1999