subshells.html

Shall高级编程

	    variable</A
> lies only within the function,
	    block of code, or subshell within which it is defined.</P
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>While the <A
HREF="variables2.html#BASHSUBSHELLREF"
>$BASH_SUBSHELL</A
>
	      internal variable indicates the nesting level of a
	      subshell, the <A
HREF="variables2.html#SHLVLREF"
>$SHLVL</A
>
	      variable <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>shows no change</I
></SPAN
> within
	      a subshell.</P
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;echo " \$BASH_SUBSHELL outside subshell       = $BASH_SUBSHELL"           # 0
   2&nbsp;  ( echo " \$BASH_SUBSHELL inside subshell        = $BASH_SUBSHELL" )     # 1
   3&nbsp;  ( ( echo " \$BASH_SUBSHELL inside nested subshell = $BASH_SUBSHELL" ) ) # 2
   4&nbsp;# ^ ^                           *** nested ***                        ^ ^
   5&nbsp;
   6&nbsp;echo
   7&nbsp;
   8&nbsp;echo " \$SHLVL outside subshell = $SHLVL"       # 3
   9&nbsp;( echo " \$SHLVL inside subshell  = $SHLVL" )   # 3 (No change!)</PRE
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            </P
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><P
>Directory changes made in a subshell do not carry over to the
        parent shell.</P
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CLASS="EXAMPLE"
><HR><A
NAME="ALLPROFS"
></A
><P
><B
>Example 20-2. List User Profiles</B
></P
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CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# allprofs.sh: print all user profiles
   3&nbsp;
   4&nbsp;# This script written by Heiner Steven, and modified by the document author.
   5&nbsp;
   6&nbsp;FILE=.bashrc  #  File containing user profile,
   7&nbsp;              #+ was ".profile" in original script.
   8&nbsp;
   9&nbsp;for home in `awk -F: '{print $6}' /etc/passwd`
  10&nbsp;do
  11&nbsp;  [ -d "$home" ] || continue    # If no home directory, go to next.
  12&nbsp;  [ -r "$home" ] || continue    # If not readable, go to next.
  13&nbsp;  (cd $home; [ -e $FILE ] &#38;&#38; less $FILE)
  14&nbsp;done
  15&nbsp;
  16&nbsp;#  When script terminates, there is no need to 'cd' back to original directory,
  17&nbsp;#+ because 'cd $home' takes place in a subshell.
  18&nbsp;
  19&nbsp;exit 0</PRE
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><P
>A subshell may be used to set up a <SPAN
CLASS="QUOTE"
>"dedicated
	      environment"</SPAN
> for a command group.
	      <TABLE
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><TR
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;COMMAND1
   2&nbsp;COMMAND2
   3&nbsp;COMMAND3
   4&nbsp;(
   5&nbsp;  IFS=:
   6&nbsp;  PATH=/bin
   7&nbsp;  unset TERMINFO
   8&nbsp;  set -C
   9&nbsp;  shift 5
  10&nbsp;  COMMAND4
  11&nbsp;  COMMAND5
  12&nbsp;  exit 3 # Only exits the subshell!
  13&nbsp;)
  14&nbsp;# The parent shell has not been affected, and the environment is preserved.
  15&nbsp;COMMAND6
  16&nbsp;COMMAND7</PRE
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>

    As seen here, the <A
HREF="internal.html#EXITREF"
>exit</A
>
    command only terminates the subshell in which it is running,
    <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> the parent shell or script.</P
><P
>One application of such a <SPAN
CLASS="QUOTE"
>"dedicated environment"</SPAN
>
        is testing whether a variable is defined.
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if (set -u; : $variable) 2&#62; /dev/null
   2&nbsp;then
   3&nbsp;  echo "Variable is set."
   4&nbsp;fi     #  Variable has been set in current script,
   5&nbsp;       #+ or is an an internal Bash variable,
   6&nbsp;       #+ or is present in environment (has been exported).
   7&nbsp;
   8&nbsp;# Could also be written [[ ${variable-x} != x || ${variable-y} != y ]]
   9&nbsp;# or                    [[ ${variable-x} != x$variable ]]
  10&nbsp;# or                    [[ ${variable+x} = x ]]
  11&nbsp;# or                    [[ ${variable-x} != x ]]</PRE
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><P
>Another application is checking for a lock file:
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><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;if (set -C; : &#62; lock_file) 2&#62; /dev/null
   2&nbsp;then
   3&nbsp;  :   # lock_file didn't exist: no user running the script
   4&nbsp;else
   5&nbsp;  echo "Another user is already running that script."
   6&nbsp;exit 65
   7&nbsp;fi
   8&nbsp;
   9&nbsp;#  Code snippet by St閜hane Chazelas,
  10&nbsp;#+ with modifications by Paulo Marcel Coelho Aragao.</PRE
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>
      </P
><P
>+</P
><P
>Processes may execute in parallel within different
        subshells. This permits breaking a complex task into subcomponents
        processed concurrently.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="PARALLEL-PROCESSES"
></A
><P
><B
>Example 20-3. Running parallel processes in subshells</B
></P
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CLASS="PROGRAMLISTING"
>   1&nbsp;	(cat list1 list2 list3 | sort | uniq &#62; list123) &#38;
   2&nbsp;	(cat list4 list5 list6 | sort | uniq &#62; list456) &#38;
   3&nbsp;	# Merges and sorts both sets of lists simultaneously.
   4&nbsp;	# Running in background ensures parallel execution.
   5&nbsp;	#
   6&nbsp;	# Same effect as
   7&nbsp;	#   cat list1 list2 list3 | sort | uniq &#62; list123 &#38;
   8&nbsp;	#   cat list4 list5 list6 | sort | uniq &#62; list456 &#38;
   9&nbsp;	
  10&nbsp;	wait   # Don't execute the next command until subshells finish.
  11&nbsp;	
  12&nbsp;	diff list123 list456</PRE
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><P
>Redirecting I/O to a subshell uses the <SPAN
CLASS="QUOTE"
>"|"</SPAN
> pipe
	  operator, as in <TT
CLASS="USERINPUT"
><B
>ls -al | (command)</B
></TT
>.</P
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CLASS="NOTE"
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HSPACE="5"
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VALIGN="TOP"
><P
>A command block between <TT
CLASS="REPLACEABLE"
><I
>curly
	  braces</I
></TT
> does <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
> launch
	  a subshell.</P
><P
>{ command1; command2; command3; . . . commandN; }</P
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><H3
CLASS="FOOTNOTES"
>Notes</H3
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ALIGN="LEFT"
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><A
NAME="FTN.AEN16686"
HREF="subshells.html#AEN16686"
>[1]</A
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VALIGN="TOP"
WIDTH="95%"
><P
>An external command invoked with an <A
HREF="internal.html#EXECREF"
>exec</A
> does <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>not</I
></SPAN
>
             (usually) fork off a subprocess / subshell.</P
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