Question

In an answer to a previous question:

How can I use 'do I have root access?' as a conditional in bash?

The suggestion to ‘try to do something and detect if it fails’ instead of ‘check permission and then do something’

I have found plenty of rationale for this e.g.:

• Thorough use of 'if' statements or 'try/catch' blocks?
• What is the advantage of using try {} catch {} versus if {} else {}

However, I have found very little clear information about how to implementing try/catch in bash. I would guess that it is too easy, except that what I have found seems rather complicated- using functions or other scripts:

• Error handling in Bash
• Executing code in if-statement (Bash)

I am relatively new to bash but confused that there is not a simple function similar to the try function in other languages.

specifically, I would like to do the following:

CMD=`./path/to/script.sh`
if [ <echo $CMD | grep error is true> ]; then
        .. do this ..
else 
        .. do that ..
fi   

Answer 1

if sudo chmod a-x /etc/shadow 2>/dev/null
then : Yes - I have root permissions
else : No - I do not have root permissions or /etc/shadow does not exist or ...
fi

This chooses an operation that does no damage if it succeeds (the shadow password file is not supposed to be executable; you could do something like chmod o-w / – remove public write permission from the root directory if you prefer), and check that it worked by looking at the exit status of the command. This throws away the error message – you have to decide whether that matters.

The ‘sudo’ is there to raise the privileges; if you think the user should already be ‘root’, then omit the ‘sudo’.