2.1. Invoking the script

Having written the script, you can invoke it by sh scriptname, ¹ or alternatively bash scriptname. (Not recommended is using sh <scriptname, since this effectively disables reading from stdin within the script.) Much more convenient is to make the script itself directly executable with a chmod.

Either:

    chmod 555 scriptname (gives everyone read/execute permission) ²

or

    chmod +rx scriptname (gives everyone read/execute permission)

    chmod u+rx scriptname (gives only the script owner read/execute permission)

Having made the script executable, you may now test it by ./scriptname. ³ If it begins with a "sha-bang" line, invoking the script calls the correct command interpreter to run it.

As a final step, after testing and debugging, you would likely want to move it to /usr/local/bin (as root, of course), to make the script available to yourself and all other users as a systemwide executable. The script could then be invoked by simply typing scriptname [ENTER] from the command-line.

Notes  

1. ^ Caution: invoking a Bash script by sh scriptname turns off Bash-specific extensions, and the script may therefore fail to execute.
2. ^ A script needs read, as well as execute permission for it to run, since the shell needs to be able to read it.
3. ^ Why not simply invoke the script with scriptname? If the directory you are in ($PWD) is where scriptname is located, why doesn't this work? This fails because, for security reasons, the current directory (./) is not by default included in a user's $PATH. It is therefore necessary to explicitly invoke the script in the current directory with a ./scriptname.