33.1. Interactive and non-interactive shells and scripts

An interactive shell reads commands from user input on a tty. Among other things, such a shell reads startup files on activation, displays a prompt, and enables job control by default. The user can interact with the shell.

A shell running a script is always a non-interactive shell. All the same, the script can still access its tty. It is even possible to emulate an interactive shell in a script.

#!/bin/bash
MY_PROMPT='$ '
while :
do
  echo -n "$MY_PROMPT"
  read line
  eval "$line"
  done

exit 0

# This example script, and much of the above explanation supplied by
# Stéphane Chazelas (thanks again).

Let us consider an interactive script to be one that requires input from the user, usually with read statements (see Example 14-3). "Real life" is actually a bit messier than that. For now, assume an interactive script is bound to a tty, a script that a user has invoked from the console or an xterm.

Init and startup scripts are necessarily non-interactive, since they must run without human intervention. Many administrative and system maintenance scripts are likewise non-interactive. Unvarying repetitive tasks cry out for automation by non-interactive scripts.

Non-interactive scripts can run in the background, but interactive ones hang, waiting for input that never comes. Handle that difficulty by having an expect script or embedded here document feed input to an interactive script running as a background job. In the simplest case, redirect a file to supply input to a read statement (read variable <file). These particular workarounds make possible general purpose scripts that run in either interactive or non-interactive modes.

If a script needs to test whether it is running in an interactive shell, it is simply a matter of finding whether the prompt variable, $PS1 is set. (If the user is being prompted for input, then the script needs to display a prompt.)

if [ -z $PS1 ] # no prompt?
then
  # non-interactive
  ...
else
  # interactive
  ...
fi

case $- in
*i*)    # interactive shell
;;
*)      # non-interactive shell
;;
# (Courtesy of "UNIX F.A.Q.," 1993)

Scripts may be forced to run in interactive mode with the -i option or with a #!/bin/bash -i header. Be aware that this can cause erratic script behavior or show error messages even when no error is present.