Interactive and login shells in Bash

Interactive and login shells in Bash

You might ever get confused because some bash config scripts are not executed, but they are sometimes executed multiple times.

This post will reveal the reason for these phenomena.

The basic definitions

When being invoked with none of these options: --noprofile, --norc, --rcfile, --init-file, a bash session defines the startup scripts to be loaded and their orders based on two boolean properties: interactivity (interactive/non-interactive) and login type (login/non-login).

From bash's man page

Interactive shell is a shell whose input, error, output are connected to terminals (TTYAn interactive shell is one started without non-option arguments (unless -s is specified) and without the -c option whose standard input and error are both connected to terminals (as determined by isatty(3)), or one started with the -i option.  PS1 is set and $-       includes i if bash is interactive, allowing a shell script or a startup file to test this state.).
-s     If the -s option is present, or if no arguments remain after option processing, then commands are read from the standard input.  This option allows the positional parameters to be set when invoking an interactive shell or when reading input through a pipe.
-c     If the -c option is present, then commands are read from the first non-option argument command_string.  If there are arguments after the command_string, the first argument is assigned to $0 and any remaining arguments are assigned to the positional parameters.  The assignment to $0 sets the name of the shell, which is used in warning and error

Typically, an interactive shell reads from/writes to a terminal. The -i option is used to set interactive mode explicitly. On the other hand, [[ -z "$PS1" ]] is a handy condition to check if a shell is in interactive mode. Likewise, this can be checked via whether $- includes i character, i.e., [[ $- == *i* ]].

ebash-5.0$ echo $PS1
\s-\v\$
bash-5.0$ echo $-
himBHs
Example of $PS1, $- values

Login shell: when being invoked with the --login (or -l) option, the shell becomes a login shell.

From bash's man page

A login shell is one whose first character of argument zero is a -, or one started with the --login option.

Startup scripts and their execution orders

If "a shell is a login shell" and "the option --noprofile is not used when the shell is invoked", the startup scripts' execution orders become:

  • /etc/profile (if exist)
  • ~/.bash_profile, ~/.bash_login, ~/.profile: only the first one that exists and is readable is used. If none of them is executed, no error occurs.

In addition, when a login shell exits, it executes ~/.bash_logout (if exists).

If "a shell is invoked in interactive mode"  and "the --norc option is not used", ~/.bashrc is read and executed if it exists. The script path can be specified with the --rcfile alternative_file.sh option.

If a shell is invoked in non-interactive mode (running a command), the BASH_ENV environment variable is expanded. If there is a value that matches an existing readable file, that file is executed.

When a graphical session (gnome, unity, kdm, kdm, ..) starts, their behaviors highly depend on the graphical programs, which are mostly configurable. There is no consistent rule for all graphical programs. Nonetheless, with the same program, they vary from version to version. Typically, ~/.profile is read, but you should not rely on this behavior.

Some Linux distributions have a default ~/.profile which sources ~/.bashrc, which explains why ~/.bashrc gets run twice. Besides, you should also check /etc/bashrc, /etc/bash.bashrc, /etc/profile. It is worth noting that it is common in many distributions that all scripts in /etc/profile.d/ directory are loaded.

Full bash's man page

Link

INVOCATION         top
A login shell is one whose first character of argument zero is a -, or one started with the --login option.

An interactive shell is one started without non-option arguments (unless -s is specified) and without the -c option whose standard input and error are both connected to terminals (as determined by isatty(3)), or one started with the -i option.  PS1 is set and $-       includes i if bash is interactive, allowing a shell script or a startup file to test this state.

The following paragraphs describe how bash executes its startup files.  If any of the files exist but cannot be read, bash       reports an error.  Tildes are expanded in filenames as described below under Tilde Expansion in the EXPANSION section.

When bash is invoked as an interactive login shell, or as a non- interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists.  After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.

When an interactive login shell exits, or a non-interactive login shell executes the exit builtin command, bash reads and executes commands from the file ~/.bash_logout, if it exists.

When an interactive shell that is not a login shell is started, bash reads and executes commands from ~/.bashrc, if that file exists.  This may be inhibited by using the --norc option.  The --rcfile file option will force bash to read and execute commands from file instead of ~/.bashrc.

When bash is started non-interactively, to run a shell script, for example, it looks for the variable BASH_ENV in the environment, expands its value if it appears there, and uses the expanded value as the name of a file to read and execute.  Bash       behaves as if the following command were executed: if [ -n "$BASH_ENV" ]; then . "$BASH_ENV"; fi but the value of the PATH variable is not used to search for the filename.

If bash is invoked with the name sh, it tries to mimic the startup behavior of historical versions of sh as closely as possible, while conforming to the POSIX standard as well.  When invoked as an interactive login shell, or a non-interactive shell with the --login option, it first attempts to read and execute commands from /etc/profile and ~/.profile, in that order.  The --noprofile option may be used to inhibit this behavior.  When invoked as an interactive shell with the name sh, bash looks for the variable ENV, expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as sh does not attempt to read and execute commands from any other startup files, the --rcfile option has no effect.  A non-interactive shell invoked with the name sh does not attempt to read any other startup files.  When invoked as sh, bash enters posix mode after the startup files are read. When bash is started in posix mode, as with the --posix command line option, it follows the POSIX standard for startup files.  In this mode, interactive shells expand the ENV variable and commands are read and executed from the file whose name is the expanded value.  No other startup files are read.

Bash attempts to determine when it is being run with its standard input connected to a network connection, as when executed by the remote shell daemon, usually rshd, or the secure shell daemon sshd.  If bash determines it is being run in this fashion, it reads and executes commands from ~/.bashrc, if that file exists and is readable.  It will not do this if invoked as sh.  The --norc option may be used to inhibit this behavior, and the --rcfile option may be used to force another file to be read, but neither rshd nor sshd generally invoke the shell with those options or allow them to be specified.

If the shell is started with the effective user (group) id not equal to the real user (group) id, and the -p option is not supplied, no startup files are read, shell functions are not inherited from the environment, the SHELLOPTS, BASHOPTS, CDPATH, and GLOBIGNORE variables, if they appear in the environment, are ignored, and the effective user id is set to the real user id. If the -p option is supplied at invocation, the startup behavior is the same, but the effective user id is not reset.