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Old 06-29-2008, 01:00 AM   #1
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variable in a tail command

hey there all

i'm writing a script that outputs the last X bytes of results in a log file ... so im using the -c flag with my tail command... pretty simple, except i don't know that the X number will be beforehand, so i have to figure it out by comparing the log file size over a length of time (say 30 seconds)

so i take the log file size at time 0, wait 30 seconds and get the size of the logfile again (which will now be larger as there is always activity on the log file).... i then minus the two numbers to find out how much new information has been written to the log file in the last 30 seconds.... i save this number to a variable called $sizediff... i then want to tail the last $sizediff bytes from the log file, and output these results to a new file called templog

something like this:
tail -$sizediffc logfile > templog

when i run the script it just hangs at the above command... if i replace the $sizediff with an actual number, it of course then works... the problem is, is that i don't know what that number will be beforehand, so i need to use a variable

if i run:
tail -$sizediff c logfile > templog

i then get the following error: tail: invalid option -- 7 ... the $sizediff variable would be 720 at this point, so its getting the variable correctly, but because the 'c' isn't beside it liek it should be, it doesn't know what to do with the flag

does anybody know a way around this? where you can use a variable in a command?

Last edited by jadeddog; 06-29-2008 at 01:03 AM.
Old 06-29-2008, 01:24 AM   #2
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nm, i figured it out... all i needed to do was put the variable in quotations, so:
tail -"$sizediff"c maillog > templog

i can't believe i didn't try that before posting
Old 06-29-2008, 01:40 AM   #3
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I see no need for quotes:

$ tail -c -4 /usr/share/dict/words
$ sizediff=4
$ tail -c -$sizediff /usr/share/dict/words
Old 06-29-2008, 09:50 PM   #4
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tail -${sizediff}c maillog > templog
Old 06-29-2008, 10:35 PM   #5
Mr. C.
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Originally Posted by chrism01 View Post

tail -${sizediff}c maillog > templog
Using arguments this way has some unforeseen traps. Consider if sizediff is negative:

$ sizediff=-10
$ tail -${sizediff}c /usr/share/dict/words 
tail: unrecognized option `--10c'
Try `tail --help' for more information.

$ tail -c $sizediff /usr/share/dict/words 
It is generally best to adhere to standard option/argument behavior:

-opt arg
--longopt arg


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