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Subject: Csh Programming Considered Harmful

This article was archived around: 6 Oct 1996 14:03:18 GMT

All FAQs in Directory: unix-faq/shell
All FAQs posted in: comp.unix.shell, comp.unix.questions, comp.unix.programmer, comp.infosystems.www.authoring.cgi
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Archive-name: unix-faq/shell/csh-whynot Version: $Id: csh-faq,v 1.7 95/09/28 12:52:17 tchrist Exp Locker: tchrist $
The following periodic article answers in excruciating detail the frequently asked question "Why shouldn't I program in csh?". It is available for anon FTP from perl.com in /pub/perl/versus/csh.whynot.gz *** CSH PROGRAMMING CONSIDERED HARMFUL *** Resolved: The csh is a tool utterly inadequate for programming, and its use for such purposes should be strictly banned! I am continually shocked and dismayed to see people write test cases, install scripts, and other random hackery using the csh. Lack of proficiency in the Bourne shell has been known to cause errors in /etc/rc and .cronrc files, which is a problem, because you *must* write these files in that language. The csh is seductive because the conditionals are more C-like, so the path of least resistance is chosen and a csh script is written. Sadly, this is a lost cause, and the programmer seldom even realizes it, even when they find that many simple things they wish to do range from cumbersome to impossible in the csh. 1. FILE DESCRIPTORS The most common problem encountered in csh programming is that you can't do file-descriptor manipulation. All you are able to do is redirect stdin, or stdout, or dup stderr into stdout. Bourne-compatible shells offer you an abundance of more exotic possibilities. 1a. Writing Files In the Bourne shell, you can open or dup arbitrary file descriptors. For example, exec 2>errs.out means that from then on, stderr goes into errs file. Or what if you just want to throw away stderr and leave stdout alone? Pretty simple operation, eh? cmd 2>/dev/null Works in the Bourne shell. In the csh, you can only make a pitiful attempt like this: (cmd > /dev/tty) >& /dev/null But who said that stdout was my tty? So it's wrong. This simple operation *CANNOT BE DONE* in the csh. Along these same lines, you can't direct error messages in csh scripts out stderr as is considered proper. In the Bourne shell, you might say: echo "$0: cannot find $file" 1>&2 but in the csh, you can't redirect stdout out stderr, so you end up doing something silly like this: sh -c 'echo "$0: cannot find $file" 1>&2' 1b. Reading Files In the csh, all you've got is $<, which reads a line from your tty. What if you've redirected stdin? Tough noogies, you still get your tty, which you really can't redirect. Now, the read statement in the Bourne shell allows you to read from stdin, which catches redirection. It also means that you can do things like this: exec 3<file1 exec 4<file2 Now you can read from fd 3 and get lines from file1, or from file2 through fd 4. In modern, Bourne-like shells, this suffices: read some_var 0<&3 read another_var 0<&4 Although in older ones where read only goes from 0, you trick it: exec 5<&0 # save old stdin exec 0<&3; read some_var exec 0<&4; read another_var exec 0<&5 # restore it 1c. Closing FDs In the Bourne shell, you can close file descriptors you don't want open, like 2>&-, which isn't the same as redirecting it to /dev/null. 1d. More Elaborate Combinations Maybe you want to pipe stderr to a command and leave stdout alone. Not too hard an idea, right? You can't do this in the csh as I mentioned in 1a. In a Bourne shell, you can do things like this: exec 3>&1; grep yyy xxx 2>&1 1>&3 3>&- | sed s/file/foobar/ 1>&2 3>&- grep: xxx: No such foobar or directory Normal output would be unaffected. The closes there were in case something really cared about all its FDs. We send stderr to sed, and then put it back out 2. Consider the pipeline: A | B | C You want to know the status of C, well, that's easy: it's in $?, or $status in csh. But if you want it from A, you're out of luck -- if you're in the csh, that is. In the Bourne shell, you can get it, although doing so is a bit tricky. Here's something I had to do where I ran dd's stderr into a grep -v pipe to get rid of the records in/out noise, but had to return the dd's exit status, not the grep's: device=/dev/rmt8 dd_noise='^[0-9]+\+[0-9]+ records (in|out)$' exec 3>&1 status=`((dd if=$device ibs=64k 2>&1 1>&3 3>&- 4>&-; echo $? >&4) | egrep -v "$dd_noise" 1>&2 3>&- 4>&-) 4>&1` exit $status; The csh has also been known to close all open file descriptors besides the ones it knows about, making it unsuitable for applications that intend to inherit open file descriptors. 2. COMMAND ORTHOGONALITY 2a. Built-ins The csh is a horrid botch with its built-ins. You can't put them together in many reasonable ways. Even simple little things like this: % time | echo which while nonsensical, shouldn't give me this message: Reset tty pgrp from 9341 to 26678 Others are more fun: % sleep 1 | while while: Too few arguments. [5] 9402 % jobs [5] 9402 Done sleep | Some can even hang your shell. Try typing ^Z while you're sourcing something, or redirecting a source command. Just make sure you have another window handy. Or try % history | more on some systems. Aliases are not evaluated everywhere you would like them do be: % alias lu 'ls -u' % lu HISTORY News bin fortran lib lyrics misc tex Mail TEX dehnung hpview logs mbox netlib % repeat 3 lu lu: Command not found. lu: Command not found. lu: Command not found. % time lu lu: Command not found. 2b. Flow control You can't mix flow-control and commands, like this: who | while read line; do echo "gotta $line" done You can't combine multiline constructs in a csh using semicolons. There's no easy way to do this alias cmd 'if (foo) then bar; else snark; endif' You can't perform redirections with if statements that are evaluated solely for their exit status: if ( { grep vt100 /etc/termcap > /dev/null } ) echo ok And even pipes don't work: if ( { grep vt100 /etc/termcap | sed 's/$/###' } ) echo ok But these work just fine in the Bourne shell: if grep vt100 /etc/termcap > /dev/null ; then echo ok; fi if grep vt100 /etc/termcap | sed 's/$/###/' ; then echo ok; fi Consider the following reasonable construct: if ( { command1 | command2 } ) then ... endif The output of command1 won't go into the input of command2. You will get the output of both commands on standard output. No error is raised. In the Bourne shell or its clones, you would say if command1 | command2 ; then ... fi 2c. Stupid parsing bugs Certain reasonable things just don't work, like this: % kill -1 `cat foo` `cat foo`: Ambiguous. But this is ok: % /bin/kill -1 `cat foo` If you have a stopped job: [2] Stopped rlogin globhost You should be able to kill it with % kill %?glob kill: No match but % fg %?glob works. White space can matter: if(expr) may fail on some versions of csh, while if (expr) works! Your vendor may have attempted to fix this bug, but odds are good that their csh still won't be able to handle if(0) then if(1) then echo A: got here else echo B: got here endif echo We should never execute this statement endif 3. SIGNALS In the csh, all you can do with signals is trap SIGINT. In the Bourne shell, you can trap any signal, or the end-of-program exit. For example, to blow away a tempfile on any of a variety of signals: $ trap 'rm -f /usr/adm/tmp/i$$ ; echo "ERROR: abnormal exit"; exit' 1 2 3 15 $ trap 'rm tmp.$$' 0 # on program exit 4. QUOTING You can't quote things reasonably in the csh: set foo = "Bill asked, \"How's tricks?\"" doesn't work. This makes it really hard to construct strings with mixed quotes in them. In the Bourne shell, this works just fine. In fact, so does this: cd /mnt; /usr/ucb/finger -m -s `ls \`u\`` Dollar signs cannot be escaped in double quotes in the csh. Ug. set foo = "this is a \$dollar quoted and this is $HOME not quoted" dollar: Undefined variable. You have to use backslashes for newlines, and it's just darn hard to get them into strings sometimes. set foo = "this \ and that"; echo $foo this and that echo "$foo" Unmatched ". Say what? You don't have these problems in the Bourne shell, where it's just fine to write things like this: echo 'This is some text that contains several newlines.' As distributed, quoting history references is a challenge. Consider: % mail adec23!alberta!pixel.Convex.COM!tchrist alberta!pixel.Convex.COM!tchri: Event not found. 5. VARIABLE SYNTAX There's this big difference between global (environment) and local (shell) variables. In csh, you use a totally different syntax to set one from the other. In the Bourne shell, this VAR=foo cmds args is the same as (export VAR; VAR=foo; cmd args) or csh's (setenv VAR; cmd args) You can't use :t, :h, etc on envariables. Watch: echo Try testing with $SHELL:t It's really nice to be able to say ${PAGER-more} or FOO=${BAR:-${BAZ}} to be able to run the user's PAGER if set, and more otherwise. You can't do this in the csh. It takes more verbiage. You can't get the process number of the last background command from the csh, 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 $!. The csh is also flaky about what it does when it imports an environment variable into a local shell variable, as it does with HOME, USER, PATH, and TERM. Consider this: % setenv TERM '`/bin/ls -l / > /dev/tty`' % csh -f And watch the fun! 6. EXPRESSION EVALUATION Consider this statement in the csh: if ($?MANPAGER) setenv PAGER $MANPAGER Despite your attempts to only set PAGER when you want to, the csh aborts: MANPAGER: Undefined variable. That's because it parses the whole line anyway AND EVALUATES IT! You have to write this: if ($?MANPAGER) then setenv PAGER $MANPAGER endif That's the same problem you have here: if ($?X && $X == 'foo') echo ok X: Undefined variable This forces you to write a couple nested if statements. This is highly undesirable because it renders short-circuit booleans useless in situations like these. If the csh were the really C-like, you would expect to be able to safely employ this kind of logic. Consider the common C construct: if (p && p->member) Undefined variables are not fatal errors in the Bourne shell, so this issue does not arise there. While the csh does have built-in expression handling, it's not what you might think. In fact, it's space sensitive. This is an error @ a = 4/2 but this is ok @ a = 4 / 2 The ad hoc parsing csh employs fouls you up in other places as well. Consider: % alias foo 'echo hi' ; foo foo: Command not found. % foo hi 7. ERROR HANDLING Wouldn't it be nice to know you had an error in your script before you ran it? That's what the -n flag is for: just check the syntax. This is especially good to make sure seldom taken segments of code code are correct. Alas, the csh implementation of this doesn't work. Consider this statement: exit (i) Of course, they really meant exit (1) or just exit 1 Either shell will complain about this. But if you hide this in an if clause, like so: #!/bin/csh -fn if (1) then exit (i) endif The csh tells you there's nothing wrong with this script. The equivalent construct in the Bourne shell, on the other hand, tells you this: #!/bin/sh -n if (1) then exit (i) endif /tmp/x: syntax error at line 3: `(' unexpected RANDOM BUGS Here's one: fg %?string ^Z kill %?string No match. Huh? Here's another !%s%x%s Coredump, or garbage. If you have an alias with backquotes, and use that in backquotes in another one, you get a coredump. Try this: % repeat 3 echo "/vmu*" /vmu* /vmunix /vmunix What??? Here's another one: % mkdir tst % cd tst % touch '[foo]bar' % foreach var ( * ) > echo "File named $var" > end foreach: No match. 8. SUMMARY While some vendors have fixed some of the csh's bugs (the tcsh also does much better here), many have added new ones. Most of its problems can never be solved because they're not actually bugs per se, but rather the direct consequences of braindead design decisions. It's inherently flawed. Do yourself a favor, and if you *have* to write a shell script, do it in the Bourne shell. It's on every UNIX system out there. However, behavior can vary. There are other possibilities. The Korn shell is the preferred programming shell by many sh addicts, but it still suffers from inherent problems in the Bourne shell's design, such as parsing and evaluation horrors. The Korn shell or its public-domain clones and supersets (like bash) aren't quite so ubiquitous as sh, so it probably wouldn't be wise to write a sharchive in them that you post to the net. When 1003.2 becomes a real standard that companies are forced to adhere to, then we'll be in much better shape. Until then, we'll be stuck with bug-incompatible versions of the sh lying about. The Plan 9 shell, rc, is much cleaner in its parsing and evaluation; it is not widely available, so you'd be significantly sacrificing portability. No vendor is shipping it yet. If you don't have to use a shell, but just want an interpreted language, many other free possibilities present themselves, like Perl, REXX, TCL, Scheme, or Python. Of these, Perl is probably the most widely available on UNIX (and many other) systems and certainly comes with the most extensive UNIX interface. Increasing numbers vendors ship Perl with their standard systems. (See the comp.lang.perl FAQ for a list.) If you have a problem that would ordinarily use sed or awk or sh, but it exceeds their capabilities or must run a little faster, and you don't want to write the silly thing in C, then Perl may be for you. You can get at networking functions, binary data, and most of the C library. There are also translators to turn your sed and awk scripts into Perl scripts, as well as a symbolic debugger. Tchrist's personal rule of thumb is that if it's the size that fits in a Makefile, it gets written in the Bourne shell, but anything bigger gets written in Perl. See the comp.lang.{perl,rexx,tcl} newsgroups for details about these languages (including FAQs), or David Muir Sharnoff's comparison of freely available languages and tools in comp.lang.misc and news.answers. NOTE: Doug Hamilton <hamilton@bix.com> has a program that he sells for profit for little toy non-UNIX systems. He calls it 'csh' or the 'hamilton csh', but it's not a csh as it's neither bug nor feature compatible with the real csh. Actually, he's fixed a great deal, but in doing so, has created a totally different shell. -- Tom Christiansen Perl Consultant, Gamer, Hiker tchrist@mox.perl.com "Espousing the eponymous /cgi-bin/perl.exe?FMH.pl execution model is like reading a suicide note -- three days too late." --Tom Christiansen <tchrist@mox.perl.com>