How do I daemonize an arbitrary script in unix?

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You can daemonize any executable in Unix by using nohup and the & operator:

nohup yourScript.sh script args&

The nohup command allows you to shut down your shell session without it killing your script, while the & places your script in the background so you get a shell prompt to continue your session. The only minor problem with this is standard out and standard error both get sent to ./nohup.out, so if you start several scripts in this manor their output will be intertwined. A better command would be:

nohup yourScript.sh script args >script.out 2>script.error&

This will send standard out to the file of your choice and standard error to a different file of your choice. If you want to use just one file for both standard out and standard error you can us this:

nohup yourScript.sh script args >script.out 2>&1 &

The 2>&1 tells the shell to redirect standard error (file descriptor 2) to the same file as standard out (file descriptor 1).

To run a command only once and restart it if it dies you can use this script:

#!/bin/bash

if [[ $# < 1 ]]; then
    echo "Name of pid file not given."
    exit
fi

# Get the pid file's name.
PIDFILE=$1
shift

if [[ $# < 1 ]]; then
    echo "No command given."
    exit
fi

echo "Checking pid in file $PIDFILE."

#Check to see if process running.
PID=$(cat $PIDFILE 2>/dev/null)
if [[ $? = 0 ]]; then
    ps -p $PID >/dev/null 2>&1
    if [[ $? = 0 ]]; then
        echo "Command $1 already running."
        exit
    fi
fi

# Write our pid to file.
echo $$ >$PIDFILE

# Get command.
COMMAND=$1
shift

# Run command until we're killed.
while true; do
    $COMMAND "$@"
    sleep 10 # if command dies immediately, don't go into un-ctrl-c-able loop
done

The first argument is the name of the pid file to use. The second argument is the command. And all other arguments are the command’s arguments.

If you name this script restart.sh this is how you would call it:

nohup restart.sh pidFileName yourScript.sh script args >script.out 2>&1 &

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