ch47_02.htm

the unix power tools

><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>kill %?glob</B
></CODE
>
kill: No match</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>but:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>fg %?glob</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>works.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56366"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56369"
></A
>White space can matter:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>if(expr)</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>may fail on some versions of csh, while:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>if (expr)</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>works!</P
></DIV
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-393-SECT-1.3"
>47.2.3 Signals </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56381"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56383"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56386"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56389"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56392"
></A
>In the C shell, all you can do with signals is trap <EM
CLASS="emphasis"
>SIGINT</EM
>.
In the Bourne shell, you can trap any signal, or the end-of-program exit.
For example, to blow away a temporary file on any of a variety of signals:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>trap 'rm -f /usr/adm/tmp/i$$ ;
    echo &quot;ERROR: abnormal exit&quot;;
    exit' 1 2 3 15
trap 'rm tmp.$$' 0   # on program exit</PRE
></BLOCKQUOTE
></P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-393-SECT-1.4"
>47.2.4 Quoting </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56401"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56404"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56407"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56410"
></A
>You can't quote things reasonably in the <EM
CLASS="emphasis"
>csh</EM
>:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>set foo = &quot;Bill asked, \&quot;How's tricks?\&quot;&quot;</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>doesn't work.
This makes it
<SPAN
CLASS="link"
>really hard (<A
CLASS="linkend"
HREF="ch10_08.htm"
TITLE="Fix Quoting in csh Aliases with makealias and quote "
>10.8</A
>)</SPAN
>
to construct strings with mixed quotes in them.
In the Bourne shell, this works just fine. 
In fact, so does this:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>cd /mnt; /usr/ucb/finger -m -s `ls \`u\``</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Dollar signs (<CODE
CLASS="literal"
>$</CODE
>) cannot be escaped in double quotes in the
<EM
CLASS="emphasis"
>csh</EM
>.
Ugh.</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>set foo = &quot;this is a \$dollar quoted and this is $HOME not quoted&quot; 
dollar: Undefined variable.</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>You have to use backslashes (<CODE
CLASS="literal"
>\</CODE
>) for newlines, and it's just darn
hard to
get them into strings sometimes.</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
><A
CLASS="indexterm"
NAME="AUTOID-56429"
></A
>% <CODE
CLASS="userinput"
><B
>set foo = &quot;this \
and that&quot;;</B
></CODE
>
% <CODE
CLASS="userinput"
><B
>echo $foo</B
></CODE
>
this  and that
% <CODE
CLASS="userinput"
><B
>echo &quot;$foo&quot;</B
></CODE
>
Unmatched &quot;.</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Say what?
You don't have these problems in the Bourne shell, where it's
just fine to write things like this:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>echo    'This is 
     some text that contains
     several newlines.'</PRE
></BLOCKQUOTE
></P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-393-SECT-1.5"
>47.2.5 Variable Syntax </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56441"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56444"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56447"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56450"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56453"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56456"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56459"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56462"
></A
>There's this big difference between global -vironment) and local
(shell) variables.
In <EM
CLASS="emphasis"
>csh</EM
>, you use a totally different syntax 
to set one from the other.</P
><P
CLASS="para"
>In Bourne shell, this: </P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>VAR=foo cmds args</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>is the same as:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>(export VAR; VAR=foo; cmd args)</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>or <EM
CLASS="emphasis"
>csh</EM
>'s:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>(setenv VAR; cmd args)</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>You can't use
<SPAN
CLASS="link"
><CODE
CLASS="literal"
>:t</CODE
>, <CODE
CLASS="literal"
>:h</CODE
>, etc. (<A
CLASS="linkend"
HREF="ch09_06.htm"
TITLE="String Editing (Colon) Operators "
>9.6</A
>)</SPAN
>
on environment variables.
Watch:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>echo Try testing with $SHELL:t</B
></CODE
>
Try testing with /bin/csh:t</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>It's really nice to be able to say
<SPAN
CLASS="link"
><CODE
CLASS="literal"
>${PAGER-more}</CODE
> or <CODE
CLASS="literal"
>FOO=${BAR:-${BAZ}}</CODE
> (<A
CLASS="linkend"
HREF="ch45_12.htm"
TITLE="Parameter Substitution "
>45.12</A
>)</SPAN
>
to be able to run the user's <EM
CLASS="emphasis"
>PAGER</EM
> if set, and <EM
CLASS="emphasis"
>more</EM
> otherwise.
You can't do this in the <EM
CLASS="emphasis"
>csh</EM
>.
It takes more verbiage.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56491"
></A
>You can't get the process number of the last background command from the
C shell, something you might like to do if you're starting up several jobs in
the background.
In the Bourne shell, the PID of the last command put in
the background is available in <CODE
CLASS="userinput"
><B
>$!</B
></CODE
>.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56496"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56499"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56502"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56505"
></A
>The <EM
CLASS="emphasis"
>csh</EM
> is also flaky about what it does when it imports an 
environment variable into a local shell variable, as it does
with <EM
CLASS="emphasis"
>HOME</EM
>, <EM
CLASS="emphasis"
>USER</EM
>, <EM
CLASS="emphasis"
>PATH</EM
>, and <EM
CLASS="emphasis"
>TERM</EM
>.
Consider this:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>setenv TERM '`/bin/ls -l / &gt; /dev/tty`'</B
></CODE
>
% <CODE
CLASS="userinput"
><B
>csh -f</B
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>And watch the fun!</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-393-SECT-1.6"
>47.2.6 Expression Evaluation </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56521"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56524"
></A
>Consider this statement in the <EM
CLASS="emphasis"
>csh</EM
>:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>if ($?MANPAGER) setenv PAGER $MANPAGER</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Despite your attempts to set only <EM
CLASS="emphasis"
>PAGER</EM
> when you want
to, the <EM
CLASS="emphasis"
>csh</EM
> aborts:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>MANPAGER: Undefined variable.</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56536"
></A
>That's because it parses the whole line anyway <EM
CLASS="emphasis"
>and evaluates it</EM
>!
You have to write this:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>if ($?MANPAGER) then
    setenv PAGER $MANPAGER
endif</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>That's the same problem you have here:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>if ($?X &amp;&amp; $X == 'foo') echo ok</B
></CODE
>
X: Undefined variable</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56547"
></A
>This forces you to write a couple of nested <EM
CLASS="emphasis"
>if</EM
> statements.
This is highly undesirable 
because it renders short-circuit Booleans useless in situations like these.
If the <EM
CLASS="emphasis"
>csh</EM
> were really C-like, you would expect to be able
to safely employ this kind of logic.
Consider the common C construct:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>if (p &amp;&amp; p-&gt;member)</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56555"
></A
>Undefined variables are not fatal errors in the Bourne shell, so 
this issue does not arise there.</P
><P
CLASS="para"
>While the <EM
CLASS="emphasis"
>csh</EM
> does have built-in expression handling, it's not
what you might think.
In fact, it's space-sensitive.
This is an error:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>@ a = 4/2</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>but this is okay:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>@ a = 4 / 2</PRE
></BLOCKQUOTE
></P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-393-SECT-1.7"
>47.2.7 Error Handling </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-56568"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56571"
></A
>Wouldn't it be nice to know you had an error in your script before
you ran it?
That's what the <EM
CLASS="emphasis"
>-n</EM
> flag is for: just check the syntax.
This is especially good to make sure seldom taken segments of 
code are correct.
Alas, the <EM
CLASS="emphasis"
>csh</EM
> implementation of this doesn't work.
Consider this statement:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>exit (i)</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Of course, they really meant:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>exit (1)</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>or just:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>exit 1</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Either shell will complain about this.
But if you hide this in an <EM
CLASS="emphasis"
>if</EM
> clause, like so:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>#!/bin/csh -fn
if (1) then
    exit (i)
endif</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>the C shell tells you there's nothing wrong with this script.
The equivalent
construct in the Bourne shell, on the other hand, tells you this:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>#!/bin/sh -n
if (1) then
    exit (i)
endif

/tmp/x: syntax error at line 3: `(' unexpected</PRE
></BLOCKQUOTE
></P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-393-SECT-1.8"
>47.2.8 Random Bugs </A
></H3
><P
CLASS="para"
>Here's one:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>!%s%x%s</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>Core dump, or garbage.</P
><P
CLASS="para"
>If you have an alias with backquotes (<CODE
CLASS="literal"
>``</CODE
>), and use that in backquotes in 
another one, you get a core dump.</P
><P
CLASS="para"
>Try this:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>% <CODE
CLASS="userinput"
><B
>repeat 3 echo &quot;/vmu*&quot;</B
></CODE
>
/vmu*
/vmunix
/vmunix</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>What???</P
><P
CLASS="para"
>While some vendors have fixed some of the <EM
CLASS="emphasis"
>csh</EM
>'s bugs (the
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>tcsh</EM
> (<A
CLASS="linkend"
HREF="ch08_03.htm"
TITLE="Introduction to tcsh"
>8.3</A
>)</SPAN
>
also
does much better here), most of its problems can never be solved
because they're a result of braindead design decisions.
Do yourself
a favor, and if you have to write a shell script, do it in the 
Bourne shell.<A
CLASS="indexterm"
NAME="AUTOID-56608"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56609"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56610"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56611"
></A
><A
CLASS="indexterm"
NAME="AUTOID-56612"
></A
></P
></DIV
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>TC</SPAN
></P
></DIV
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