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
Let us consider an interactive script to be one that requires input
from the user, usually with read statements (see Example
15-3). "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? ### if [ -v PS1 ] # On Bash 4.2+ ... then # non-interactive ... else # interactive ... fi
Alternatively, the script can test for the presence of option "i" in
the $- flag.
case $- in *i*) # interactive shell ;; *) # non-interactive shell ;; # (Courtesy of "UNIX F.A.Q.," 1993)
However, John Lange describes an alternative method, using the
-t test operator.
# Test for a terminal!
fd=0 # stdin
# As we recall, the -t test option checks whether the stdin, [ -t 0 ], #+ or stdout, [ -t 1 ], in a given script is running in a terminal. if [ -t "$fd" ] then echo interactive else echo non-interactive fi
# But, as John points out: # if [ -t 0 ] works ... when you're logged in locally # but fails when you invoke the command remotely via ssh. # So for a true test you also have to test for a socket.
if [[ -t "$fd" || -p /dev/stdin ]] then echo interactive else echo non-interactive fi
Note: 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.