[Bash] Command inside a variable

yvovandoorn · 01-17-2007, 12:37 PM

In a script I am writing I have it write to an error log when it doesn't complete certain tasks. I have the following set as a variable:

Code:

WRITERR="tail -n50 $LOG | grep PRODBACKUP-knik > $ERRLOG"

However what seems to be happening when I call for $WRITERR is that tail tries to tail the entire command, thus it errors out with:

Code:

tail: cannot open `|' for reading: No such file or directory
tail: cannot open `grep' for reading: No such file or directory
tail: cannot open `PRODBACKUP-knik' for reading: No such file or directory
tail: cannot open `>' for reading: No such file or directory

When I run the command inside the script but not as a variable, thus like this:

Code:

tail -n50 $LOG | grep PRODBACKUP-knik > $ERRLOG

Then it works fine, however I have the $WRITERR all over the place in the script and it just seems a lot simpler to go about it that way.

What am I doing wrong?

Below is an excerpt from the script where this is used in a real life scenario:

Code:

HOSTNAME="knik"
LOG="/var/log/backup.log"
ERRLOG="/var/log/backup-err.log"
WRITERR="tail -n50 $LOG | grep PRODBACKUP-knik > $ERRLOG"
ADMINS="emailhere"
TEMPDUMP="/usr/local/dumps/prodbackup"

if [ -f $TEMPDUMP/$HOSTNAME.1.tar.gz ];
	then
		echo "PRODBACKUP-$HOSTNAME: Gzipped file has been downloaded successfully!" >> $LOG
	else
		echo "PRODBACKUP-$HOSTNAME: No gzipped file found!" >> $LOG
		echo "PRODBACKUP-$HOSTNAME: FAILURE!!" >> $LOG
		$WRITERR
		mail -s "Weekly Production Backup for $HOSTNAME FAILED" $ADMINS < $ERRLOG
		exit 2
fi

pwc101 · 01-17-2007, 12:52 PM

try using backticks ` (not apostrophes ') to surround the tail command:

Code:

WRITERR=`tail -n50 $LOG | grep PRODBACKUP-knik > $ERRLOG`

You can also use VARIABLE=$(command):

Code:

WRITERR=$(tail -n50 $LOG | grep PRODBACKUP-knik > $ERRLOG)

These two examples are synonymous.

I'm not sure about writing to another variable ($ERRLOG) which is a file though. I'd try getting rid of > $ERRLOG. You would then have a variable WRITERR which contains exactly what $ERRLOG would contain. If you wanted to write that to a log, a $WRITERR > $ERRLOG would need to be put into the script somewhere.

I think.

edit: To execute the variable so that you could write it to a log file, you'd need to use echo (since all that's contained in the variable is text). Thus:

Code:

echo "PRODBACKUP-$HOSTNAME: FAILURE!!" >> $LOG
echo $WRITERR > $ERRLOG
mail -s "Weekly Production Backup for $HOSTNAME FAILED" $ADMINS < $ERRLOG

Again, I think.

yvovandoorn · 01-17-2007, 01:52 PM

Awesome with your tips I was able to get it to work.

End result:

Code:

-WRITERR="tail -n50 $LOG | grep PRODBACKUP-knik > $ERRLOG"
+WRITERR=`tail -n50 $LOG | grep PRODBACKUP-knik > $ERRLOG`

Code:

echo "PRODBACKUP-$HOSTNAME: FAILURE!!" >> $LOG
-$WRITERR
+echo $WRITERR
mail -s "Weekly Production Backup for $HOSTNAME FAILED" $ADMINS < $ERRLOG

Thanks for the help.

pwc101 · 01-17-2007, 04:20 PM

No worries

Glad I got it right! I was learning as much as you were

unSpawn · 01-17-2007, 07:08 PM

I'd suggest making it a function because when using a variable like "$WRITERR" it gets filled at the time the interpreter parses the position in the script (do a "sh -x scriptname" to see) and not when you need actually it (of course this is more useful if the log fills rapidly).

Code:

function WRITERR() { 
 tail -n50 "${LOG:=/var/log/backup.log}" 2>/dev/null \
 | grep PRODBACKUP-`hostname -s 2>/dev/null` \
 >> "${ERRLOG:=/var/log/backup-err.log}" 2>/dev/null
 }

Making it a function you can
- use it in the same spot you do now,
- reuse it whenever necessary,
- keep your code easy to read and
- easier extendable.

osvaldomarques · 01-20-2007, 06:48 PM

Just as a complement for this topic, we have to remember the "eval" builtin shell command. It evaluates the content of a variable and then executes it.

In your first question, all you would need execute "eval $WRITERR" to obtain the expected result.

This command is useful when we have to programmatically assemble a command line; we can even use apostrophes to delay command substitution.