A shell script is an ASCII text file containing one or more commands.
# My example bash script
echo "Hello World"
The first line contains a shebang #! followed by the path to the shell, in this case bash - this acts as an interpreter directive and ensures that the script is executed under the correct shell.
The "#" character indicates a comment, so the shebang line is ignored by bash when running the script.
Next you need to make the script executable with chmod
$ chmod u+x my_shell_script.sh
You can now run the script by prefixing it with ./
If you get an error like "#!/bin/bash: No such file or directory", that is typically an indication that the File encoding or line endings are wrong, use an editor like VI or TextWrangler (shows line encodings at the bottom of the edit window) the script file should be Unicode (UTF-8) and Unix (LF) line endings
If you will be writing a few shell scripts then it's worth creating a folder, perhaps called "scripts" and adding that to the system path:
$ mkdir -p $HOME/scripts
$ export PATH="$PATH:~/scripts"
Even better is to edit your .bash_profile file to include export PATH="$PATH:~/scripts" that will keep the "scripts" folder in your path every time you log in.
With the script saved in the folder, you can now run it with just:
exec - Execute a command