If ever posed by this question by your seniors, Do put this thing up front, Since most sites with time go missing , or are asked to retire due to the maintenance costs, I thought rather use Google Blogging platform to keep many of the things that keep finding ( rather handy things ) here .
Well, coming back to using and not using C Shell, Choice is not to use,
For the following reasons .
======================================================================
Top Ten Reasons not to use the C shell
======================================================================
Written by Bruce Barnett
With MAJOR help from
Peter Samuelson
Chris F.A. Johnson
Jesse Silverman
Ed Morton
and of course Tom Christiansen
Updated:
September 22, 2001
November 26, 2002
July 12, 2004
February 27, 2006
October 3, 2006
January 17. 2007
November 22, 2007
March 1, 2008
In the late 80's, the C shell was 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 (http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/
), 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 confusing. 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 sub-processes
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 should 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 executes 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 )
is fine, but
% if(1)
generates a syntax error. Or that the above works, the "while" command:
% while(1)
fails.
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 (! $?variable ) ...
Which variable must begin with a letter? Give up? The solution -
you must add a space before the "!" character to fix the "error." The
examples in the manual page don't mention that spaces are required.
Sigh...
Here's another one. I wanted to search for a string at the end
of a line, using grep. That is
% set var = "string"
% grep "$var$" < file
Most shells treat this as
% grep "string$" <file
Great. Does the C shell do this? As John Belushi would say, "Noooooo!"
Instead, we get
Variable name must contain alphanumeric characters.
Ah. So we back quote (backslash) it.
% grep "$var\$" <file
This doesn't work. The same thing happens. One work-around is
% grep "$var"'$' <file
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.
Here's another:
if ( $zero ) then
if the C shell has a real parser - complain
endif
In other words, you can have a script that works fine for months, and
THEN reports a syntax error. Your customers will love this
"professionalism."
And here's another I just found today (October 2006).
Create a script that has
#/bin/csh -f
if (0)
endif
And make sure there is no "newline" character after the endif.
Execute this and you get the error
then: then/endif not found.
Now add one line in the middle
#/bin/csh -f
if (0)
else
endif
and again - make sure there is no newline character at the end of the
last line. Run it again and it works fine.
And we are just getting warmed up. The C shell 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 backslash is needed. One line has two backslashes, 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 backslashes! 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. Sometimes I start
writing an AWK script, like
#!/bin/awk -f
BEGIN {A=123;}
etc...
And if I want to convert this to a shell script (because I want to
specify the value of 123 as an argument), I simply replace the first line with:
#!/bin/sh
awk '
BEGIN {A=123'}
etc.
If I used the C shell, I'd have to add a \ before the end of each line.
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.
One more thing - normally a shell allows you to put the quotes anywhere on a line:
echo abc"de"fg
is the same as
echo "abcdefg"
That's because the quote toggles the INTERPRET/DON'T INTERPRET parser.
However, you cannot put a quote right before the backslash if it follows a
variable name whose value has a space. These next two lines generates
a syntax error:
% set a = "a b"
% set a = $a"\
c"
All I wanted to do was to append a "<new line>c" to the $a variable.
It only works if the current value does NOT have a space.
In other words
% set a = "a_b"
% set a = $a"\
c"
is fine. Changing "_" to a space causes a syntax error. Another
surprise. That's the C shell - one never knows where the next surprise
will be.
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.
What works and what doesn't is no longer simple and easy to understand.
As an example, look at the backslash quote. The Bourne shell uses the
backslash to escape everything except the newline. In the C shell, it
also escapes the backslash and the dollar sign. Suppose you want to enclose
$HOME in double quotes. Try typing:
% echo "$HOME"
/home/barnett
Logic tells us to put a backslash in front. So we try
% echo "\$HOME"
\/home/barnett
Sigh.
So there is no way to escape a variable in a double quote. What about
single quotes?
% echo '$HOME'
$HOME
works fine. But here's another exception.
% echo MONEY$
MONEY$
% echo 'MONEY$'
MONEY$
% echo "MONEY$"
Illegal variable name.
The last one is illegal. So adding double quotes CAUSES a syntax error.
With single quotes, "!" character is special, as is
the "~" character. Using single quotes (the strong quotes) the
command
% echo '!1'
1: Event not found.
will give you the error
A backslash is needed because the single quotes won't quote the
exclamation mark. On some versions of the C shell,
echo hi!
works, but
echo 'hi!'
doesn't. A backslash is required in front:
echo 'hi\!'
or if you wanted to put a ! before the word:
echo '\!hi'
Now suppose you type
% set a = "~"
% echo $a
/home/barnett
% echo '$a'
$a
% echo "$a"
~
The echo commands output THREE different values depending on the quotes.
So no matter what type of quotes you use, there are exceptions.
Those exceptions can drive you mad.
And then there's dealing with spaces.
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". A simple = does not copy a variable
correctly if there are spaces involved. Double quotes don't help.
It's time to use the fourth form of quoting - which is only useful
when displaying (not set) the value:
% set b = ( $*:q )
Here's another. Let's saw you had nested backticks.
Some shells use $(program1 $(program2)) to allow this.
The C shell does not, so you have to use nested backticks.
I would expect this to be
`program1 \`program2\` `
but what works is the illogical
`program1 ``program2``
Got it? It gets worse. Try to pass back-slashes to an alias
You need billions and billions of them. Okay. I exaggerate.
A little. But look at Dan Bernstein'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*"
You use this to make sure you get quotes correctly specified in aliases.
Larry Wall calls this backslashitis. What a royal 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 a stream of data in the script?
(i.e. using a pipe). 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 of the script, even though you used a pipe on that line.
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 data-stream 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 is - 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
middle of 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. On a scale
of 1 to 10, it scores -1000.
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 filenames 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 (which can be created in the script). Also - what
if you want to have a script read from STDIN, create some data in the
middle of the script, and use $< to read from the new file. Can't do
it. There is no way to do
set ans = $< <newfile
or
set ans = $< </dev/tty
or
echo ans | set ans = $<
$< is only STDIN, and cannot change for the duration of the script.
The workaround usually means creating several smaller scripts instead
of one script.
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 backslashes, etc. I'm
SOOOO tired of hitting my head against a wall.
Functions allow you to simplify scripts. Anything more sophisticated
than an alias that would require function requires a separate csh
script/file.
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 directed 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. See
http://www.faqs.org/faqs/unix-faq/shell/csh-whynot/
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 (or need) 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 with a Control-C, but the background processes are
still running. 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. Much 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 ( $?A ) then
% echo variable A exists
% endif
However, there is no simple way to determine if the variable has a
value. The C shell test
% if ($?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 seven:
* 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 consistent. 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
built-in commands is asking for trouble., i.e.
echo "!1"
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.
Here are some more examples. You can have an array in the C shell, but
if you try add a new element, you get strange errors.
% set a = ()
% @ a[1] = 2
@: Subscript out of range.
So if you wants to add to an existing array, you have to use something like
set a = ( $a 2 )
Now this works
@ arrayname[1] = 4
but try to store a string in the array.
@ arrayname[1] = "a"
and you get
@: Badly formed number.
Another bug - from Aleksandar Radulovic - If the last line of the C
shell script does not have a new line character, it never gets
exeucted.
I just discoveed another odd bug with the C shell - thank's to a
posting from "yusufm":
Guess what the following script will generate
setenv A 1
echo $A
setenv A=2
echo $A
setenv B=3
echo $B
setenv B=4
echo $B
I'm not going to tell you what the bug is, or how many there are. I
think it's more fun to let you discover it yourself.
I can add some more reasons. Jesse Silverman says reason #0 is that
it's not POSIX compliant. True. But the C shell was written before
the standard existed. This is a historical flaw, and not a braindead
stupid lazy dumb-ass flaw.
-------------
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 fighting 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
super-set 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.
I found myself developing more and more bad habits over time because I
was using the C shell. I would use
foreach a ( `cat file` )
instead of redirection. I would use several smaller scripts to work
around problems in one script. And most importantly, I put off
learning the Bourne shell for years as I struggled with the C
shell. Don't make the same mistake I made.
