What is the use of UNIX source command under bash or any other shell? How do I use source command under UNIX / OS X / Linux operating systems?

Bash shell comes with source command which is used to read and execute commands from given FILENAME and return. If any ARGUMENTS are supplied, they become the positional parameters when FILENAME is executed. This is useful to create your own shell scripting libraries and call them using the source command. The syntax is as follows:

source /path/to/fileName

Create a shell script called functions.sh as follows:

#!/bin/bash
JAIL=/jaildir
 
addusertojail(){
   echo "Adding users..."
}
 
setupjail(){
   echo "Setting up jail"
}

Now create a file called test.sh:

#!/bin/bash
source functions.sh
 
# access $JAIL defined in functions.sh
echo "Jail dir : $JAIL"
 
# call functions
addusertojail
setupjail