6.3 Interactive Shells

6.3.1 What is an Interactive Shell?

An interactive shell is one started without non-option arguments, unless `-s' is specified, without specifiying the `-c' option, and whose input and error output are both connected to terminals (as determined by isatty(3)), or one started with the `-i' option.

An interactive shell generally reads from and writes to a user's terminal.

The `-s' invocation option may be used to set the positional parameters when an interactive shell is started.

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

6.3.3 Interactive Shell Behavior

When the shell is running interactively, it changes its behavior in several ways.

1. Startup files are read and executed as described in Bash Startup Files.
2. Job Control (see section Job Control) is enabled by default. When job control is in effect, Bash ignores the keyboard-generated job control signals SIGTTIN, SIGTTOU, and SIGTSTP.
3. Bash expands and displays PS1 before reading the first line of a command, and expands and displays PS2 before reading the second and subsequent lines of a multi-line command.
4. Bash executes the value of the PROMPT_COMMAND variable as a command before printing the primary prompt, $PS1 (see section Bash Variables).
5. Readline (@pxref{Command Line Editing}) is used to read commands from the user's terminal.
6. Bash inspects the value of the ignoreeof option to set -o instead of exiting immediately when it receives an EOF on its standard input when reading a command (see section The Set Builtin).
7. Command history (@pxref{Bash History Facilities}) and history expansion (@pxref{History Interaction}) are enabled by default. Bash will save the command history to the file named by $HISTFILE when an interactive shell exits.
8. Alias expansion (see section Aliases) is performed by default.
9. In the absence of any traps, Bash ignores SIGTERM (see section Signals).
10. In the absence of any traps, SIGINT is caught and handled ((see section Signals). SIGINT will interrupt some shell builtins.
11. An interactive login shell sends a SIGHUP to all jobs on exit if the hupoxexit shell option has been enabled (see section Signals).
12. The `-n' invocation option is ignored, and `set -n' has no effect (see section The Set Builtin).
13. Bash will check for mail periodically, depending on the values of the MAIL, MAILPATH, and MAILCHECK shell variables (see section Bash Variables).
14. Expansion errors due to references to unbound shell variables after `set -u' has been enabled will not cause the shell to exit (see section The Set Builtin).
15. The shell will not exit on expansion errors caused by var being unset or null in ${var:?word} expansions (see section Shell Parameter Expansion).
16. Redirection errors encountered by shell builtins will not cause the shell to exit.
17. When running in POSIX mode, a special builtin returning an error status will not cause the shell to exit (see section Bash POSIX Mode).
18. A failed exec will not cause the shell to exit (see section Bourne Shell Builtins).
19. Parser syntax errors will not cause the shell to exit.
20. Simple spelling correction for directory arguments to the cd builtin is enabled by default (see the description of the cdspell option to the shopt builtin in Bash Builtin Commands).
21. The shell will check the value of the TMOUT variable and exit if a command is not read within the specified number of seconds after printing $PS1 (see section Bash Variables).