To run a simple shell script continuously in the background, it is convenient to register it as a systemd service. Here, as an example, we create a script at /opt/hello.sh that writes “hello world” to a log file every second, and manage it via systemd.
Also, when SELinux is enabled, placing the script under /tmp may cause it to lack the proper context and prevent execution via systemd. By placing the script under /opt, the usr_t label is applied, allowing it to run without issues.
cat <<'EOFSH' > /opt/hello.sh
#!/bin/bash
while true
do
echo hello world >> /tmp/hello.log
sleep 1
done
EOFSH
chmod 744 /opt/hello.sh
cat <<'EOFSERVICE' > /etc/systemd/system/hello.service
[Unit]
Description = hello daemon
[Service]
ExecStart = /opt/hello.sh
Restart = always
[Install]
WantedBy = multi-user.target
EOFSERVICE
# setting
systemctl enable hello --now
systemctl start hello
# check
sleep 3 ; tail -F /tmp/hello.log
Note on SELinux Contexts
If the script is located in /tmp, SELinux may prevent its execution. When placed in /opt as shown below, the usr_t label is applied, allowing it to be properly called from a systemd service.
[root@localhost ~]# ls -Z /opt/hello.sh
unconfined_u:object_r:usr_t:s0 /opt/hello.sh
[root@localhost ~]#
By writing ExecStart=/usr/bin/sh -c <Shell>, you don’t need to worry about the SELinux context.