The Answer Gang 
By Jim Dennis, Karl-Heinz Herrmann, Breen, Chris, and... (meet the Gang) ... the Editors of Linux Gazette... and You!
finding then catting
From Greg Messer
Answered By: Neil Youngman, Jason Creighton, Thomas Adam, Benjamin Okopnik
Answer Gang,
I run a smallish mail server and am using Squirelmail for web-based email. I use spamassassin/procmail to move emails that are borderline spam to /home/username/Trash.
My users have been instructed to occasionaly log into squirelmail and look through their Trash folder and empty it occassionaly even if they're popping in with the dreaded Outlook Express. They of course don't, and it's becoming a problem. I need to run a command that will find all the files called Trash in the user's home directories and empty them. I can't simply delete this file as it causes Squiremail to generate an error and I would get many many phone calls even though squirelmail will fix this problem on their next login.
This is my third attempt at automating this procedure and my third failure.
I can do this:
find /home -name Trash -exec ls -al {} \;
and this:
find /home -name Trash -exec rm {} \;
but not this:
find /home -name Trash -exec cat /dev/null > {} \;
[Neil]
It's the redirection that's the problem. If you quote the '>' thus:
find /hometest -name Trash -exec cat /dev/null '>' {} \;
it will work, with the caveat that you may still hit some "trash" files in subdirectories.
Check where you ran the command. You will find an empty file called "{}", created by your redirection. The command you ran was equivalent to
find /hometest -name Trash -exec cat /dev/null \; > {}
That will empty anything called Trash in subdirectories as well as in the login directories. To only hit files in the login directories you should use a for loop, e.g.
for file in /home/*/Trash do echo -n > $file done
Before trying this put another echo in front of echo echo -n > $file, so you can see the commands it will run and sanity check them before running it for real.
What errors are you getting? Do you have permissions to write to these files?
or this:
find /home -name Trash | xargs cat > /dev/null
While root, when I do this:
find /hometest -name -Trash -exec cat /dev/null > {} \;
it runs and exists after a second giving me a new prompt (a carriage return) and no errror messages.
When I run this:
find /hometest -name Trash -exec ls -s {} \;
I get this:
60 /hometest/accounting.test/Trash 264 /hometest/adam.test/Trash 3120 /hometest/agency.test/Trash 164 /hometest/joh.doe/Trash 4976 /hometest/alice.test/Trash
so obviously it didn't work but I didn't get any errors.
Your "for" script worked great and is short and sweet. I'm very greatful, however, for my own information, I'd still like to understand what's wrong with my find syntax/structure. If you guys post this solution on the website you should put in the keywords "empty files". I've googled for all kinds of crazy things and never found a solution.
find /hometest -name -Trash -exec cat /dev/null > {} \;
This runs "find /hometest -name -Trash -exec cat /dev/null" and redirects the output to a file named "{}".
Quoting the '>' doesn't help since find doesn't use the shell to expand commands given with -exec. (That is, if you quoted the ">", cat would be run with three arguments. The first would be a file named "/dev/null". The second would a file named ">", which cat would probably complain doesn't exist. It is possible you might actually have a file named ">", but it's such a weird and confusing name that you probably don't. And the third would be the name of the file you're trying to truncate.)
If, for some reason, you needed to use "find" (perhaps to only truncate files with a certain mtime, or whatever), you could use a script like this:
#! /bin/sh
for file in "$@"; do
[ -f "$file" ] && echo -n > "$file"
done
name it truncate.sh or something, make it executable, and save it somewhere. Then you could do:
find /path/to/files -exec truncate.sh {} \;
...or use xargs, or whatever.
find . -type f -exec ./truncate {} \;
[n6tadam@station fi]$ ls -lFb total 8 -rw-r--r-- 1 n6tadam n6tadam 0 Jan 11 11:18 foo drwxr-xr-x 2 n6tadam n6tadam 4096 Jan 11 11:11 ignore/ -rw-r--r-- 1 n6tadam n6tadam 120 Jan 11 11:08 this\ has\ spaces
Ignoring the "ignore/" directory, I've got a file with spaces in the filename [1], as well as a 'normal' file. If I wanted to truncate the files in the CWD above, I might use:
find . -type f -maxdepth 1 -exec sh -c 'cat /dev/null > {}' \;
... which is fine, for the file with no spaces. Of course, the truncate.sh script you wrote is fine for handling that (you actually quoted the variable -- thousands do not). But just what is wrong with that command above? Well, for each file that find finds, it has to spawn a separate non-interactive shell to process it. That's slow.
xargs might improve things (I'll leave this as an exercise to the reader to use 'time'):
find . -type f -maxdepth 1 -print0 | xargs -0i sh -c "cat /dev/null > \"{}\""
Note the quoting. It's paramount that this is done, because even though the '-print0' option to find splits file names ending '\0' (and xargs re-interprets them again at the other end), we're still having to quote the filename (this will still fail if the filename contains a '"' character, though). Why? Because by the time it gets passed through to the shell to handle it, we're back to the our old tricks of: '"\"use\" more quo\"t\"es'.
So is using find(1) any better than using a plain shell script that globs a given directory for files to truncate? No. Because find blindly exec()'s whatever we pass to it (and we're having to use shell redirection) we must invoke the shell for it to work. The only advantage to using find is that it would handle some strange files, nothing more (in this particular application of it, anyway).
I suppose you could make that find command more efficient:
find . -type f -maxdepth 1 -not -empty -print0 | xargs -0i sh -c "cat /dev/null > \"{}\""
... which just ensures that the files we pass to it have a filesize greater than zero. The "best" solution that I personally can see, is using the following:
find . -type f -maxdepth 1 -not -empty -print0 | xargs -0i cp /dev/null {}
This obliterates the need to fork a subshell to perform any redirection -- and as with any "find .. | xargs" combination, it'll be quite fast, too. But the main reason for using it is that by avoiding any shell-redirection-mangle-filename techniques, we don't have to worry about quoting. The delimiter of '\0' via find and xargs should be enough to protect it.
Also note that cat'ting /dev/null is nonsensical in this instance.
[1] Remember that there is nothing "illegal" about using such characters. Any character is a valid one for filenames at the filesystem level. What defines them as being a pain is the shell. Nothing more.
find -print0 | xargs -0 command
Jason Creighton
Mostly, it's a REALLY good way to screw yourself up; in general, it's not a good idea to use anything outside of [a-zA-Z0-9_] as part of a filename.
But then, we're talking about us.
ben@Fenrir:/tmp/foo$ for n in `seq 0 255`; do a=$(printf "\x`printf "%x" $n`"); >"a${a}"; done
bash: a/: Is a directory
ben@Fenrir:/tmp/foo$ ls -b
a a{ a\214 a\243 a\272 a\321 a\350 a\377 a\031 ad aO
a` a} a\215 a\244 a\273 a\322 a\351 a\002 a\032 aD ap
a^ a@ a\216 a\245 a\274 a\323 a\352 a\003 a\033 ae aP
a~ a$ a\217 a\246 a\275 a\324 a\353 a\004 a\034 aE aq
a< a* a\220 a\247 a\276 a\325 a\354 a\005 a\035 af aQ
a= a\\ a\221 a\250 a\277 a\326 a\355 a\006 a\036 aF ar
a> a& a\222 a\251 a\300 a\327 a\356 a\a a\037 ag aR
a| a# a\223 a\252 a\301 a\330 a\357 a\b a0 aG as
a\ a% a\224 a\253 a\302 a\331 a\360 a\t a1 ah aS
a_ a+ a\225 a\254 a\303 a\332 a\361 a\v a2 aH at
a- a\001 a\226 a\255 a\304 a\333 a\362 a\f a3 ai aT
a, a\200 a\227 a\256 a\305 a\334 a\363 a\r a4 aI au
a; a\201 a\230 a\257 a\306 a\335 a\364 a\016 a5 aj aU
a: a\202 a\231 a\260 a\307 a\336 a\365 a\017 a6 aJ av
a! a\203 a\232 a\261 a\310 a\337 a\366 a\020 a7 ak aV
a? a\204 a\233 a\262 a\311 a\340 a\367 a\021 a8 aK aw
a. a\205 a\234 a\263 a\312 a\341 a\370 a\022 a9 al aW
a' a\206 a\235 a\264 a\313 a\342 a\371 a\023 aa aL ax
a" a\207 a\236 a\265 a\314 a\343 a\372 a\024 aA am aX
a( a\210 a\237 a\266 a\315 a\344 a\373 a\025 ab aM ay
a) a\211 a\240 a\267 a\316 a\345 a\374 a\026 aB an aY
a[ a\212 a\241 a\270 a\317 a\346 a\375 a\027 ac aN az
a] a\213 a\242 a\271 a\320 a\347 a\376 a\030 aC ao aZ
HTML script maintained by Heather Stern of Starshine Technical Services, http://www.starshine.org/
