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I have a Linux program which can write information to stdout and stderr.

I have a shell script which redirects that output to a file in /var/log. (Via >> and 2>&1.)

Is there a way to make that log file rotate? (max size, then switch to a different file, keep only a limited number of files)

I've seen a few answers which talk about the logrotate program, which sounds good, but they also seem to be focused on programs which are generating log files internally and handle HUP signals. Is there a way to make this work with a basic output redirection script?

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Why can't you just modify the script that redirects the output to contain the logic for the rotation? –  MaQleod Jun 1 '11 at 5:05
I could, if someone could tell me how to detect the size of a logfile and rotate it out from under the stdout of a process without disturbing that process. I don't have to use logrotate if there's a better option, that just sounded like a convenient starting point for discussion. –  Miral Jun 1 '11 at 5:12
You don't have to use logrotate, but using logrotate just saves time... There is usually little point reinventing the wheel. –  bubu Jun 1 '11 at 5:43
Exactly my point. So is there a way to make logrotate work with an ongoing process's redirected stdout? –  Miral Jun 1 '11 at 6:12

5 Answers 5

up vote 11 down vote accepted

As an alternative, you could pipe the output through tools designed with the primary purpose of maintaining size-capped, automatically rotated, log file sets, such as:

Tools to then process multilog-format log file sets include, amongst others:

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Thanks, multilog looks like just what I needed. –  Miral Jun 3 '11 at 1:36

If you can have it go to one of the standard log streams (syslog, daemon, cron, user, security, mail, etc.) you can use the logger command and pipe to it instead.

echo "Hello." | logger -p daemon.info

Otherwise, you may be better off piping your logged content to a custom program or script to handle it, or look at setting up the logrotate configuration.

EDIT: JdeBP's answer seems to have what you may be looking for.

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+1 for simplicity. BTW, you can also configure a custom facility (local0) instead of the standard ones (daemon in your example) –  Roger Keays Oct 9 '12 at 7:56

the rotatelogs tool shipped with apache (in the bin dir) (see docs) takes input from stdin and rotates the log after some specific amount of time

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So is there a way to make logrotate work with an ongoing process's redirected stdout?

Yes! Check out the "copytruncate" directive offered by logrotate. Specifying that instructs logrotate to handle this very situation: a simple program that keeps its log file open indefinitely.

One caveat may or may not be a problem in your situation:

Note that there is a very small time slice between copying the file and truncating it, so some logging data might be lost.

Anecdotally, I've seen some "real world" log sources that do encourage users to apply this directive. There's some discussion of this option here.

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I made simple log.py script. Which write to 1 file and rotate it every day and every 5M


[note to moderators: do not inline my work without copyright notices]

# echo test 2>&1 | log ./run.log
-rw-r--r-- 1 axet axet     850176 Feb 22 12:28 run.log
-rw-r--r-- 1 axet axet    1048595 Feb 22 09:00 run.log.1
-rw-r--r-- 1 axet axet    1319926 Feb 22 04:57 run.log.2
-rw-r--r-- 1 axet axet    2400918 Feb 22 01:17 run.log.3
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