[轉錄](fwd) Top Ten reasons not to use the C shell

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標題: (fwd) Top Ten reasons not to use the C shell
時間: Thu Sep 20 21:10:50 2001
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標題: (fwd) Top Ten reasons not to use the C shell
時間: Thu Sep 20 19:34:30 2001

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Subject: Top Ten reasons not to use the C shell
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======================================================================
		Top Ten reasons not to use the C shell
======================================================================


	Written by Bruce Barnett
	With MAJOR contributions by Peter Samuelson
        September 19, 2001


	In the 90's, the C shell the most popular interactive shell.
The Bourne shell was too "bare-bones." The Korn shell had to be
purchased, and the Bourne Again shell wasn't created yet.

	I've used the C shell for years, and on the surface it has a
lot of good points. It has arrays (the Bourne shell only has one).  It
has test(1), basename(1) and expr(1) built-in, while the Bourne shell
needed external programs.  UNIX was hard enough to learn, and spending
months to learn two shells seemed silly when the C shell seemed
adequate for the job. So many have decided that since they were using
the C shell for their interactive session, why not use it for writing
scripts.

		THIS IS A *BIG* MISTAKE.

	Oh - it's okay for a 5-line script. The world isn't going to
end if you use it. However, many of the posters on USENET treat it as
such.  I've used the C shell for very large scripts and it worked fine
in most cases. There are ugly parts, and work-arounds. But as your
script grows in sophistication, you will need more work-arounds and
eventually you will find yourself bashing your head against a wall
trying to work around the problem.

	I know of many people who have read Tom Christiansen's essay
about the C shell, and they were not really convinced. A lot of Tom's 
examples were really obscure, and frankly I've always felt Tom's
argument wasn't as convincing as it could be.  So I decided to write
my own version of this essay - as a gentle argument to a current C
shell programmer from a former C shell fan.

[Note - since I compare shells, it can be cofusing. If the line starts
with a "%" then I'm using the C shell. If in starts with a "$" then it
is the Bourne shell.

	      -------------------------------------
	      Top Ten reasons not to use the C shell
	      -------------------------------------

	1. The Ad Hoc Parser
	2. Multiple-line quoting difficult
	3. Quoting can be confusing and inconsistent
	4. If/while/foreach/read cannot use redirection
	5. Getting input a line at a time
	6. Aliases are line oriented
	7. Limited file I/O redirection
	8. Poor management of signals and subprocesses
	9. Fewer ways to test for missing variables
	10. Inconsistent use of variables and commands.




1. The Ad Hoc Parser

	The biggest problem of the C shell it its ad hoc parser.
Now this information won't make you immediately switch shells. 
But it's the biggest reason to do so. Many of the other items listed
are based on this problem. Perhaps I better elaborate.

	The parser is the code that converts the shell commands into
variables, expressions, strings, etc. High-quality programs have a
full-fledged parser that converts the input into tokens, verifies the
tokens are in the right order, and then executed the tokens.

	The C shell does not do this. It parses as it executes. You
can have expressions in many types of instructions:

%	if ( expression ) 
%	set variable = ( expression )
%	while ( expression )

	They should be treated the same. They are not. You may find out that 

%	if ( 1 )

but

%	if(1)

doesn't work.

	You never know when you will find a new bug.  As I write this
(September 2001) I ported a C shell script to another UNIX system. (It
was my .login script, okay? Sheesh!) Anyhow I got an error "Variable
name must begin with a letter" somewhere in the dozen files used when
I log in. I finally traced the problem down to the following "syntax"
error:

%	if (! $?variable ) ...

Adding a space before the "!" character fixed the "error." The
examples in the manual page don't mention that spaces are required.
Sigh...

	Most of the flaws are due to the ad hoc parser. For instance, 

%		if ( $?A ) set  B = $A

	If variable A is defined, then set B to $A.  Sounds good. The
problem? If A is not defined, you get "A: Undefined variable." 

	If you want to check a Bourne shell script for syntax errors,
use "sh -n." This doesn't execute the script. but it does check all
errors. What a wonderful idea. Does the C shell have this feature? Of
course not.  Errors aren't found until they are EXECUTED.  For
instance, the code

%	if ( $zero ) then
%		while
%		end
%	endif

will execute with no complaints. However, if $zero becomes one, then
you get the syntax error:

	while: Too few arguments.

In other words, you can have a script that works fine for months, and
THEN gets a syntax error. Your customers will love this "professionalism."
And we are just getting warmed up. It's a time bomb, gang...

	Tick... Tick... Tick...
	

2. Multiple-line quoting difficult


	The C shell complaints if strings are longer than a line.
If you are typing at a terminal, and only type one quote, it's nice to
have an error instead of a strange prompt. However, for shell
programming - it stinks like a bloated skunk.


	Here is a simple 'awk' script that adds one to the first value
of each line. I broke this simple script into three lines, because
many awk scripts are several lines long. I could put it on one line,
but that's not the point. Cut me some slack, okay?

	#!/bin/awk -f
	{print $1 + \
		2;
	}

	Calling this from a Bourne shell is simple:

	#!/bin/sh
	awk '
	{print $1 + \
		2;
	}
	'

	They look the SAME! What a novel concept. Now look at the C
shell version.

	#!/bin/csh -f
	 awk '{print $1 + \\
	 	2 ;\
	 }'


	An extra slash is needed. One line has two slashes, and the
second has one. Suppose you want to set the output to a variable.
Sounds simple? Perhaps. Look how it changes:

	#!/bin/csh -f
	set a = `echo 7 |  awk '{print $1 + \\\
		 2 ;\\
	 }'`

	Now you need three slashes!  And the second line only has two.
Keeping track of those backslashes can drive you crazy when you have
large awk and sed scripts. And you can't simply cut and paste scripts
from different shells - if you use the C shell.

	Also note that if you WANT to include a newline in a string,
strange things happen:
%	set a = 'a \
	b'
%	echo $a
	a  b

	The newline goes away. Suppose you really want a newline in
the string. Will another backslash work?

%	set a = 'a \\
	b'
%	echo $a
	a \  b

	That didn't work. Suppose you decide to quote the variable:

%	set a = 'a \
	b'
%	echo "$a"
	Unmatched ".

	Syntax error!? How bizarre.  There is a solution - use the :q
quote modifier.

%	set a = 'a \
	b'
%	echo $a:q
	a 
	b

	This can get VERY complicated when you want to make aliases
include backslash characters. More on this later. Heh. Heh.

3. Quoting can be confusing and inconsistent

	The Bourne shell has three types of quotes:

	"........" - only $, `, and \ are special.
	'.......' - Nothing is special (this includes the backslash)
	\. 	 - The next character is not special
			 (Exception: a newline)

	That's it. Very few exceptions. The C shell is another matter.
I've mentioned the newline character, but that's just the beginning.
The "!" character is special, as is the "~" character.
Using single quotes (the strong quotes) the command

%	echo '!1'

will give you the error

	1: Event not found.

A backslash is needed because the single quotes don't work.
Now suppose you type

%	set a = "~"
%	echo $a
%	echo '$a'
%	echo "$a"

The echo commands output THREE different values.
If you call a C shell script, and pass it an argument with a space:

%	myscript "a b" c

Now guess what the following script will print.

	#!/bin/csh -f
	echo $#
	set b = ( $* )
	echo $#b

	It prints "2" and then "3". Double quotes don't help.  It's
time to use the fourth form of quoting:

%	set b = ( $*:q )

	Got it? It gets worse. Try to pass backslashes to an alias
You need billions and billions of them. Okay. I exagerate.
A little. But look at Dan Berstein's two aliases used to get quoting
correct in aliases:

%	alias quote "/bin/sed -e 's/\\!/\\\\\!/g' \\
	-e  's/'\\\''/'\\\'\\\\\\\'\\\''/g' \\
	-e 's/^/'\''/' \\
	-e 's/"\$"/'\''/'"
%	alias makealias "quote | /bin/sed 's/^/alias \!:1 /' \!:2*"

	All this because quoting aliases is a pain.
	Tick.. Tick.. Tick..

4. If/while/foreach/read cannot use redirection

   The Bourne shell allows complex commands to be combined with pipes.
   The C shell doesn't. Suppose you want to choose an argument to grep.
   Example:

%	if ( $a ) then
%	   grep xxx
%	else
%	   grep yyy
%	endif

	No problem as long as the text you are grepping is piped
into the script. But what if you want to create the data
stream in the script?  Suppose you change the first line to be

%	cat $file | if ($a ) then

	Guess what? The file $file is COMPLETELY ignored. Instead, the
script use standard input.  The only standard input the "if" command
can use MUST be specified outside of the script. Therefore what can be
done in one Bourne shell file has to be done in several C shell
scripts - because a single script can't be used. The 'while' command
is the same way. For instance the following command outputs the time
with hyphens between the numbers instead of colons:

$	date | tr ':' ' ' | while read a b c d e f g
$	do
$	echo The time is  $d-$e-$f 
$	done

	You can use < as well as pipes. In other words, *ANY* command in
the Bourne shell can have the datastream redirected. That's because it
has a REAL parser [rimshot].

	Speaking of which. the Bourne shell allows you to combine
several lines onto a single line as long as semicolons are placed
between. This includes complex commands. For example - the following
is perfectly fine with the Bourne shell:

$	if  true;then grep a;else grep b; fi

	This has several advantages. Commands in a makefile - see
make(1) - have to be on one line. Trying to put a C shell "if" command
in a makefile is painful.  Also - if your shell allows you to recall
and edit previous commands, then you can use complex commands and edit
them. The C shell allows you to repeat only the first part of a
complex command, like the single line with the "if" statement. It's
much nicer recalling and editing the entire complex command. But
that's for interactive shells, and outside the scope of this essay.

5. Getting input a line at a time

	Suppose you want to read one line from a file. This simple
task is very difficult for the C shell. The C shell provides one way
to read a line:

%	set ans = $<

	The trouble it - this ALWAYS reads from standard input.  If a
terminal is attached to standard input, then it reads from the
terminal.  If a file is attached to the script, then it reads the
file.

	But what do you do if you want to specify the filename in
the script?  You can use "head -1" to get a line. but how do you read
the next line? You can create a temporary file, and read and delete
the first line. How ugly and extremely inefficient. 

	Now what if you want to read a file, and ask the user
something during this?  As an example - suppose you want to read a
list of files from a pipe, and ask the user what to do with some of
them? Can't do this with the C shell - $< reads from standard input. Always.
The Bourne shell does allow this. Simply use

$	read ans </dev/tty

to read from a terminal, and

$	read ans

to read from a pipe.


6. Aliases are line oriented

	Aliases MUST be one line. However, the "if" WANTS to be on
multiple lines, and quoting multiple lines is a pain. Clearly the work
of a masochist. You can get around this if you bash your head enough,
or else ask someone else with a soft spot for the C shell:

%	alias X 'eval "if (\!* =~ 'Y') then \\
	      echo yes \\
	      else \\
	      echo no \\
	      endif"'
	
	Notice that the "eval" command was needed. The Bourne shell
function is more flexible than aliases, simpler and can easily fit on
one line if you wish.

$	X() { if [ "$1" = "Y" ]; then  echo yes; else echo no; fi;}


If you can write a Bourne shell script, you can write a function.
Same syntax.  There is no need to use special "\!:1" arguments, extra
shell processes, special quoting, multiple backslahses, etc.  I'm
SOOOO tired of hitting my head against a wall.

Tick..Tick..Tick..

7. Limited file I/O redirection

	The C shell has one mechanism to specify standard output and
standard error, and a second to combine them into one stream. It can
be directored to a file or to a pipe.

	That's all you can do. Period. That's it. End of story.

	It's true that for 90% to 99% of the scripts this is all you need to
do. However, the Bourne shell can do much much more:

	You can close standard output, or standard error.
	You can redirect either or both to any file.
	You can merge output streams
	You can create new streams

	As an example, it's easy to send standard error to a file, and
leave standard output alone. But the C shell can't do this very well.

	Tom Christiansen gives several examples in his essay.
I suggest you read his examples.


8. Poor management of signals and subprocesses

	The C shell has very limited signal and process management.

	Good software can be stopped gracefully. If an error occurs,
or a signal is sent to it, the script should clean up all temporary
files. The C shell has one signal trap:

%	onintr label

	To ignore all signals, use

%	onintr - 

	The C shell can be used to catch all signals, or ignore all signals. 
All or none. That's the choice. That's not good enough.

	Many programs have sophisticated signal handling. Sending a
-HUP signal might cause the program to re-read configuration
files. Sending a -USR1 signal may cause the program to turn debug mode
on and off. And sending -TERM should cause the program to
terminate. The Bourne shell can have this control. The C shell cannot.

	Have you ever had a script launch several sub-processes and
then try to stop them when you realized you make a mistake?
You can kill the main script, but you have to use "ps" to find the
other processes and kill them one at a time. That's the best the C
shell can do. The Bourne shell can do better.

	A good programmer makes sure all of the child processes are
killed when the parent is killed.  Here is a fragment of a Bourne
shell program that launches three child processes, and passes a -HUP
signal to all of them so they can restart.

$	PIDS=
$	program1 & PIDS="$PIDS $!"
$	program2 & PIDS="$PIDS $!"
$	program3 & PIDS="$PIDS $!"
$	trap "kill -1 $PIDS" 1 

If the program wanted to exit on signal 15, and echo its process ID, a
second signal handler can be added by adding:

$	trap "echo PID $$ terminated;kill -TERM $PIDS;exit" 15

You can also wait for those processes to terminate using the wait
command:

$	wait "$PIDS"

	Notice you have precise control over which children you are
waiting for. The C shell waits for all child processes. Again - all or
none - those are your choices. But that's not good enough.  Here is an
example that executes three processes. If they don't finish in 30
seconds, they are terminated - an easy job for the Bourne shell:

$	MYID=$$
$	PIDS=
$	(sleep 30; kill -1 $MYID) &
$	(sleep 5;echo A) & PIDS="$PIDS $!"
$	(sleep 10;echo B) & PIDS="$PIDS $!"
$	(sleep 50;echo C) & PIDS="$PIDS $!"
$	trap "echo TIMEOUT;kill $PIDS" 1
$	echo waiting for $PIDS
$	wait $PIDS
$	echo everything OK


	There are several variations of this. You can have child
processes start up in parallel, and wait for a signal for synchronization.

	There is also a special "0" signal. This is the end-of-file
condition. So the Bourne shell can easily delete temporary
files when done:

	trap "/bin/rm $tempfiles" 0

	The C shell lacks this. There is no way to get the process ID
of a child process and use it in a script. The wait command
waits for ALL processes, not the ones your specify. It just can't
handle the job.

9. Fewer ways to test for missing variables

	The C shell provides a way to test if a variable exists -
   using the $?var name:

%	if ( $?A ) then
%	   echo variable A exists
%	endif

However, there is no simple way to determine if the varible has a
value.  The C shell test

%     if ($?A && ("$A" =~ ?*)) then

Returns the error:

    A: undefined variable.

You can use nested "if" statements  using:

%	if ( $?A ) then
%		if ( "$A" =~ ?* ) then
%		   # okay
%		else
%			echo "A exists but does not have a value"
%		endif
%	else
%			echo "A does not exist"
%	endif
	
The Bourne shell is much easier to use. You don't need complex "if"
commands. Test the variable while you use it:

$	echo ${A?'A does not have a value'}

If the variable exists with no value, no error occurs. If you want to
add a test for the "no-value" condition, add the colon:

$	echo ${A:?'A is not set or does not have a value'}
	
Besides reporting errors, you can have default values:

$	B=${A-default}

You can also assign values if they are not defined:

$	echo ${A=default}

	These also support the ":" to test for null values.

10. Inconsistent use of variables and commands.

	The Bourne shell has one type of variable. The C shell has five:

	* Regular variables - $a
	* Wordlist variables - $a[1]
	* Environment variables - $A
	* Alias arguments	- !1
	* History arguments	- !1
	* Sub-process variables - %1
	* Directory variables - ~user

	These are not treated the same. For instance, you can use the
:r modifier on regular variables, but on some systems you cannot use
it on environment variables without getting an error. Try to get the
process ID of a child process using the C shell:

	program &
	echo "I just created process %%"

	It doesn't work. And forget using ~user variables for anything
complicated. Can you combine the :r with history variables? No. I've
already mentioned that quoting alias arguments is special. These
variables and what you can do with them is not consistant.  Some have
very specific functions. The alias and history variables use the same
character, but have different uses.

	This is also seen when you combine built-ins. If you have an
alias "myalias" then the following lines may generate strange 
errors (as Tom has mentioned before):
	

	repeat 3 myalias
	kill -1 `cat file`
	time | echo

	In general, using pipes, backquotes and redirection with
builtin commands  is asking for trouble., i.e. 

	set j = ( `jobs` )
	kill -1 $PID || echo process $PID not running
	
There are many more cases. It's hard to predict how these commands
will interact. You THINK it should work, but when you try it, it fails.

		   -------------
		   In conclusion
		   -------------

I've listed the reasons above in what I feel to be order of
importance. You can work around many of the issues, but you have to
consider how many hours you have to spend finghting the C shell,
finding ways to work around the problems. It's frustrating, and
frankly - spending some time to learn the basics of the Bourne shell
are worth every minute. Every UNIX system has the Bourne shell or a
superset of it.  It's predictable, and much more flexible than the C
shell. If you want a script that has no hidden syntax errors, properly
cleans up after itself, and gives you precise control over the
elements of the script, and allows you to combine several parts into a
large script, use the Bourne shell.


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