Example help

[chudak]

Hello and thank you for your FLoM program !

I have a cron_wrapper bash script (see below) that appends to a log file and in theory crontab entries may run at the same time.

Would you be so kind and suggest an example for this case?

Thanks!

================

!/bin/bash

/nightlies/cron_wrapper.sh

check for no argument case and stop

if [ -z $1 ]; then
echo "need argument"
exit 1
fi

set permanent $LOG file var

LOG="/var/log/crontab-nightlies-log/crontab.log"

temp files to store sdtout and stderr

named with the PID of this script in their name so they'll be unique

STDERR="/var/tmp/stderr.$$"
STDOUT="/var/tmp/stdout.$$"

$STDOUT and $STDERR are removed when the script exits for any reason

trap "rm -f $STDOUT $STDERR" 0

echo "Running command: $@" >> $LOG

run a command from this script's argument

redirect stdout to $STDOUT file and redirect stderr to $STDERR file

"$@" > $STDOUT 2> $STDERR

get return code from the command run`

code=$?

if [ $code != 0 ] ; then
# echoing to stdout/stderr makes cron send email
echo "stdout:"
cat $STDOUT
echo "stderr:"
cat $STDERR
else
# normal exit: just log stdout
DATE=$(date)
echo -n "$DATE: " >> $LOG
cat $STDOUT >> $LOG
fi

[Christian Ferrari]

Dear Yuri,
from my understanding of your script, you want to synchronize the access to the $LOG file.
The resource is accessed by 3 different operations:

echo "Running command: $@" >> $LOG
[...]
        commands with unpredictable duration
[...]
echo -n "$DATE: " >> $LOG
cat $STDOUT >> $LOG

interleaved by some commands with unpredictable duration.
If your intent is just to avoid overlapped log, your script could be changed with a further tmp file:

LOG2=$(mktemp)
[...]
echo "Running command: $@" >> $LOG2
[...]
echo -n "$DATE: " >> $LOG2
cat $STDOUT >> $LOG2
flom -r $LOG -- bash -c "cat $LOG2 >> $LOG"
rm $LOG2

the clue is

flom -r $LOG -- bash -c "cat $LOG2 >> $LOG"

it synchronizes the execution of the command "cat ..." with an exclusive lock related to the hierarchical resource $LOG.

You can't use:

flom -r $LOG -- cat $LOG2 >> $LOG

because the synchronized command needs a shell environment for ">>" operator.