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# Interactive Shell Behavior (Bash Reference Manual)

## 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](/code-gists/bash/bash-startup-files-bash-reference-manual.md#Bash-Startup-Files).
2.  Job Control (see [Job Control](/code-gists/bash/job-control-bash-reference-manual.md#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. Bash expands and displays `PS0` after it reads a command but before executing it. See [Controlling the Prompt](/code-gists/bash/controlling-the-prompt-bash-reference-manual.md#Controlling-the-Prompt), for a complete list of prompt string escape sequences.
4.  Bash executes the value of the `PROMPT_COMMAND` variable as a command before printing the primary prompt, `$PS1` (see [Bash Variables](/code-gists/bash/bash-variables-bash-reference-manual.md#Bash-Variables)).
5.  Readline (see [Command Line Editing](/code-gists/bash/command-line-editing-bash-reference-manual.md#Command-Line-Editing)) is used to read commands from the user’s terminal.
6.  Bash inspects the value of the `ignoreeof` option to `set -o` instead of exiting immediately when it receives an `EOF` on its standard input when reading a command (see [The Set Builtin](/code-gists/bash/the-set-builtin-bash-reference-manual.md#The-Set-Builtin)).
7.  Command history (see [Bash History Facilities](/code-gists/bash/bash-history-facilities-bash-reference-manual.md#Bash-History-Facilities)) and history expansion (see [History Interaction](/code-gists/bash/history-interaction-bash-reference-manual.md#History-Interaction)) are enabled by default. Bash will save the command history to the file named by `$HISTFILE` when a shell with history enabled exits.
8.  Alias expansion (see [Aliases](/code-gists/bash/aliases-bash-reference-manual.md#Aliases)) is performed by default.
9.  In the absence of any traps, Bash ignores `SIGTERM` (see [Signals](/code-gists/bash/signals-bash-reference-manual.md#Signals)).
10.  In the absence of any traps, `SIGINT` is caught and handled (see [Signals](/code-gists/bash/signals-bash-reference-manual.md#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](/code-gists/bash/signals-bash-reference-manual.md#Signals)).
12.  The -n invocation option is ignored, and ‘set -n’ has no effect (see [The Set Builtin](/code-gists/bash/the-set-builtin-bash-reference-manual.md#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](/code-gists/bash/bash-variables-bash-reference-manual.md#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 [The Set Builtin](/code-gists/bash/the-set-builtin-bash-reference-manual.md#The-Set-Builtin)).
15.  The shell will not exit on expansion errors caused by var being unset or null in `${var:?word}` expansions (see [Shell Parameter Expansion](/code-gists/bash/shell-parameter-expansion-bash-reference-manual.md#Shell-Parameter-Expansion)).
16.  Redirection errors encountered by shell builtins will not cause the shell to exit.
17.  When running in POSIX mode, a special builtin returning an error status will not cause the shell to exit (see [Bash POSIX Mode](/code-gists/bash/bash-posix-mode-bash-reference-manual.md#Bash-POSIX-Mode)).
18.  A failed `exec` will not cause the shell to exit (see [Bourne Shell Builtins](/code-gists/bash/bourne-shell-builtins-bash-reference-manual.md#Bourne-Shell-Builtins)).
19.  Parser syntax errors will not cause the shell to exit.
20.  Simple spelling correction for directory arguments to the `cd` builtin is enabled by default (see the description of the `cdspell` option to the `shopt` builtin in [The Shopt Builtin](/code-gists/bash/the-shopt-builtin-bash-reference-manual.md#The-Shopt-Builtin)).
21.  The shell will check the value of the `TMOUT` variable and exit if a command is not read within the specified number of seconds after printing `$PS1` (see [Bash Variables](/code-gists/bash/bash-variables-bash-reference-manual.md#Bash-Variables)).