variables2.html

Shall高级编程

HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>As of version 2.05 of Bash,
	    filename globbing no longer distinguishes between lowercase
	    and uppercase letters in a character range between
	    brackets. For example, <B
CLASS="COMMAND"
>ls [A-M]*</B
>
	    would match both <TT
CLASS="FILENAME"
>File1.txt</TT
>
	    and <TT
CLASS="FILENAME"
>file1.txt</TT
>. To revert to
	    the customary behavior of bracket matching, set
	    <TT
CLASS="VARNAME"
>LC_COLLATE</TT
> to <TT
CLASS="OPTION"
>C</TT
>
	    by an <TT
CLASS="USERINPUT"
><B
>export LC_COLLATE=C</B
></TT
>
	    in <TT
CLASS="FILENAME"
>/etc/profile</TT
> and/or
	    <TT
CLASS="FILENAME"
>~/.bashrc</TT
>.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><TT
CLASS="VARNAME"
>$LC_CTYPE</TT
></DT
><DD
><P
>This internal variable controls character interpretation
	    in <A
HREF="globbingref.html"
>globbing</A
> and pattern
	    matching.</P
></DD
><DT
><A
NAME="LINENOREF"
></A
><TT
CLASS="VARNAME"
>$LINENO</TT
></DT
><DD
><P
>This variable is the line number of the shell
	    script in which this variable appears. It has significance only
	    within the script in which it appears, and is chiefly useful for
	    debugging purposes.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;# *** BEGIN DEBUG BLOCK ***
   2&nbsp;last_cmd_arg=$_  # Save it.
   3&nbsp;
   4&nbsp;echo "At line number $LINENO, variable \"v1\" = $v1"
   5&nbsp;echo "Last command argument processed = $last_cmd_arg"
   6&nbsp;# *** END DEBUG BLOCK ***</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><A
NAME="MACHTYPEREF"
></A
><TT
CLASS="VARNAME"
>$MACHTYPE</TT
></DT
><DD
><P
>machine type</P
><P
>Identifies the system hardware.</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $MACHTYPE</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>i686</TT
></PRE
></TD
></TR
></TABLE
></DD
><DT
><A
NAME="OLDPWD"
></A
><TT
CLASS="VARNAME"
>$OLDPWD</TT
></DT
><DD
><P
>old working directory
	  (<SPAN
CLASS="QUOTE"
>"OLD-print-working-directory"</SPAN
>,
	    previous directory you were in)</P
></DD
><DT
><TT
CLASS="VARNAME"
>$OSTYPE</TT
></DT
><DD
><P
>operating system type</P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $OSTYPE</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>linux</TT
></PRE
></TD
></TR
></TABLE
></DD
><DT
><A
NAME="PATHREF"
></A
><TT
CLASS="VARNAME"
>$PATH</TT
></DT
><DD
><P
>path to binaries, usually 
	      <TT
CLASS="FILENAME"
>/usr/bin/</TT
>, 
	      <TT
CLASS="FILENAME"
>/usr/X11R6/bin/</TT
>, 
	      <TT
CLASS="FILENAME"
>/usr/local/bin</TT
>, etc.</P
><P
>When given a command, the shell automatically does
	      a hash table search on the directories listed in the
	      <I
CLASS="FIRSTTERM"
>path</I
> for the executable. The path
	      is stored in the <A
HREF="othertypesv.html#ENVREF"
>environmental
	      variable</A
>, <TT
CLASS="VARNAME"
>$PATH</TT
>, a list
	      of directories, separated by colons. Normally,
	      the system stores the <TT
CLASS="VARNAME"
>$PATH</TT
>
	      definition in <TT
CLASS="FILENAME"
>/etc/profile</TT
>
	      and/or <TT
CLASS="FILENAME"
>~/.bashrc</TT
> (see <A
HREF="files.html"
>Appendix G</A
>).</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><B
CLASS="COMMAND"
>echo $PATH</B
>
 <TT
CLASS="COMPUTEROUTPUT"
>/bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin</TT
></PRE
></TD
></TR
></TABLE
>
              </P
><P
><TT
CLASS="USERINPUT"
><B
>PATH=${PATH}:/opt/bin</B
></TT
> appends
	      the <TT
CLASS="FILENAME"
>/opt/bin</TT
>
	      directory to the current path. In a script, it may be
	      expedient to temporarily add a directory to the path
	      in this way. When the script exits, this restores the
	      original <TT
CLASS="VARNAME"
>$PATH</TT
> (a child process, such
	      as a script, may not change the environment of the parent
	      process, the shell).</P
><P
><A
NAME="CURRENTWDREF"
></A
></P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The current <SPAN
CLASS="QUOTE"
>"working directory"</SPAN
>, 
	      <TT
CLASS="FILENAME"
>./</TT
>, is usually
	      omitted from the <TT
CLASS="VARNAME"
>$PATH</TT
> as a security
	      measure.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="PIPESTATUSREF"
></A
><TT
CLASS="VARNAME"
>$PIPESTATUS</TT
></DT
><DD
><P
><A
HREF="arrays.html#ARRAYREF"
>Array</A
> variable holding
	    exit status(es) of last executed
	    <I
CLASS="FIRSTTERM"
>foreground</I
> <A
HREF="special-chars.html#PIPEREF"
>pipe</A
>. Interestingly enough, this
	    does not necessarily give the same result as the <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
> of the last
	    executed command.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $PIPESTATUS</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>0</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -al | bogus_command</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: bogus_command: command not found</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $PIPESTATUS</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>141</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>ls -al | bogus_command</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: bogus_command: command not found</TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $?</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>127</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
><P
>The members of the <TT
CLASS="VARNAME"
>$PIPESTATUS</TT
> array
		hold the exit status of each respective command
		executed in a pipe. <TT
CLASS="VARNAME"
>$PIPESTATUS[0]</TT
>
		holds the exit status of the first command in the pipe,
		<TT
CLASS="VARNAME"
>$PIPESTATUS[1]</TT
> the exit status of
		the second command, and so on.</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"
>$PIPESTATUS</TT
> variable
              may contain an erroneous <SPAN
CLASS="ERRORCODE"
>0</SPAN
> value
              in a login shell (in releases prior to 3.0 of Bash).
              </P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>tcsh% </TT
><TT
CLASS="USERINPUT"
><B
>bash</B
></TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>who | grep nobody | sort</B
></TT
>
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ${PIPESTATUS[*]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>0</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
><P
>	      The above lines contained in a script would produce the expected
	      <TT
CLASS="COMPUTEROUTPUT"
>0 1 0</TT
> output.
              </P
><P
>	      Thank you, Wayne Pollock for pointing this out and supplying the
	      above example.
	      </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <TT
CLASS="VARNAME"
>$PIPESTATUS</TT
> variable gives
	        unexpected results in some contexts.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo $BASH_VERSION</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>3.00.14(1)-release</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>$ ls | bogus_command | wc</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: bogus_command: command not found
 0       0       0</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ${PIPESTATUS[@]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>141 127 0</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
><P
>Chet Ramey attributes the above output to the behavior of
		<A
HREF="external.html#LSREF"
>ls</A
>. If <I
CLASS="FIRSTTERM"
>ls</I
>
		writes to a <I
CLASS="FIRSTTERM"
>pipe</I
> whose output is not
		read, then SIGPIPE kills it, and its <A
HREF="exit-status.html#EXITSTATUSREF"
>exit status</A
> is
		<SPAN
CLASS="RETURNVALUE"
>141</SPAN
>. Otherwise its
		exit status is <SPAN
CLASS="RETURNVALUE"
>0</SPAN
>,
		as expected. This likewise is the case for <A
HREF="textproc.html#TRREF"
>tr</A
>.</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
><TT
CLASS="VARNAME"
>$PIPESTATUS</TT
> is a
		    <SPAN
CLASS="QUOTE"
>"volatile"</SPAN
> variable. It needs to be
		    captured immediately after the pipe in question, before
		    any other command intervenes.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>$ ls | bogus_command | wc</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>bash: bogus_command: command not found
 0       0       0</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ${PIPESTATUS[@]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>0 127 0</TT
>
 
 <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>echo ${PIPESTATUS[@]}</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>0</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	      </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <A
HREF="bashver3.html#PIPEFAILREF"
>pipefail option</A
>
		    may be useful in cases where
		    <TT
CLASS="VARNAME"
>$PIPESTATUS</TT
> does not give the desired
		    information.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="PPIDREF"
></A
><TT
CLASS="VARNAME"
>$PPID</TT
></DT
><DD
><P
></P
><P
>The <TT
CLASS="VARNAME"
>$PPID</TT
> of a process is
	    the process ID (<TT
CLASS="VARNAME"
>pid</TT
>) of its parent process.

	      <A
NAME="AEN4419"
HREF="#FTN.AEN4419"
>[1]</A
>
	      </P
><P
>Compare this with the <A
HREF="system.html#PIDOFREF"
>pidof</A
> command.</P
></DD
><DT
><TT
CLASS="VARNAME"
>$PROMPT_COMMAND</TT
></DT
><DD
><P
>A variable holding a command to be executed
	      just before the primary prompt, <TT
CLASS="VARNAME"
>$PS1</TT
>
	      is to be displayed.</P
></DD
><DT
><A
NAME="PS1REF"
></A
><TT
CLASS="VARNAME"
>$PS1</TT
></DT
><DD
><P
>This is the main prompt, seen at the command line.</P
></DD
><DT
><A
NAME="SECPROMPTREF"
></A
><TT
CLASS="VARNAME"
>$PS2</TT
></DT
><DD
><P
>The secondary prompt, seen when additional input is
	      expected. It displays as <SPAN
CLASS="QUOTE"
>"&#62;"</SPAN
>.</P
></DD
><DT
><TT
CLASS="VARNAME"
>$PS3</TT
></DT
><DD
><P
>The tertiary prompt, displayed in a
	    <A
HREF="testbranch.html#SELECTREF"
>select</A
> loop (see <A
HREF="testbranch.html#EX31"
>Example 10-29</A
>).</P
></DD
><DT
><TT
CLASS="VARNAME"
>$PS4</TT
></DT
><DD
><P
>The quartenary prompt, shown at the beginning of
	    each line of output when invoking a script with the
	    <SPAN
CLASS="TOKEN"
>-x</SPAN
> <A
HREF="options.html#OPTIONSREF"
>option</A
>.
	    It displays as <SPAN
CLASS="QUOTE"
>"+"</SPAN
>.</P
></DD
><DT
><A
NAME="PWDREF"
></A
><TT
CLASS="VARNAME"
>$PWD</TT
></DT
><DD
><P
>working directory (directory you are in at the time)</P
><P
>This is the analog to the <A
HREF="internal.html#PWD2REF"
>pwd</A
>
	    builtin command.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"