Interactive Shells

Interactive Shells#

interactive shell

shell, interactive

What is an Interactive Shell?#

An interactive shell is one started without non-option arguments (unless -s is specified) and without specifying the -c option, 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 starts.

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

Interactive Shell Behavior#

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

1. Bash reads and executes startup files as described in Bash Startup Files.

2. Job Control (see 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 executes the values of the set elements of the PROMPT_COMMAND array variable as commands before printing the primary prompt, $PS1 (see Bash Variables).

4. 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. Bash expands and displays PS0 after it reads a command but before executing it. See Controlling the Prompt, for a complete list of prompt string escape sequences.

5. Bash uses Readline (see Command Line Editing) 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 The Set Builtin).

7. Bash enables Command history (see Bash History Facilities) and history expansion (see History Interaction) by default. When a shell with history enabled exits, Bash saves the command history to the file named by $HISTFILE.

8. Alias expansion (see Aliases) is performed by default.

9. In the absence of any traps, Bash ignores SIGTERM (see Signals).

10. In the absence of any traps, SIGINT is caught and handled (see Signals). SIGINT will interrupt some shell builtins.

11. An interactive login shell sends a SIGHUP to all jobs on exit if the huponexit shell option has been enabled (see Signals).

12. The -n option has no effect, whether at invocation or when using ‘set -n’ (see The Set Builtin).

13. Bash will check for mail periodically, depending on the values of the MAIL, MAILPATH, and MAILCHECK shell variables (see Bash Variables).

14. The shell will not exit on expansion errors due to references to unbound shell variables after ‘set -u’ has been enabled (see The Set Builtin).

15. The shell will not exit on expansion errors caused by \<var> being unset or null in ${var:?word} expansions (see 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 Bash POSIX Mode).

18. A failed exec will not cause the shell to exit (see Bourne Shell Builtins).

19. Parser syntax errors will not cause the shell to exit.

20. If the cdspell shell option is enabled, the shell will attempt simple spelling correction for directory arguments to the cd builtin (see the description of the cdspell option to the shopt builtin in The Shopt Builtin). The cdspell option is only effective in interactive shells.

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 Bash Variables).