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I wrote a script that listens for commands from a FIFO and collects couple of files for debugging on my raspberry pi, all working fine until i decided to write a systemd unit file for it. running systemctl start crashcollector will start the script but it never reads anything from the pipe it's running but not doing anything, systemctl stop crashcollector takes forever to stop the service so i'm guessing something wrong with my unit file even though i wrote many that work just fine.

bash script:

#!/bin/bash

collectCrashReport() {
    # teleport to cache dir
    cd /mnt/cache
    # remove any previous report
    rm -rf crash/crashreport_*
    # create report directory
    directory=crashreport_`cat /etc/hostname`_$(date +%Y%m%d%H%M%S)
    mkdir $directory
    cd $directory
    # snatch all the logs
    cp -r /var/log/ .
    ps --no-headers -ax > processes.dump
    # dump dmesg to file 
    dmesg > dmesg.dump
    # create crash dir
    mkdir crash
    # zip the whole thing
    zip -qr /mnt/cache/crash/"$directory.zip" .
    # remove collection dir
    rm -rf /mnt/cache/crashreport_*
    # done and done
}

# check if already running and kill
PIDFILE=/var/run/crashcollector.pid
# check if already running
if [ -f $PIDFILE ]; then
    kill `cat $PIDFILE`
fi
echo $$ > $PIDFILE

cleanup() {
    rm -f $PIDFILE
    exit 0
}
# trap them signals 
trap "cleanup" INT TERM EXIT

that_awesome_pipe="CCPipe"
[ -p $that_awesome_pipe ] || mkfifo $that_awesome_pipe
chmod 766 $that_awesome_pipe
while :; do
    if read line; then
        case $line in
        "collect")
            collectCrashReport
            ;;
        *)
            log "Received bad command $line, ignoring..."
            ;;
        esac    
    fi
done <"$that_awesome_pipe"

Systemd unit file:

[Unit]
Description=Crash report collector
After=network.target

[Service]
Type=simple
ExecStart=/usr/bin/crashCollector
ExecStop=/usr/bin/kill -9 `cat /var/run/crashcollector.pid`
KillMode=process
Restart=always
RestartSec=3

[Install]
WantedBy=multi-user.target

not sure what exactly i'm missing here removing that if read line block makes everything work just fine.

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  • 1
    Try replacing commands enclosed in back ticks (`) with $() as backticks are rather dated now. – Raman Sailopal Sep 25 '17 at 12:13
  • Also, you could perhaps try "while read line do" as opposed to using if. – Raman Sailopal Sep 25 '17 at 12:14
  • changing the if read to while read didn't solve it – Tychus Sep 25 '17 at 12:53
  • ^ No... remove the if read...fi construct entirely (but keep its contents), and change the while :; do to while read line; do. – roaima Sep 27 '17 at 18:23
  • You really also ought to quote your variables when they're used. For example case $line in should be case "$line" in. – roaima Sep 27 '17 at 18:25
3

There are so many things wrong, there.

  • Do not treat ExecStart and ExecStop as if they are shell command lines. They are not. The systemd manual does warn against this. Shell expansions such as command substitution are not available in these service unit file settings. It is not the shell language.
  • Do not create a PID file mechanism when the place that you are starting from is having an actual service manager to hand. PID files are a mechanism that we have known to be broken since the 1980s, and that proper service management completely does away with the need for. You have a service manager. It will ensure that at most only one instance of the service runs at any time. It will track the process ID. It will deal with sending termination signals to the service process when the service is shut down.
  • Do not put basic server-side stuff into the server process, such as creating the FIFO and opening its server end.

Do things this way:

  • Create a socket unit to go alongside the service unit.
  • Take all of the explicit handling of the FIFO out of crashCollector.
  • Use ListenFIFO in the socket unit. This will of course need an absolute pathname.
  • Use StandardInput=socket in the service unit.
  • Make your crashcollector script just read from its standard input.
  • Take all of the PID file mucking about out of crashCollector.
  • Take all of the signal trapping and cleanup() stuff out of crashCollector.
  • Take all of the explicit killing with ExecStop, and the consequent bodge that you tried to do with KillMode, out of the service unit.
  • Use StandardError=journal in the service unit.
  • Replace log with a simple echo 1>&2 to standard error.

Further reading

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0

Everything was fixed after I moved the fifo to /tmp; not sure what that changes, but things work smoothly now.

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