How can I check a file exists and execute a command if not?

Question

I have a daemon I have written using Python. When it is running, it has a PID file located at /tmp/filename.pid. If the daemon isn't running then PID file doesn't exist.

On Linux, how can I check to ensure that the PID file exists and if not, execute a command to restart it?

The command would be

python daemon.py restart

which has to be executed from a specific directory.

The "which has to be executed from a specific directory" part of your description sounds like a recipe for trouble. Beware - rethink if at all possible. — Jonathan Leffler, Mar 21 '09 at 18:09
duplicate: http://stackoverflow.com/questions/638975/how-do-i-tell-if-a-file-does-not-exist-in-bash/ — , Mar 21 '09 at 19:36
/tmp is a bad location to put PID files, since some distributions have cleaner processes that delete files from /tmp, and users may delete files from there to make space. — Douglas Leeder, Jul 28 '15 at 09:29

score 86 · Accepted Answer · edited Mar 21 '09 at 18:52

86

[ -f /tmp/filename.pid ] || python daemon.py restart

-f checks if the given path exists and is a regular file (just -e checks if the path exists)

the [] perform the test and returns 0 on success, 1 otherwise

the || is a C-like or, so if the command on the left fails, execute the command on the right.

So the final statement says, if /tmp/filename.pid does NOT exist then start the daemon.

edited Mar 21 '09 at 18:52

John Feminella

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answered Mar 21 '09 at 18:05

eduffy

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What do you mean by "regular file"? What are the other options? – Barakuda Jun 04 '18 at 09:47
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@Barakuda - A non-regular file could be a directory, named pipe, network socket, character device, symbolic link... – eduffy Jun 04 '18 at 17:27
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Must be && instead of || – EthraZa Aug 02 '20 at 20:18
I think && is if the condition is met, while || is if it's not met. [ -f my_file ] || echo 'absent' && echo 'exists' – Jautis Nov 23 '22 at 21:59

score 18 · Answer 2 · edited Jul 31 '17 at 14:46

18

test -f filename && daemon.py restart || echo "File doesn't exists"

edited Jul 31 '17 at 14:46

Messa

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answered Nov 26 '14 at 07:14

harveyD

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if only the file needs to be checked for existence : test -f filename && echo "exists" || echo "does not exist" – lode Jun 28 '18 at 23:10

Brimstedt · Answer 3 · 2015-05-04T19:41:24.683

11

If it is bash scripting you are wondering about, something like this would work:

if [ ! -f "$FILENAME" ]; then
   python daemon.py restart
fi

A better option may be to look into lockfile

edited May 04 '15 at 19:41

answered Mar 21 '09 at 18:07

Brimstedt

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score 6 · Answer 4 · answered Mar 21 '09 at 18:10

6

The other answers are fine for detecting the existence of the file. However for a complete solution you probably should check that the PID in the pidfile is still running, and that it's your program.

answered Mar 21 '09 at 18:10

Douglas Leeder

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gahooa · Answer 5 · 2019-02-14T22:03:37.577

5

Another approach to solving the problem is a script that ensures that your daemon "stays" alive...

Something like this (note: signal handling should be added for proper startup/shutdown):

$PIDFILE = "/path/to/pidfile"

if [ -f "$PIDFILE" ]; then
    echo "Pid file exists!"
    exit 1
fi

while true; do
    # Write it's own pid file
    python your-server.py ;

    # force removal of pid in case of unexpected death.
    rm -f $PIDFILE;

    # sleep for 2 seconds
    sleep 2;

done

In this way, the server will stay alive even if it dies unexpectedly.

edited Feb 14 '19 at 22:03

answered Mar 21 '09 at 18:51

gahooa

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`if [ ! -f "$PIDFILE" ]; then` should actually be `if [ -f "$PIDFILE" ]; then`. I tried to edit it, but the edit is too small for stackoverflow. :/ – logidelic Feb 13 '19 at 21:50

score 1 · Answer 6 · answered Mar 21 '14 at 05:23

1

ls /tmp/filename.pid

It returns true if file exists. Returns false if file does not exist.

answered Mar 21 '14 at 05:23

KBIIX

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score 1 · Answer 7 · answered Mar 21 '09 at 18:56

1

You can also use a ready solution like Monit.

answered Mar 21 '09 at 18:56

Anonymous

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How can I check a file exists and execute a command if not?

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