ash.1

操作系统设计与实现源码

  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.


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  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
     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


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         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
         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


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     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.

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,


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     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.

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.







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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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