Converting watch into a unit file systemd

I’ve got a shell script as follows

ss.sh

#!/bin/bash
opFile="custom.data"
sourceFile="TestOutput"
./fc app test > $sourceFile
grep -oP '[0-9.]+(?=%)|[0-9.]+(?=[A-Z]+ of)' "$sourceFile" | tr 'n' ',' > $opFile
sed -i 's/,$//' $opFile

The requirement is that I need to use this script with the watch command. And I’d like to make this into a systemctl service. I did it as so.

sc.sh

#!/bin/bash
watch -n 60 /root/ss.sh

And in my /etc/systemd/system,

log_info.service

[Unit]

Description="Test Desc"
After=network.target

[Service]
ExecStart=/root/sc.sh
Type=simple

[Install]
WantedBy=default.target

When I run systemctl start log_info.service, It runs but not continuously the way I’d like it to.

On running sytemctl status log_info.service,

info_log.service - "Test Desc"
   Loaded: loaded (/etc/systemd/system/info_log.service; disabled; vendor preset: enabled)
   Active: failed (Result: exit-code) since Mon 2016-09-12 08:17:02 UTC; 2min 18s ago
  Process: 35555 ExecStart=/root/sc.sh (code=exited, status=1/FAILURE)
 Main PID: 35555 (code=exited, status=1/FAILURE)

Sep 12 08:17:02 mo-b428aa6b4 systemd[1]: Started "Test Desc".
Sep 12 08:17:02 mo-b428aa6b4 sc.sh[35654]: Error opening terminal: unknown.
Sep 12 08:17:02 mo-b428aa6b4 systemd[1]: info_log.service: Main process exited, code=exited, status=1/FAILURE
Sep 12 08:17:02 mo-b428aa6b4 systemd[1]: info_log.service: Unit entered failed state.
Sep 12 08:17:02 mo-b428aa6b4 systemd[1]: info_log.service: Failed with result 'exit-code'.

Any ideas as to why it’s not running right? Any help would be appreciated!

Answer

So the reason I learnt (from superuser) for this failing was exactly what was in my error console, i.e,

Error opening terminal: unknown

Watch can only be executed from the terminal because it requires access to a terminal, while services don’t have that access.

A possible alternative to watch could be using a command that doesn’t require the terminal, like screen or tmux. Or, another alternative which worked for me, that was suggested by grawity on superuser, was

# foo.timer
[Unit]
Description=Do whatever

[Timer]
OnActiveSec=60
OnUnitActiveSec=60

[Install]
WantedBy=timers.target

The logic behind this was that, the need was to run the script every 60 seconds, not to use watch. Hence, grawity suggested that I use a timer unit file that calls the service file every 60 seconds instead. If the service unit was a different name from the timer unit, [Timer] Unit= can be used.

Hope this helped you and +1 to grawity and Eric Renouf from superuser for the answers!