Normally, running a shell script starts a new shell process, often running a different shell than our interactive shell. For example, our login shell may be tcsh or bash, and a script may be run by sh because it starts with '#!/bin/sh -e".

In some circumstances, we might not want a script to be executed by a separate shell process. For example, suppose we just made some changes to our .cshrc or .bashrc file that would affect PATH or some other important environment variable, or the prompt shell variable that describes our shell prompt.

If we run the start up script by typing ~/.cshrc or ~/.bashrc, a new shell process will be started which will execute the commands in the script and then terminate. Commands in the script may alter the shell variables and environment variables of the child process, but the parent process (the process from which you invoked the script), will be unaffected.

In order to make the "current" shell process run the commands in a script, we must source it. This is done using the internal shell command source in all shells except Bourne shell, which uses ".". Bourne shell derivatives support both "." and "source".

Hence, to source .cshrc, we would run

shell-prompt: source ~/.cshrc
        

To source .bashrc, we would run

shell-prompt: source ~/.bashrc
        

. ~/.bashrc
        

To source .shrc from a basic Bourne shell, we would have to run

. ~/.shrc