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Csh Programming Considered Harmful
- From: tchrist@cs.Colorado.EDU (Tom Christiansen)
- Date: 1 Oct 93 19:26:41 GMT
- Newsgroups: comp.unix.shell,comp.unix.questions,comp.unix.programmer,news.answers
- Subject: Csh Programming Considered Harmful
- Archive-name: unix-faq/shell/csh-whynot
- Version: $Id: csh-faq,v 1.5 93/10/01 13:26:05 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 convex.com in /pub/csh.whynot .
- *** 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!
- 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.
- --
- Tom Christiansen tchrist@cs.colorado.edu
- Consultant
- Boulder Colorado 303-444-3212
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