sha-bang.html

Shall高级编程

>sh &#60;scriptname</B
></TT
>,
	since this effectively disables reading from
	<A
HREF="ioredirintro.html#STDINOUTDEF"
><TT
CLASS="FILENAME"
>stdin</TT
></A
>
	within the script.) Much more convenient is to make
	the script itself directly executable with a <A
HREF="external.html#CHMODREF"
>chmod</A
>.

	<DIV
CLASS="VARIABLELIST"
><DL
><DT
>Either:</DT
><DD
><P
><TT
CLASS="USERINPUT"
><B
>chmod 555 scriptname</B
></TT
> (gives
	      everyone read/execute permission)
	        <A
NAME="AEN303"
HREF="#FTN.AEN303"
>[9]</A
>
	      </P
></DD
><DT
>or</DT
><DD
><P
><TT
CLASS="USERINPUT"
><B
>chmod +rx scriptname</B
></TT
> (gives
	      everyone read/execute permission)</P
><P
><TT
CLASS="USERINPUT"
><B
>chmod
	      u+rx scriptname</B
></TT
> (gives only the
		script owner read/execute permission)</P
></DD
></DL
></DIV
>
      </P
><P
>Having made the script executable, you may now test it by
	<TT
CLASS="USERINPUT"
><B
>./scriptname</B
></TT
>.
	
	  <A
NAME="AEN315"
HREF="#FTN.AEN315"
>[10]</A
>

	If it begins with a <SPAN
CLASS="QUOTE"
>"sha-bang"</SPAN
> line, invoking the
	script calls the correct command interpreter to run it.</P
><P
>As a final step, after testing and debugging,
	you would likely want to move it to <TT
CLASS="FILENAME"
>/usr/local/bin</TT
> (as
	<I
CLASS="FIRSTTERM"
>root</I
>, of course), to make the script
	available to yourself and all other users as a systemwide
	executable.  The script could then be invoked by simply typing
	<B
CLASS="COMMAND"
>scriptname</B
> <B
CLASS="KEYCAP"
>[ENTER]</B
> from the
	command line.</P
></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.AEN192"
HREF="sha-bang.html#AEN192"
>[1]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Also seen in the literature as
	  <I
CLASS="FIRSTTERM"
>she-bang</I
> or <I
CLASS="FIRSTTERM"
>sh-bang</I
>.
	  This derives from the concatenation of the tokens
	  <I
CLASS="FIRSTTERM"
>sharp</I
> (<SPAN
CLASS="TOKEN"
>#</SPAN
>) and
	  <I
CLASS="FIRSTTERM"
>bang</I
> (<SPAN
CLASS="TOKEN"
>!</SPAN
>).</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN202"
HREF="sha-bang.html#AEN202"
>[2]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Some flavors of UNIX (those based on 4.2 BSD)
          allegedly take a four-byte magic number, requiring
          a blank after the <SPAN
CLASS="TOKEN"
>!</SPAN
> --
	  <TT
CLASS="USERINPUT"
><B
>#! /bin/sh</B
></TT
>. <A
HREF="http://www.in-ulm.de/~mascheck/various/shebang/#details"
TARGET="_top"
>	  According to Sven Mascheck</A
> this is probably a myth.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN214"
HREF="sha-bang.html#AEN214"
>[3]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>The <SPAN
CLASS="TOKEN"
>#!</SPAN
> line in a shell script
	      will be the first thing the command interpreter
	      (<B
CLASS="COMMAND"
>sh</B
> or <B
CLASS="COMMAND"
>bash</B
>)
	      sees. Since this line begins with a <SPAN
CLASS="TOKEN"
>#</SPAN
>,
	      it will be correctly interpreted as a comment when the
	      command interpreter finally executes the script. The
	      line has already served its purpose - calling the command
	      interpreter.</P
><P
>If, in fact, the script includes an
	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>extra</I
></SPAN
> <SPAN
CLASS="TOKEN"
>#!</SPAN
> line, then
	      <B
CLASS="COMMAND"
>bash</B
> will interpret it as a comment.
	        <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/bash
   2&nbsp;
   3&nbsp;echo "Part 1 of script."
   4&nbsp;a=1
   5&nbsp;
   6&nbsp;#!/bin/bash
   7&nbsp;# This does *not* launch a new script.
   8&nbsp;
   9&nbsp;echo "Part 2 of script."
  10&nbsp;echo $a  # Value of $a stays at 1.</PRE
></TD
></TR
></TABLE
></P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN230"
HREF="sha-bang.html#AEN230"
>[4]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>This allows some cute tricks.</P
><P
><TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="100%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;#!/bin/rm
   2&nbsp;# Self-deleting script.
   3&nbsp;
   4&nbsp;# Nothing much seems to happen when you run this... except that the file disappears.
   5&nbsp;
   6&nbsp;WHATEVER=65
   7&nbsp;
   8&nbsp;echo "This line will never print (betcha!)."
   9&nbsp;
  10&nbsp;exit $WHATEVER  # Doesn't matter. The script will not exit here.
  11&nbsp;                # Try an echo $? after script termination.
  12&nbsp;                # You'll get a 0, not a 65.</PRE
></TD
></TR
></TABLE
></P
><P
>Also, try starting a <TT
CLASS="FILENAME"
>README</TT
> file with a
        <TT
CLASS="USERINPUT"
><B
>#!/bin/more</B
></TT
>, and making it executable.
        The result is a self-listing documentation file. (A <A
HREF="here-docs.html#HEREDOCREF"
>here document</A
> using
	<A
HREF="external.html#CATREF"
>cat</A
> is possibly a better alternative
	-- see <A
HREF="here-docs.html#EX71"
>Example 18-3</A
>).</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN244"
HREF="sha-bang.html#AEN244"
>[5]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
><A
NAME="POSIX2REF"
></A
><SPAN
CLASS="strong"
><B
CLASS="EMPHASIS"
>P</B
></SPAN
>ortable
	 <SPAN
CLASS="strong"
><B
CLASS="EMPHASIS"
>O</B
></SPAN
>perating
	 <SPAN
CLASS="strong"
><B
CLASS="EMPHASIS"
>S</B
></SPAN
>ystem <SPAN
CLASS="bold"
><B
CLASS="EMPHASIS"
>I</B
></SPAN
>nterface, an attempt to
	 standardize UNI<SPAN
CLASS="strong"
><B
CLASS="EMPHASIS"
>X</B
></SPAN
>-like
	 OSes. The POSIX specifications are listed on the <A
HREF="http://www.opengroup.org/onlinepubs/007904975/toc.htm"
TARGET="_top"
>Open
	 Group site</A
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN257"
HREF="sha-bang.html#AEN257"
>[6]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>To avoid this possibility, a script may begin
	  with a <A
HREF="system.html#ENVV2REF"
>#!/bin/env bash</A
>
	  <I
CLASS="FIRSTTERM"
>sha-bang</I
> line. This may be
	  useful on UNIX machines where <I
CLASS="FIRSTTERM"
>bash</I
>
	  is not located in <TT
CLASS="FILENAME"
>/bin</TT
></P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN267"
HREF="sha-bang.html#AEN267"
>[7]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>If <I
CLASS="FIRSTTERM"
>Bash</I
> is your default
	shell, then the <SPAN
CLASS="TOKEN"
>#!</SPAN
> isn't necessary at the
	beginning of a script.	However, if launching a script from
	a different shell, such as <I
CLASS="FIRSTTERM"
>tcsh</I
>,
	then you <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>will</I
></SPAN
> need the
	<SPAN
CLASS="TOKEN"
>#!</SPAN
>.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN288"
HREF="sha-bang.html#AEN288"
>[8]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Caution: invoking a <I
CLASS="FIRSTTERM"
>Bash</I
>
	  script by <TT
CLASS="USERINPUT"
><B
>sh scriptname</B
></TT
> turns off
	  Bash-specific extensions, and the script may therefore fail
	  to execute.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN303"
HREF="sha-bang.html#AEN303"
>[9]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>A script needs <I
CLASS="FIRSTTERM"
>read</I
>, as
		well as execute permission for it to run, since the shell
		needs to be able to read it.</P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN315"
HREF="sha-bang.html#AEN315"
>[10]</A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>Why not simply invoke the script with
	  <TT
CLASS="USERINPUT"
><B
>scriptname</B
></TT
>? If the directory you
	  are in (<A
HREF="variables2.html#PWDREF"
>$PWD</A
>) is where
	  <TT
CLASS="FILENAME"
>scriptname</TT
> is located, why doesn't
	  this work? This fails because, for security reasons, the
	  current directory (<TT
CLASS="FILENAME"
>./</TT
>)
	  is not by default included in a user's <A
HREF="variables2.html#PATHREF"
>$PATH</A
>. It is therefore necessary to
	  explicitly invoke the script in the current directory with
	  a <TT
CLASS="USERINPUT"
><B
>./scriptname</B
></TT
>.</P
></TD
></TR
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