

SH(1)                     Minix Programmer's Manual                      SH(1)


NAME
     ash - a shell

SYNOPSIS
     ash [ -efIijnsxz ] [ +efIijnsxz ] [ -c command ] [ arg ] ...

COPYRIGHT
     Copyright 1989 by Kenneth Almquist.

DESCRIPTION
     Ash is a version of sh with features similar to those  of  the  System  V
     shell.   This  manual page lists all the features of ash but concentrates
     on the ones not in other shells.

  Invocation

     If the -c options is given, then the shell executes the  specified  shell
     command.   The -s flag cause the shell to read commands from the standard
     input (after executing any command specified  with  the  -c  option.   If
     neither  the -s or -c options are set, then the first arg is taken as the
     name of a file to read commands from.   If  this  is  impossible  because
     there  are  no  arguments following the options, then ash will set the -s
     flag and will read commands from the standard input.

     The shell sets the initial value of the positional  parameters  from  the
     args  remaining  after  any arg used as the name of a file of commands is
     deleted.

     The flags (other than -c) are  set  by  preceding  them  with  ``-''  and
     cleared  by  preceding them with ``+''; see the set builtin command for a
     list of flags.  If no value is specified for the -i flag, the -s flag  is
     set,  and  the  standard  input  and output of the shell are connected to
     terminals, then the -i flag will be set.  If no value  is  specified  for
     the -j flag, then the -j flag will be set if the -i flag is set.

     When the shell is invoked with the -c option,  it  is  good  practice  to
     include  the  -i flag if the command was entered interactively by a user.
     For compatibility with the System V shell,  the  -i  option  should  come
     after the -c option.

     If the first character of argument zero to the shell is ``-'', the  shell
     is  assumed  to be a login shell, and the files /etc/profile and .profile
     are read if they exist.  If the environment variable  SHINIT  is  set  on
     entry  to  the  shell,  the  commands  in  SHINIT are normally parsed and
     executed.  SHINIT is not examined if the shell is a login shell, or if it
     the  shell  is  running a shell procedure.   (A shell is considered to be
     running a shell procedure if neither the -s nor the -c options are set.)

  Control Structures

     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.  (This differs from the System V shell, which  requires
     a list to contain at least one command in most cases.)  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


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     onto the next command; otherwise it waits for the  command  to  terminate
     before proceeding to the next one.

     ``&&'' and ``||''  are  binary  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 ``|'' operator is a binary operator which feeds the  standard  output
     of  the first command into the standard input of the second command.  The
     exit status of the ``|'' operator  is  the  exit  status  of  the  second
     command.  ``|'' has a higher priority than ``||'' or ``&&''.

     An if command looks like

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


     A while command looks like

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

     The for command looks like

         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 break and continue commands look like

         break [ num ]
         continue [ num ]

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





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     The case command looks like

         case word in
         pattern) list ;;
         ...
         esac

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

     Commands may be grouped by writing either

         (list)

     or

         { list; }

     The first of these executes the commands in a subshell.

     A function definition looks like

         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 staement of a function, and
     looks like

         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.   Ash  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 return command looks like

         return [ exitstatus ]

     It terminates the currently executing function.  Return is implemented as
     a builtin command.



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  Simple Commands

     A simple command is a sequence of  words.   The  execution  of  a  simple
     command  proceeds  as  follows.   First,  the  leading  words of the form
     ``name=value'' are stripped off and assigned to the  environment  of  the
     command.   Second,  the  words  are expanded.  Third, the first remaining
     word is taken as the command name that command is located.   Fourth,  any
     redirections  are performed.  Fifth, the command is executed.  We look at
     these operations in reverse order.

     The execution of the command varies with the type of command.  There  are
     three  types  of commands:  shell functions, builtin commands, and normal
     programs.

     When a shell function is executed, all of the shell positional parameters
     (except  $0,  which  remains  unchanged) are set to the parameters to 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  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.

     When a normal program is executed, the shell runs  the  program,  passing
     the  parameters  and the environment to the program.  If the program is a
     shell procedure, the shell will interpret 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  commands  located  in  the  parent shell will be
     remembered by the child.  If the program is a file beginning with ``#!'',
     the remainder of the first line specifies an interpreter for the program.
     The shell (or the operating system, under Berkeley  UNIX)  will  run  the
     interpreter  in this case.  The arguments to the interpreter will consist
     of any arguments given on the first line of the program, followed by  the
     name of the program, followed by the arguments passed to the program.

  Redirection

     Input/output redirections can be intermixed with the words  in  a  simple
     command  and  can  be  placed  following any of the other commands.  When
     redirection  occurs,  the  shell  saves  the  old  values  of  the   file
     descriptors  and  restores  them  when the command completes.  The ``<'',
     ``>'', and ``>>''  redirections  open  a  file  for  input,  output,  and
     appending, respectively.  The ``<&digit'' and ``>&digit'' makes the input
     or output a duplicate of the file descriptor numbered by the digit.  If a
     minus  sign  is  used in place of a digit, the standard input or standard
     output are closed.

     The ``<< word'' redirection takes input from a  here  document.   As  the
     shell encounters ``<<'' redirections, it collects them.  The next time it
     encounters an unescaped newline, it reads the  documents  in  turn.   The
     word  following  the  ``<<''  specifies  the  contents  of  the line that
     terminates the document.  If none of the quoting methods ('', "",  or  \)


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     are used to enter the word, then the document  is  treated  like  a  word
     inside double quotes:  ``$'' and backquote are expanded and backslash can
     be used to escape these and to continue  long  lines.   The  word  cannot
     contain  any  variable  or  command  substitutions, and its length (after
     quoting) must be in the range of 1 to 79 characters.  If ``<<-'' is  used
     in  place  of ``<<'', then leading tabs are deleted from the lines of the
     document.  (This is to allow you do indent  shell  procedures  containing
     here documents in a natural fashion.)

     Any of the preceding redirection operators may be preceded  by  a  single
     digit  specifying  the file descriptor to be redirected.  There cannot be
     any white space between the digit and the redirection operator.

  Path Search

     When locating a command, the shell first looks to see if it has  a  shell
     function  by  that  name.   Then,  if  PATH does not contain an entry for
     "%builtin", 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, or a directory name
     followed by a flag beginning with a percent sign.  The current  directory
     should be indicated by an empty directory name.

     If no percent sign is present, then the entry causes the shell to  search
     for  the command in the specified directory.  If the flag is ``%builtin''
     then the list of shell builtin commands is  searched.   If  the  flag  is
     ``%func''  then  the  directory  is  searched for a file which is read as
     input to the shell.  This file should define a function whose name is the
     name of the command being searched for.

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

  The Environment

     The environment of a command is a set  of  name/value  pairs.   When  the
     shell  is  invoked,  it  reads  these  names  and  values, sets the shell
     variables with these names to the corresponding  values,  and  marks  the
     variables as exported.  The export command can be used to mark additional
     variables as exported.

     The environment of a command is constructed  by  constructing  name/value
     pairs  from all the exported shell variables, and then modifying this set
     by the assignments which precede the command, if any.

  Expansion

     The process of evaluating words when a shell  procedure  is  executed  is
     called   expansion.    Expansion   consists   of  four  steps:   variable
     substitution,  command  substitution,  word  splitting,  and  file   name
     generation.   If  a  word  is the expression following the word case in a
     case statement, the file name which follows a redirection symbol,  or  an
     assignment to the environment of a command, then the word cannot be split
     into multiple words.  In these cases, the last two steps of the expansion


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     process are omitted.

  Variable Substitution

     To be written.

  Command Substitution

     Ash accepts two syntaxes for command substitution:

         `list`

     and

         $(list)

     Either  of  these  may  be  included  in  a  word.   During  the  command
     substitution process, the command (syntactly a list) will be executed and
     anything that the command writes to the standard output will be  captured
     by  the shell.  The final newline (if any) of the output will be deleted;
     the rest of the output will be substituted for the command in the word.

  Word Splitting

     When the value of a variable or the output of a command  is  substituted,
     the  resulting  text is subject to word splitting, unless the dollar sign
     introducing the variable or backquotes containing the text were  enclosed
     in  double  quotes.   In addition, ``$@'' is subject to a special type of
     splitting, even in the presence of double quotes.

     Ash uses two different splitting algorithms.  The normal approach,  which
     is  intended  for splitting text separated by which space, is used if the
     first character of the shell variable  IFS  is  a  space.   Otherwise  an
     alternative   experimental  algorithm,  which  is  useful  for  splitting
     (possibly empty) fields separated by a separator character, is used.

     When performing splitting, the shell scans the replacement  text  looking
     for  a  character (when IFS does not begin with a space) or a sequence of
     characters (when IFS does begin with a space), deletes the  character  or
     sequence  of  characters,  and  spits  the  word into two strings at that
     point.  When IFS begins with a space, the shell  deletes  either  of  the
     strings  if they are null.  As a special case, if the word containing the
     replacement text is the null string, the word is deleted.

     The variable ``$@'' is special in two ways.  First, splitting takes place
     between the positional parameters, even if the text is enclosed in double
     quotes.  Second, if the word containing the replacement text is the  null
     string  and there are no positional parameters, then the word is deleted.
     The result of these rules is that "$@" is equivalent  to  "$1"  "$2"  ...
     "$n",  where  n  is the number of positional parameters.  (Note that this
     differs from the System V shell.  The System V documentation claims  that
     "$@"  behaves  this way; in fact on the System V shell "$@" is equivalent
     to "" when there are no positional paramteters.)

  File Name Generation



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     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.

     If a word fails to match any files and the -z flag is not set,  then  the
     word  will  be  left  unchanged  (except that the meta-characters will be
     converted to normal characters).  If the -z flag is set, then the word is
     only  left unchanged if none of the patterns contain a character that can
     match anything besides itself.  Otherwise the -z flag forces the word  to
     be  replaced  with  the names of the files that it matches, even if there
     are zero names.

  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.

     Two exclamation points at the  beginning  of  a  pattern  function  as  a
     ``not''  operator,  causing  the  pattern  to  match  any string that the
     remainder of the pattern does not match.  Other occurances of exclamation
     points in a pattern match exclamation points.  Two exclamation points are
     required rather than one to decrease the incompatibility with the  System
     V shell (which does not treat exclamation points specially).

     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.

  The /u Directory

     By convention, the name ``/u/user'' refers to the home directory  of  the
     specified  user.   There  are  good  reasons  why  this feature should be
     supported by the file system (using a feature  such  as  symbolic  links)
     rather  than  by the shell, but ash is capable of performing this mapping
     if the file system doesn't.  If the mapping is done by ash,  setting  the
     -f flag will turn it off.



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  Character Set

     Ash silently discards  nul  characters.   Any  other  character  will  be
     handled  correctly  by  ash, including characters with the high order bit
     set.

  Job Names and Job Control

     The term job refers to a process created by a shell command,  or  in  the
     case of a pipeline, to the set of processes in the pipeline.  The ways to
     refer to a job are:

         %number
         %string
         %%
         process_id

     The first form identifies a job by job number.  When a  command  is  run,
     ash  assigns it a job number (the lowest unused number is assigned).  The
     second form identifies a job by giving a prefix of the  command  used  to
     create  the  job.   The prefix must be unique.  If there is only one job,
     then the null prefix will identify the job, so you can refer to  the  job
     by writing ``%''.  The third form refers to the current job.  The current
     job is the last job to be stopped while it was in the  foreground.   (See
     the  next  paragraph.)   The  last  form  identifies  a job by giving the
     process id of the last process in the job.

     If the operating system that ash is running on supports job control,  ash
     will  allow  you  to  use it.  In this case, typing the suspend character
     (typically ^Z) while running a command will return you to  ash  and  will
     make  the  suspended  command the current job.  You can then continue the
     job in the background by typing  bg,  or  you  can  continue  it  in  the
     foreground by typing fg.

  Atty

     If the shell variable ATTY is set, and the shell variable TERM is not set
     to  ``emacs'', then ash generates appropriate escape sequences to talk to
     atty(1).

  Exit Statuses

     By tradition, an exit status of zero means that a command  has  succeeded
     and  a  nonzero  exit  status indicates that the command failed.  This is
     better than no convention at all, but in practice it is extremely  useful
     to  allow  commands  that  succeed  to  use  the  exit  status  to return
     information to the caller.  A variety of  better  conventions  have  been
     proposed,  but  none  of  them  has  met  with  universal  approval.  The
     convention used  by  ash  and  all  the  programs  included  in  the  ash
     distribution is as follows:
                 0           Success.
                 1           Alternate success.
                 2           Failure.
                 129-...     Command terminated by a signal.
     The alternate success return is used  by  commands  to  indicate  various
     conditions which are not errors but which can, with a little imagination,


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     be conceived of as less successful than plain success.  For example, test
     returns  1  when the tested condition is false and getopts returns 1 when
     there  are  no  more  options.   Because  this  convention  is  not  used
     universally, the -e option of ash causes the shell to exit when a command
     returns 1 even though that contradicts the convention described here.

     When a command is terminated by a signal, the uses 128  plus  the  signal
     number as the exit code for the command.

  Builtin Commands

     This concluding 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
     (catf,  echo,  expr,  line,  nlecho,  test,  ``:'',  and  true) which can
     optionally be compiled into the shell.  The  builtin  commands  described
     below that accept options use the System V Release 2 getopt(3) syntax.


  bg [ job ] ...
     Continue the specified jobs (or the current job if no jobs are given)  in
     the  background.   This command is only available on systems with Bekeley
     job control.

  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 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.

  . file
     The commands in the specified file are read and executed by the shell.  A
     path  search is not done to find the file because the directories in PATH
     generally contain files that are intended to be executed, not read.

  eval string...
     The strings are parsed as shell commands  and  executed.   (This  differs
     from  the  System V shell, which concatenates the arguments (separated by
     spaces) and parses the result as a single 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.  If the
     command is not found, the exec command causes the shell to exit.



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  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.  Ash 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.

  fg [ job ]
     Move the specified job or  the  current  job  to  the  foreground.   This
     command is only available on systems with Bekeley job control.

  getopts optstring var
     The System V 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 command to delete all the entries in the
     hash table except for functions.

  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 splitting above, and the pieces are


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     assigned to the variables in  order.   If  there  are  more  pieces  than
     variables,  the  remaining  pieces (along with the characters in IFS that
     separated them) are assigned to the last variable.   If  there  are  more
     variables  than  pieces,  the  remaining  variables are assigned the null
     string.

     The -e  option  causes  any  backslashes  in  the  input  to  be  treated
     specially.   If  a  backslash is followed by a newline, the backslash and
     the newline will be deleted.  If a backslash is  followed  by  any  other
     character, the backslash will be deleted and the following character will
     be treated as though it were not in IFS, even if it is.

  readonly name...
     The specified names are marked as read  only,  so  that  they  cannot  be
     subsequently modified or unset.  Ash 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
     if  the  option  flags are introduced with a + rather than a -.  Only the
     first argument to set can contain options.  The possible options are:

     -e  Causes the shell to exit when a command  terminates  with  a  nonzero
         exit status, except when the exit status of the command is explicitly
         tested.  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  Turn off file name generation.

     -I  Cause the shell to ignore end  of  file  conditions.   (This  doesn't
         apply when the shell a script sourced using the ``.''  command.)  The
         shell will in fact exit if it gets 50 eof's in a row.

     -i  Make the shell interactive.  This causes  the  shell  to  prompt  for
         input,  to trap interrupts, to ignore quit and terminate signals, and
         to return to the main command loop rather than exiting on error.

     -j  Turns on Berkeley job control, on systems that support it.  When  the
         shell starts up, the -j is set by default if the -i flag is set.

     -n  Causes the shell to read commands but not  execute  them.   (This  is
         marginally useful for checking the syntax of scripts.)

     -s  If this flag is set  when  the  shell  starts  up,  the  shell  reads
         commands  from  its  standard  input.   The shell doesn't examine the


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         value of this flag any other time.

     -x  If this flag is set, the shell will print  out  each  command  before
         executing it.

     -z  If this flag is set, the file name generation  process  may  generate
         zero  files.   If  it is not set, then a pattern which does not match
         any files will be replaced by a quoted version of the pattern.

     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 leave the value of
     the positional parameters unchanged, so to set the positional  parameters
     to set of values that may be empty, execute the command

         shift $#

     first to clear out the old values of the positional parameters.

  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.

  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.




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EXAMPLES
     The following function redefines the cd command:

         cd() {
             if command cd "$@"
             then if test -f .enter
             then . .enter
             else return 0
             fi
             fi
         }

     This function causes the file ``.enter'' to be  read  when  you  enter  a
     directory,  if it exists.  The command command is used to access the real
     cd command.  The ``return 0'' ensures that the function  will  return  an
     exit  status  of zero if it successfully changes to a directory that does
     not contain a ``.enter''  file.   Redefining  existing  commands  is  not
     always a good idea, but this example shows that you can do it if you want
     to.

     The suspend function distributed with ash looks like

         # Copyright (C) 1989 by Kenneth Almquist.  All rights reserved.
         # This file is part of ash, which is distributed under the terms
         # specified by the Ash General Public License.

         suspend() {
             local -
             set +j
             kill -TSTP 0
         }

     This turns off job control and then sends a stop signal  to  the  current
     process group, which suspends the shell.  (When job control is turned on,
     the shell ignores the TSTP signal.)  Job control will be turned  back  on
     when  the function returns because ``-'' is local to the function.  As an
     example of what not to do, consider an earlier version of suspend:

         suspend() {
             suspend_flag=$-
             set +j
             kill -TSTP 0
             set -$suspend_flag
         }

     There are two problems  with  this.   First,  suspend_flag  is  a  global
     variable  rather  than  a  local  one,  which  will cause problems in the
     (unlikely) circumstance that the user is using  that  variable  for  some
     other  purpose.   Second,  consider  what  happens  if  shell received an
     interrupt signal after it executes the first set command  but  before  it
     executes  the  second  one.   The  interrupt  signal will abort the shell
     function, so that the second set command will never be executed  and  job
     control  will  be  left  off.   The  first version of suspend avoids this
     problem by turning job control off only in a  local  copy  of  the  shell
     options.   The  local  copy  of  the  shell options is discarded when the
     function is terminated, no matter how it is terminated.


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HINTS
     Shell variables can be used to provide abbreviations for things which you
     type frequently.  For example, I set
                 export h=$HOME
     in my .profile so that I can type the name of my home directory simply by
     typing ``$h''.

     When writing shell procedures, try not to make assumptions about what  is
     imported  from  the  environment.   Explicitly  unset  or  initialize all
     variables, rather than assuming they will be unset.  If you use cd, it is
     a good idea to unset CDPATH.

     People sometimes use ``<&-'' or ``>&-'' to provide no input to a  command
     or  to  discard  the  output of a command.  A better way to do this is to
     redirect the input or output of the command to /dev/null.

     Word splitting and file name generation are performed by default, and you
     have  to explicitly use double quotes to suppress it.  This is backwards,
     but you can learn to live with it.  Just get  in  the  habit  of  writing
     double  quotes  around  variable and command substitutions, and omit them
     only when you really want word splitting and file  name  generation.   If
     you want word splitting but not file name generation, use the -f option.

AUTHORS
     Kenneth Almquist

SEE ALSO
     echo(1), expr(1), line(1), pwd(1), true(1).

BUGS
     When command substitution occurs inside a  here  document,  the  commands
     inside  the  here document are run with their standard input closed.  For
     example, the following will not word because the standard  input  of  the
     line command will be closed when the command is run:

         cat <<-!
         Line 1: $(line)
         Line 2: $(line)
         !


     Unsetting a function which is currently being executed may cause  strange
     behavior.

     The shell syntax allows a here document to be terminated  by  an  end  of
     file  as  well  as by a line containing the terminator word which follows
     the ``<<''.  What this means is that if you mistype the terminator  line,
     the  shell  will  silently  swallow  up the rest of your shell script and
     stick it in the here document.









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