Useless Use of Cat Award

If you've been reading comp.unix.shell or any of the related groups (comp.unix.questions inter alia) for any amount of time, this should be a familiar topic.

I made this web page on the topic primarily so I'd have a simpler URL than one of those ghastly Deja News searches to hand to people. I've tried to reconstruct Randal's standard form letter from looking at his postings (see end) and added some comments of my own.

If you came here looking for material about abuse of feline animals, try this Alta Vista search instead.

Contents:

• Useless Use of cat
• Useless Use of kill -9
• Useless Use of echo
• Useless Use of ls *
• Useless Use of wc -l
• Useless Use of grep | awk
• Useless Use of Backticks
• Useless Use of test
• Assorted Other Gripes
• Related Resources
• Reconstructions of Randal's Form Letters

The Useless Use of Cat Award

The oldest article Deja News finds is from 1995, but it's actually a followup to an earlier article. By Internet standards, this is thus an Ancient Tradition.

Exercise: Try to find statistically significant differences between the followups from 1995 and the ones being posted today.

(See below for a reconstruction of the Award text.)

Briefly, here's the collected wisdom on using cat:

The purpose of cat is to concatenate (or "catenate") files. If it's only one file, concatenating it with nothing at all is a waste of time, and costs you a process.

Of course, as Heiner points out, using cat on a single file to view it from the command line is a valid use of cat (but you might be better off if you get accustomed to using less for this instead).

In a recent thread on comp.unix.shell, the following example was posted by Andreas Schwab as another Useful Use of Cat on a lone file:

	{ foo; bar; cat mumble; baz } | whatever

Other Fun Awards

Useless Use of Kill -9

(See below for a reconstruction of the Award text. It explains the issues clearly enough.)

Useless Use of echo

This is really a special case of Useless Use of Backticks but it deserves its own section because it's something you see fairly frequently.

The canonical form of this is something like

	variable="something here, or perhaps even the result of backticks"
	some command -options `echo $variable`

Depending a little bit on what exactly you have the variable for, this can be reduced at least to

	variable="something here, or perhaps even the result of backticks"
	some command -options $variable

and there is often no real reason to even think of using echo in backticks when the simpler construct will do.

(There is a twist: echo will "flatten" any whitespace in $variable into a single space -- unless you double-quote $variable, of course --, and sometimes you can legitimately use echo in backticks for this side effect. But that's rarely necessary or useful, and so most often, this is just a misguided use of echo.)

There is another example in the next section, and a longer rant about Useless Use of Backticks further down the page. There is also a parallel, slightly different example on the Backticks Example page

Useless Use of ls *

Very clever. Usually this is seen as part of a for loop:

	for f in `ls *`; do
		command "$f"   # newbies will often forget the quotes, too
	done

Here's a related but slightly more benign error (because echo is often built into the shell):

	for f in `echo *`; do
		command "$f"
	done

	for f in *; do
		command "$f"
	done

	command *

Finally, as Aaron Crane points out, the result of ls * will usually be the wrong thing if you do it in a directory with subdirectories; ls will list the contents of those directories, not just their names.

Useless Use of wc -l

Anything that looks like

	something | grep '..*' | wc -l

	something | grep -c .   # Notice that . is better than '..*'

	something | grep . >/dev/null && ...

If something is reasonably coded, it might even already be setting its exit code to tell you whether it succeeded in doing what you asked it to do; in that case, all you have to check is the exit code:

	something && ...

I used to have a really wretched example of clueless code (which I had written up completely on my own, to protect the innocent) which I've moved to a separate page and annotated a little bit. It expands on the above and also has a bit about useless use of backticks (q.v.)

Here's a contribution I got from Aaron Crane (thanks!):

grep -c can actually solve a large class of problems that grep | wc -l can't. If what interests you is the count for each of a group of files, then the only way to do it with grep | wc -l is to put a loop round it. So where I had this:

	grep -c "^~h" [A-Z]*/hmm[39]/newMacros

the naive solution using wc -l would have been

	for f in [A-Z]*/hmm[39]/newMacros; do
	    # or worse, for f in `ls [A-Z]*/hmm[39]/newMacros` ...
	    echo -n "$f:"
	    # so that we know which file's results we're looking at
	    grep "^~h" "$f" | wc -l
	    # gag me with a spoon
	done

and notice that we also had to fiddle to get the output in a convenient form.

Useless Use of grep | awk and grep | sed

ps -l | grep -v '[g]rep' | awk '{print $2}'

(Of course, this is merely an example. If you have lsof it's probably a better solution to this particular problem; also the output of ps varies wildly from system to system so you might want to print something else than $2 and use completely different options to ps.)

Remember that sed and awk are glorified variants of grep. So why use grep at all?

ps -l | awk '!/[a]wk/{print $2}'

True Story from Real Life: an older version of the GNATS system would think my real name was "System Operator" because it just went looking for the first occurrence of the letters e-r-a in the /etc/passwd file. (Well, actually, it thought my name was "System Era". It took me a while to figure out how it arrived at this somewhat whimsical conclusion. Incidentally, you also have to wonder why the author thought my real name was worth knowing, and if this is the right way to get that information. The end goal was to produce a template for an e-mail message -- perhaps my MUA would already know my real name, and even be able to produce nice e-mail headers for GNATS?)

Useless Use of Backticks

The cluelessness example (I've put it on a separate page) contains a lot of badly chosen backticks and a somewhat longer discussion of what exactly they can accomplish, and some ideas for how to do it differently.

Obviously, backticks are a valid construction, and you can put them to good use in many shell scripts. I have simply noticed that newbie scripters often generously treat backticks as the hammer for all those nails they see.

Dangerous Backticks

	for f in `cat file`; do
	    ...
	done

Incidentally, this is also one of the Very Ancient Recurring Threads in comp.unix.shell so don't make the mistake of posting anything that resembles this.

The right way to do this is

	while read f; do
	    ...
	done <file

	command that used to be in backticks |
	while read f; do
	    ...
	done

	command `other command`

	other command | xargs command

	# Note that we use -print0 instead of -print
	# This is a GNU find -specific option
	# Similarly, the -0 option to xargs is a GNU feature
	find -options -more -options -print0 |
	xargs -r0 command

In case something is still not obvious, the only disallowed file name characters under Unix are the null character and slash. If a file is called something like /tmp/moo<newline>/etc/passwd, normal find /tmp -print would output

/tmp/moo
/etc/passwd

Useless Use of Test

All these UUOC postings remind me of one of my least favourite constructs in (Bourne, FTSOE) shell scripts:

	mangle the world
	if [ $? -ne 0 ] ; then 
	    echo "Oh dear, mangling the world failed horribly" \
	        >&2  # shortened, stderr redirection by your humble HTMLizer
	    putback the world
	    exit 255
	fi

when

	if mangle the world ; then : ; else
	    echo "Oh dear, mangling the world failed horribly" \
	        >&2 # I'm still here
	    putback the world
	    exit 255
	fi

seems to me much cleaner (even when the "then" clause can't be used in a more economical way, or, as with some shells, omitted).

Assorted Other Gripes

(I'll integrate it better with this page as soon as I have the time; I've been keeping it in my inbox for an embarrassing amount of time so I thought I'd better at least move it here where people can see it.)

Some things that bug me:

• Regular expressions used for searching (not substituting) that begin or end with '.*'. Actually 'anything*' or 'anything?'. If you are willing to accept "zero repetitions" of the anything, why specify it?
• Awk scripts that are basically cut unless reordering of fields is needed.
• Case conversions in comp.unix.shell (ex. how do I change my file names from UC to LC?) using tr/sed/awk/??? when some shells have builtin case conversions.
• Complex schemes to basically eliminate certain chars. For example DOS lines to UNIX lines. Sure read dos2unix(1), but using sed/awk/... when "tr -d '^M'" is all that is needed.
• Global changes to a file using sed to create a tmp file and renaming the tmp file, when an "ed(1)" here document would do fine.

Frederick J. Sena comments;

I really hate useless "kill -9"'s and "rm -fr"'s! After that, the next most annoying thing is people who use a useless "chmod 777" to make a file writable.

Frederick also remarks:

I disagree with your awk/cut comment, as I often use awk for everything and cut for nothing because the syntax for awk is so much cleaner for one liners and I don't have to RTFM so much.

Despite the looks of this embarrassing section, I do appreciate comments and additional ideas for this page. Send me mail with your suggestions!

Related Resources

• Csh Programming Considered Harmful by Tom Christiansen
• GUIs Considered Harmful (ditto)

• Welcome to comp.unix.shell
• Unix shell differences and how to change your shell
• Unix Frequently Asked Questions

I have a small collection of Unix links with some more information, too.

Useless Use of Cat Award form letter

And of course, if you've been following along for a week or two, you know
that this (BING!) is a Useless Use of Cat!

Rememeber, nearly all cases where you have:

        cat file | some_command and its args ...

you can rewrite it as:

        <file some_command and its args ...

and in some cases, such as this one, you can move the filename
to the arglist as in:

        some_command and its args ... file

Just another Useless Use of Usenet,

Useless Use of Kill -9 form letter

No no no.  Don't use kill -9.

It doesn't give the process a chance to cleanly:

1) shut down socket connections

2) clean up temp files

3) inform its children that it is going away

4) reset its terminal characteristics

and so on and so on and so on.

Generally, send 15, and wait a second or two, and if that doesn't
work, send 2, and if that doesn't work, send 1.  If that doesn't,
REMOVE THE BINARY because the program is badly behaved!

Don't use kill -9.  Don't bring out the combine harvester just to tidy
up the flower pot.

Just another Useless Use of Usenet,

)-: ylwols hguoht( depoh dah I naht retteb neve ,dekrow ti yltnerappA -- .egap "elpoeP looC" s'ladnaR no dedulcni ti teg ot si egap siht fo esoprup elohw eht ,yllaer oN

era eriksson * home page