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Manual of the 4.4BSD-Lite Almquist shell

SH(1)                                                                    SH(1)

       sh - command interpreter (shell)

       sh [-/+aCefnuvxIimsVEb] [-/+o longname] [arg ...]

       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 (run GNU's bash if you
       want that).  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.


       The shell is a command that reads lines from either a file or the ter-
       minal, 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 stor-
       age, 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 and the file
       can be executed directly by the shell.


       If no args are present and if the standard input of the shell is con-
       nected to a terminal (or if the -i flag is set), the shell is consid-
       ered an interactive shell.  An interactive shell generally prompts
       before each command and handles programming and command errors differ-
       ently (as described below).  When first starting, the shell inspects
       argument 0, and if it begins with a dash '-', the shell is also consid-
       ered 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 restrict-
       ing commands in the ENV file to interactive invocations.  Place com-
       mands within the ``case'' and ``esac'' below (these commands are
       described later):

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

       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    noclobber
              Don't overwrite existing files with ``>''.  (UNIMPLEMENTED for

       -e    errexit
              If not interactive, exit immediately if any untested command
              fails.  The exit status of a command is considered to be explic-
              itly 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.

       -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 builtin vi(1) command line editor (disables -E if it
              has been set).

       -E    emacs
              Enable the builtin 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 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.


       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.

       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.


       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 argu-
       ments.  They can also be used to create lexically obscure code.  This
       use is discouraged.


       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 follow-
       ing actions:

       1) Leading words of the form ``name=value'' are stripped off and
       assigned to the environment of the simple command.  Redirection opera-
       tors 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 1) affect the current

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


       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 redirec-
       tion is:

                 [n] redir-op file

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

       [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.

              Duplicate standard input (or n1) from file descriptor n2.

              Close standard input (or n).

              Duplicate standard output (or n) from n2.

              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

       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 parame-
       ters (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.

       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 a shell procedure, the shell will inter-
       pret the program in a subshell.  The shell will reinitialize itself in
       this case, so that the effect will be as if a new shell had been
       invoked to handle the shell procedure, except that the location of com-
       mands located in the parent shell will be remembered by the child.

       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.  Finally, it 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 by an empty directory name.

       Command names containing a slash are simply executed without performing
       any of the above searches.

       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 function.

       Complex Commands

       Complex commands are combinations of simple commands with control oper-
       ators 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.


       A pipeline is a sequence of one or more commands separated by the con-
       trol operator |.  The standard output of all but the last command is
       connected to the standard input of the next command.

       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 speci-
       fied 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.  Oth-
       erwise, 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 sta-
       tus is 1; if the last command returns greater than zero, the exit sta-
       tus 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.

       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; oth-
       erwise it waits for the command to terminate before proceeding to the
       next one.

       ``&&'' 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.

       The syntax of the if command is

           if list
           then list
           [ elif list
           then    list ] ...
           [ else list ]

       The syntax of the while command is

           while list
           do   list

       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 repeat until the exit status of the first list
       is zero.

       The syntax of the for command is

           for variable in word...
           do   list

       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 con-
       tinues with the next iteration of the innermost loop.  These are imple-
       mented as builtin commands.

       The syntax of the case command is

           case word in
           pattern) list ;;

       The pattern can actually be one or more patterns (see Shell Patterns
       described later), separated by ``|'' characters.

       Commands may be grouped by writing either



           { list; }

       The first of these executes the commands in a subshell.


       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 ini-
       tially 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 envi-
       ronment variables into shell variables.  New variables can be set using
       the form


       Variables set by the user must have a name consisting solely of alpha-
       betics, 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 spe-
       cial characters.  The value of the parameter is listed next to its

       *      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

       @      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 posi-
              tional 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

       Tilde expansions, parameter expansions, command substitutions, arith-
       metic 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 sin-
       gle 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 sub-
       stitution, 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:


       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:


       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-

       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.

              Use Default Values.  If parameter is unset or null, the expan-
              sion of word is substituted; otherwise, the value of parameter
              is substituted.

              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.

              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.

              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; omis-
       sion of the colon results in a test for a parameter that is only unset.

              String Length.  The length in characters of the value of parame-

       The following four varieties of parameter expansion provide for sub-
       string 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 charac-
       ters to be quoted, whereas quoting characters within the braces has
       this effect.  (UNIMPLEMENTED IN 4.4alpha)

              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

              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

              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

              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

       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:


       or (``backquoted'' version):


       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 <new-
       line>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 expan-
       sion is as follows:


       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 expan-
       sion 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 sub-
       stitution 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 start-
       ing 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 there 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 char-
       acter 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

       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


       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).

       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 the 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 actu-
              ally 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.

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

       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...
              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 vari-
              able 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.

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

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

       fc  -s [old=new] [first]
              The fc builtin lists, or edits and re-executes, commands previ-
              ously 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 vari-
              able.  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.

              Suppress command numbers when listing with -l.

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

              Re-execute the command without invoking an editor.


              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:

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

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

              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 environment variables affect the execution of fc:

              Name of the editor to use.

              The number of previous commands that are accessable.

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

       getopts optstring var
              The POSIX getopts command.

       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 com-
              mand to delete all the entries in the hash table except for

       jobid [ job ]
              Print the process id's of the processes in the job.  If the job
              argument is omitted, use the current job.

       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 ] [ -e ] 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 split-
              ting 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

              The -e option causes any backslashes in the input to be treated
              specially.  If a backslash is followed by a newline, the back-
              slash and the newline will be deleted.   If a backslash is fol-
              lowed 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.

       readonly name...
              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.

       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

              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 (equiv-
              alent 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 there are zero
              positional parameters, shifting doesn't do anything.

       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. Action may be null or omitted; 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.

       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 sta-
              tus 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

       Command Line Editing

       When sh is being used interactively from a terminal, the current
       command and the command history (see fc in Builtins) can be edited
       using vi-mode command-line editing. This mode uses commands, described
       below, similar to a subset of those described in the vi man page.  The
       command set -o vi enables vi-mode editing and place sh into vi insert
       mode.  With vi-mode enabled, sh can be switched between insert mode and
       command mode. The editor is not described in full here, but will be in
       a later document. It's similar to vi: typing <ESC> will throw you into
       command VI command mode. Hitting <return> while in command mode will
       pass the line to the shell.