testing for option in bash not working well

Question

I was trying to test for noclobber option using: if [ -o noclobber ] but it turned nothing.

I set the noclobber option on using: set +o noclobber.

prompt> cat checkoption.sh
#!/bin/bash
if [ -o noclobber ]
     then
          echo "Option is on."
fi
prompt>
prompt> set +o noclobber
prompt> set -o | grep noglobber
noclobber       on
prompt> ./checkoption.sh
prompt>

Any idea why I am not getting a message here?

Possible duplicate of How to know if extglob is enabled in the current bash session? — muru, Jan 28 '18 at 06:45
Oh wait, you're running a script. The noclobber option is not inherited by the script's shell. Try sourcing it: . ./checkoption.sh — muru, Jan 28 '18 at 06:47
That was it. Can you explain what the preceding dot did? [Edit] Ok, i got it , it was for sourcing.. — pigeon, Jan 28 '18 at 07:55
source and . are commands for running a script in the current shell. See https://unix.stackexchange.com/q/182282/70524 — muru, Jan 28 '18 at 07:56

Kusalananda · Accepted Answer · 2018-01-28T07:57:18.833

Shell options are not inherited between shell sessions, and you are dealing with two shell sessions here:

The interactive session where you set your shell option, and
The shell script's session in which you test for the option.

The shell script will never detect that the option was set in the calling interactive shell session.

Solutions:

Turn your code into a shell function (in e.g. $HOME/.bashrc):

checknoclobber () { [ -o noclobber ] && echo 'Noclobber is on'; }

Or, for the generic case,

checkoption () {
    if [ -o "$1" ]; then
        printf '%s is set\n' "$1" >&2
        return 0
    else
        printf '%s is not set (or invalid option name)\n' "$1" >&2
        return 1
    fi
}

Set the option in the script.
Source the script file in the interactive shell with source or with the . command.

testing for option in bash not working well

1 Answers1