How can a Bash script tell how it was run?

Question

I have a Bash script I was trying to make to help me run a rather complex command with small changes that it would ask me about through echo and read.

I have found solutions to force it to run a terminal to execute the command, but I'm not interested in that. What I would like it to do is, if I space out and just hit Enter on it in Nautilus (making it run with Run Software), it'll just gently pop up a notification saying "Please run this from a terminal."

I can get the popup to happen -- as in I know the command -- but I can't get the Bash script to tell if it's being run inside a terminal or not, it seems to always think so. Is it even possible?

Answer 1

From man bash under CONDITIONAL EXPRESSIONS:

-t fd  
    True if file descriptor fd is open and refers to a terminal.

Assuming fd 1 is standard out, if [ -t 1 ]; then should work for you. The Advanced Shell Scripting Guide claims that -t used this way will fail over ssh, and that the test (using stdin, not stdout) should therefore be:

if [[ -t 0 || -p /dev/stdin ]]

-p tests if a file exists and is a named pipe. However, I'd note experientially this is not true for me: -p /dev/stdin fails for both normal terminals and ssh sessions whereas if [ -t 0 ] (or -t 1) works in both cases (see also Gilles comments below about issues in that section of the Advanced Shell Scripting Guide).

---

If the primary issue is a specialized context from which you wish to call the script to behave in a way appropriate to that context, you can sidestep all these technicalities and save your self some fuss by using a wrapper and a custom variable:

!#/bin/bash

export SPECIAL_CONTEXT=1
/path/to/real/script.sh

Call this live_script.sh or whatever and double click that instead. You could of course accomplish the same thing with command line arguments, but a wrapper would still be needed to make point and click in a GUI file browser work.

Answer 2

Use the bash $SHLVL variable to detect the level of shell nesting. In a script run 'raw' by double-clicking it will be 1, in a script running within a terminal it will be 2.

#!/bin/bash
if (( SHLVL < 2 )) ; then
    echo "Please run this from a terminal."
    read -p "Press <Enter> to close this window"
    exit 1
fi
# rest of script

Note: $ is not required when testing numerical variables within (( )).

Answer 3

Although goldilocks' answer is probably correct in the typical case, it does seem that there are edge cases. In my own case, my xserver is configured to start up from tty1 and it never leaves that tty. If Xorg's stdout is a TTY then it seems the clients will have that TTY linked to their file descriptor by default.

Here's how I solved my issue:

#!/bin/bash
isxclient=$( readlink /dev/fd/2 | grep -q 'tty' && [[ -n $DISPLAY ]] ; echo $? )
if [[ ! -t 2  || $isxclient == "0" ]]; then
        notify-send "Script wasn't started from an interactive shell"
else
        echo "Script was started from an interactive shell"
fi

I haven't tested this to see if it works on a more standard X configuration, and I also doubt very much that this is the only edge case. If anyone finds a more generally applicable solution, please come back and tell us.

Answer 4

Another, using the bash options set internal variable, $-.

From .bashrc,

# If not running interactively, don't do anything
case $- in
    *i*) ;;
    *) return;;
esac