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Bash Reference Manual

6.3.2 Is this Shell Interactive?

To determine within a startup script whether or not Bash is running interactively, test the value of the `-' special parameter. It contains i when the shell is interactive. For example:

case "$-" in *i*) echo This shell is interactive ;; *) echo This shell is not interactive ;; esac

Alternatively, startup scripts may examine the variable PS1; it is unset in non-interactive shells, and set in interactive shells. Thus:

if [ -z "$PS1" ]; then echo This shell is not interactive else echo This shell is interactive fi