variables2.html

一本完整的描述Unix Shell 编程的工具书的所有范例

><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><SPAN
CLASS="QUOTE"
>"<TT
CLASS="VARNAME"
>$*</TT
>"</SPAN
> must be
	    quoted.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><TT
CLASS="VARNAME"
>$@</TT
></DT
><DD
><P
>Same as <SPAN
CLASS="TOKEN"
>$*</SPAN
>, but each parameter is a
	      quoted string, that is, the parameters are passed on
	      intact, without interpretation or expansion. This means,
	      among other things, that each parameter in the argument
	      list is seen as a separate word.</P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Of course, <SPAN
CLASS="QUOTE"
>"<TT
CLASS="VARNAME"
>$@</TT
>"</SPAN
>
	    should be quoted.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="ARGLIST"
></A
><P
><B
>Example 9-6. <B
CLASS="COMMAND"
>arglist</B
>: Listing arguments with $* and $@</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# arglist.sh
   3&nbsp;# Invoke this script with several arguments, such as "one two three".
   4&nbsp;
   5&nbsp;E_BADARGS=65
   6&nbsp;
   7&nbsp;if [ ! -n "$1" ]
   8&nbsp;then
   9&nbsp;  echo "Usage: `basename $0` argument1 argument2 etc."
  10&nbsp;  exit $E_BADARGS
  11&nbsp;fi  
  12&nbsp;
  13&nbsp;echo
  14&nbsp;
  15&nbsp;index=1          # Initialize count.
  16&nbsp;
  17&nbsp;echo "Listing args with \"\$*\":"
  18&nbsp;for arg in "$*"  # Doesn't work properly if "$*" isn't quoted.
  19&nbsp;do
  20&nbsp;  echo "Arg #$index = $arg"
  21&nbsp;  let "index+=1"
  22&nbsp;done             # $* sees all arguments as single word. 
  23&nbsp;echo "Entire arg list seen as single word."
  24&nbsp;
  25&nbsp;echo
  26&nbsp;
  27&nbsp;index=1          # Reset count.
  28&nbsp;                 # What happens if you forget to do this?
  29&nbsp;
  30&nbsp;echo "Listing args with \"\$@\":"
  31&nbsp;for arg in "$@"
  32&nbsp;do
  33&nbsp;  echo "Arg #$index = $arg"
  34&nbsp;  let "index+=1"
  35&nbsp;done             # $@ sees arguments as separate words. 
  36&nbsp;echo "Arg list seen as separate words."
  37&nbsp;
  38&nbsp;echo
  39&nbsp;
  40&nbsp;index=1          # Reset count.
  41&nbsp;
  42&nbsp;echo "Listing args with \$* (unquoted):"
  43&nbsp;for arg in $*
  44&nbsp;do
  45&nbsp;  echo "Arg #$index = $arg"
  46&nbsp;  let "index+=1"
  47&nbsp;done             # Unquoted $* sees arguments as separate words. 
  48&nbsp;echo "Arg list seen as separate words."
  49&nbsp;
  50&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
><P
>Following a <B
CLASS="COMMAND"
>shift</B
>, the
	      <TT
CLASS="VARNAME"
>$@</TT
> holds the remaining command-line
	      parameters, lacking the previous <TT
CLASS="VARNAME"
>$1</TT
>,
	      which was lost.
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;# Invoke with ./scriptname 1 2 3 4 5
   3&nbsp;
   4&nbsp;echo "$@"    # 1 2 3 4 5
   5&nbsp;shift
   6&nbsp;echo "$@"    # 2 3 4 5
   7&nbsp;shift
   8&nbsp;echo "$@"    # 3 4 5
   9&nbsp;
  10&nbsp;# Each "shift" loses parameter $1.
  11&nbsp;# "$@" then contains the remaining parameters.</PRE
></TD
></TR
></TABLE
>
            </P
><P
>The <TT
CLASS="VARNAME"
>$@</TT
> special parameter finds
	      use as a tool for filtering input into shell scripts. The
	      <B
CLASS="COMMAND"
>cat "$@"</B
> construction accepts input
	      to a script either from <TT
CLASS="FILENAME"
>stdin</TT
> or
	      from files given as parameters to the script. See <A
HREF="textproc.html#ROT13"
>Example 12-21</A
> and <A
HREF="textproc.html#CRYPTOQUOTE"
>Example 12-22</A
>.</P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <TT
CLASS="VARNAME"
>$*</TT
> and <TT
CLASS="VARNAME"
>$@</TT
>
	      parameters sometimes display inconsistent and
	      puzzling behavior, depending on the setting of <A
HREF="variables2.html#IFSREF"
>$IFS</A
>.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="INCOMPAT"
></A
><P
><B
>Example 9-7. Inconsistent <TT
CLASS="VARNAME"
>$*</TT
> and <TT
CLASS="VARNAME"
>$@</TT
> behavior</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#  Erratic behavior of the "$*" and "$@" internal Bash variables,
   4&nbsp;#+ depending on whether they are quoted or not.
   5&nbsp;#  Inconsistent handling of word splitting and linefeeds.
   6&nbsp;
   7&nbsp;
   8&nbsp;set -- "First one" "second" "third:one" "" "Fifth: :one"
   9&nbsp;# Setting the script arguments, $1, $2, etc.
  10&nbsp;
  11&nbsp;echo
  12&nbsp;
  13&nbsp;echo 'IFS unchanged, using "$*"'
  14&nbsp;c=0
  15&nbsp;for i in "$*"               # quoted
  16&nbsp;do echo "$((c+=1)): [$i]"   # This line remains the same in every instance.
  17&nbsp;                            # Echo args.
  18&nbsp;done
  19&nbsp;echo ---
  20&nbsp;
  21&nbsp;echo 'IFS unchanged, using $*'
  22&nbsp;c=0
  23&nbsp;for i in $*                 # unquoted
  24&nbsp;do echo "$((c+=1)): [$i]"
  25&nbsp;done
  26&nbsp;echo ---
  27&nbsp;
  28&nbsp;echo 'IFS unchanged, using "$@"'
  29&nbsp;c=0
  30&nbsp;for i in "$@"
  31&nbsp;do echo "$((c+=1)): [$i]"
  32&nbsp;done
  33&nbsp;echo ---
  34&nbsp;
  35&nbsp;echo 'IFS unchanged, using $@'
  36&nbsp;c=0
  37&nbsp;for i in $@
  38&nbsp;do echo "$((c+=1)): [$i]"
  39&nbsp;done
  40&nbsp;echo ---
  41&nbsp;
  42&nbsp;IFS=:
  43&nbsp;echo 'IFS=":", using "$*"'
  44&nbsp;c=0
  45&nbsp;for i in "$*"
  46&nbsp;do echo "$((c+=1)): [$i]"
  47&nbsp;done
  48&nbsp;echo ---
  49&nbsp;
  50&nbsp;echo 'IFS=":", using $*'
  51&nbsp;c=0
  52&nbsp;for i in $*
  53&nbsp;do echo "$((c+=1)): [$i]"
  54&nbsp;done
  55&nbsp;echo ---
  56&nbsp;
  57&nbsp;var=$*
  58&nbsp;echo 'IFS=":", using "$var" (var=$*)'
  59&nbsp;c=0
  60&nbsp;for i in "$var"
  61&nbsp;do echo "$((c+=1)): [$i]"
  62&nbsp;done
  63&nbsp;echo ---
  64&nbsp;
  65&nbsp;echo 'IFS=":", using $var (var=$*)'
  66&nbsp;c=0
  67&nbsp;for i in $var
  68&nbsp;do echo "$((c+=1)): [$i]"
  69&nbsp;done
  70&nbsp;echo ---
  71&nbsp;
  72&nbsp;var="$*"
  73&nbsp;echo 'IFS=":", using $var (var="$*")'
  74&nbsp;c=0
  75&nbsp;for i in $var
  76&nbsp;do echo "$((c+=1)): [$i]"
  77&nbsp;done
  78&nbsp;echo ---
  79&nbsp;
  80&nbsp;echo 'IFS=":", using "$var" (var="$*")'
  81&nbsp;c=0
  82&nbsp;for i in "$var"
  83&nbsp;do echo "$((c+=1)): [$i]"
  84&nbsp;done
  85&nbsp;echo ---
  86&nbsp;
  87&nbsp;echo 'IFS=":", using "$@"'
  88&nbsp;c=0
  89&nbsp;for i in "$@"
  90&nbsp;do echo "$((c+=1)): [$i]"
  91&nbsp;done
  92&nbsp;echo ---
  93&nbsp;
  94&nbsp;echo 'IFS=":", using $@'
  95&nbsp;c=0
  96&nbsp;for i in $@
  97&nbsp;do echo "$((c+=1)): [$i]"
  98&nbsp;done
  99&nbsp;echo ---
 100&nbsp;
 101&nbsp;var=$@
 102&nbsp;echo 'IFS=":", using $var (var=$@)'
 103&nbsp;c=0
 104&nbsp;for i in $var
 105&nbsp;do echo "$((c+=1)): [$i]"
 106&nbsp;done
 107&nbsp;echo ---
 108&nbsp;
 109&nbsp;echo 'IFS=":", using "$var" (var=$@)'
 110&nbsp;c=0
 111&nbsp;for i in "$var"
 112&nbsp;do echo "$((c+=1)): [$i]"
 113&nbsp;done
 114&nbsp;echo ---
 115&nbsp;
 116&nbsp;var="$@"
 117&nbsp;echo 'IFS=":", using "$var" (var="$@")'
 118&nbsp;c=0
 119&nbsp;for i in "$var"
 120&nbsp;do echo "$((c+=1)): [$i]"
 121&nbsp;done
 122&nbsp;echo ---
 123&nbsp;
 124&nbsp;echo 'IFS=":", using $var (var="$@")'
 125&nbsp;c=0
 126&nbsp;for i in $var
 127&nbsp;do echo "$((c+=1)): [$i]"
 128&nbsp;done
 129&nbsp;
 130&nbsp;echo
 131&nbsp;
 132&nbsp;# Try this script with ksh or zsh -y.
 133&nbsp;
 134&nbsp;exit 0
 135&nbsp;
 136&nbsp;# This example script by Stephane Chazelas,
 137&nbsp;# and slightly modified by the document author.</PRE
></TD
></TR
></TABLE
><HR></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <B
CLASS="COMMAND"
>$@</B
> and <B
CLASS="COMMAND"
>$*</B
>
	      parameters differ only when between double quotes.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="IFSEMPTY"
></A
><P
><B
>Example 9-8. <TT
CLASS="VARNAME"
>$*</TT
> and <TT
CLASS="VARNAME"
>$@</TT
> when
	        <TT
CLASS="VARNAME"
>$IFS</TT
> is empty</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;#  If $IFS set, but empty,
   4&nbsp;#+ then "$*" and "$@" do not echo positional params as expected.
   5&nbsp;
   6&nbsp;mecho ()       # Echo positional parameters.
   7&nbsp;{
   8&nbsp;echo "$1,$2,$3";
   9&nbsp;}
  10&nbsp;
  11&nbsp;
  12&nbsp;IFS=""         # Set, but empty.
  13&nbsp;set a b c      # Positional parameters.
  14&nbsp;
  15&nbsp;mecho "$*"     # abc,,
  16&nbsp;mecho $*       # a,b,c
  17&nbsp;
  18&nbsp;mecho $@       # a,b,c
  19&nbsp;mecho "$@"     # a,b,c
  20&nbsp;
  21&nbsp;#  The behavior of $* and $@ when $IFS is empty depends
  22&nbsp;#+ on whatever Bash or sh version being run.
  23&nbsp;#  It is therefore inadvisable to depend on this "feature" in a script.
  24&nbsp;
  25&nbsp;
  26&nbsp;# Thanks, Stephane Chazelas.
  27&nbsp;
  28&nbsp;exit 0</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
></DL
></DIV
><DIV
CLASS="VARIABLELIST"
><P
><B
>Other Special Parameters</B
></P
><DL
><DT
><A
NAME="FLPREF"
></A
><TT
CLASS="VARNAME"
>$-</TT
></DT
><DD
><P
>Flags passed to script (using <A
HREF="internal.html#SETREF"
>set</A
>). See <A
HREF="internal.html#EX34"
>Example 11-15</A
>.</P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>This was originally a <I
CLASS="EMPHASIS"
>ksh</I
>
	      construct adopted into Bash, and unfortunately it does not
	      seem to work reliably in Bash scripts. One possible use
	      for it is to have a script <A
HREF="miscellany.html#IITEST"
>self-test
	      whether it is interactive</A
>.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><TT
CLASS="VARNAME"
>$!</TT
></DT
><DD
><P
>PID (process ID) of last job run in background</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;LOG=$0.log
   2&nbsp;
   3&nbsp;COMMAND1="sleep 100"
   4&nbsp;
   5&nbsp;echo "Logging PIDs background commands for script: $0" &#62;&#62; "$LOG"
   6&nbsp;# So they can be monitored, and killed as necessary.
   7&nbsp;echo &#62;&#62; "$LOG"
   8&nbsp;
   9&nbsp;# Logging commands.
  10&nbsp;
  11&nbsp;echo -n "PID of \"$COMMAND1\":  " &#62;&#62; "$LOG"
  12&nbsp;${COMMAND1} &#38;
  13&nbsp;echo $! &#62;&#62; "$LOG"
  14&nbsp;# PID of "sleep 100":  1506
  15&nbsp;
  16&nbsp;# Thank you, Jacques Lederer, for suggesting this.</PRE
></TD
></TR
></TABLE
>
            </P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;possibly_hanging_job &#38; { sleep ${TIMEOUT}; eval 'kill -9 $!' &#38;&#62; /dev/null; }
   2&nbsp;# Forces completion of an ill-behaved program.
   3&nbsp;# Useful, for example, in init scripts.
   4&nbsp;
   5&nbsp;# Thank you, Sylvain Fourmanoit, for this creative use of the "!" variable.</PRE
></TD
></TR
></TABLE
>

            </P
></DD
><DT
><A
NAME="UNDERSCOREREF"
></A
><TT
CLASS="VARNAME"
>$_</TT
></DT
><DD
><P
>Special variable set to last argument of previous command
	        executed.</P
><DIV
CLASS="EXAMPLE"
><HR><A
NAME="USCREF"
></A
><P
><B
>Example 9-9. Underscore variable</B
></P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;echo $_              # /bin/bash
   4&nbsp;                     # Just called /bin/bash to run the script.
   5&nbsp;
   6&nbsp;du &#62;/dev/null        # So no output from command.
   7&nbsp;echo $_              # du
   8&nbsp;
   9&nbsp;ls -al &#62;/dev/null    # So no output from command.
  10&nbsp;echo $_              # -al  (last argument)
  11&nbsp;
  12&nbsp;:
  13&nbsp;echo $_              # :</PRE
></TD
></TR
></TABLE
><HR></DIV
></DD
><DT
><A
NAME="XSTATVARREF"
></A
><TT
CLASS="VARNAME"
>$?</TT
></DT
><DD
><P
><A
HREF="exit-status.html#EXITSTATUSREF"
>Exit status</A
>
	    of a command, <A
HREF="functions.html#FUNCTIONREF"
>function</A
>,
	    or the script itself (see <A
HREF="functions.html#MAX"
>Example 23-7</A
>)</P
></DD
><DT
><A
NAME="PROCCID"
></A
><TT
CLASS="VARNAME"
>$$</TT
></DT
><DD
><P
>Process ID of the script itself. The
	  <TT
CLASS="VARNAME"
>$$</TT
> variable often finds use
	    in scripts to construct <SPAN
CLASS="QUOTE"
>"unique"</SPAN
>
	    temp file names (see <A
HREF="contributed-scripts.html#FTPGET"
>Example A-13</A
>, <A
HREF="debugging.html#ONLINE"
>Example 29-6</A
>, <A
HREF="filearchiv.html#DERPM"
>Example 12-28</A
>, and <A
HREF="internal.html#SELFDESTRUCT"
>Example 11-25</A
>). This is usually simpler than
	    invoking <A
HREF="filearchiv.html#MKTEMPREF"
>mktemp</A
>.</P
></DD
></DL
></DIV
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN4002"
HREF="variables2.html#AEN4002"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The PID of the currently running script is
	      <TT
CLASS="VARNAME"
>$$</TT
>, of course.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN4301"
HREF="variables2.html#AEN4301"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The words <SPAN
CLASS="QUOTE"
>"argument"</SPAN
>
		and <SPAN
CLASS="QUOTE"
>"parameter"</SPAN
> are often used
		interchangeably. In the context of this document, they
		have the same precise meaning, that of a variable passed
		to a script or function.</P
></TD
></TR
></TABLE
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