I am trying to understand how the set command works on fish shell.
For example, to remove the greeting message, the fish manual suggests to:
To empty the text, with the default greeting function:
set -U fish_greetingor
set -g fish_greetingin config.fish.
But why it is suggested to use set -g fish_greeting in config.fish instead of just set fish_greeting? What is the practical purpose of -g here?
If a variable already exists, set without "-g" will update it.
I.e. if you have a local variable called "fish_greeting", then set fish_greeting will change that local variable.
If you have a universal variable called "fish_greeting", then set fish_greeting will update that. Universal variables are stored by fish, persistently in a file, so doing that will update a file and change it in other running fish sessions too.
By adding "-g" (short for "--global"), you ensure that it only modifies a global variable (and shadowing the universal one if any in the current fish shell invocation only), creating it if necessary.
set -g should always be used (in scripts at least) for global variables to remove the risk of a set var value changing a variable that happens to be universal (which could affect other scripts or interactive fish sessions by the same user)? set -x foo bar, which should be set -gx foo bar. global foo to modify them.