A Mnemonic for Changing Modes
Making a script file executable involves an incantation that has something to
chmod but what exactly?
chmod has two modes, absolute and
symbolic. In absolute mode, you set the entire set of permissions with a
numeric value like 755.
In symbolic mode, you can add or subtract permissions to either the user (u), group (g) or owner (o). To give read permissions to the group type:
> chmod g+r myfile.txt
And to give execution priviledges to the owner of a file:
> chmod o+x myscript.rb