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Shell Startup Process

Startup Methods

The startup methods of Shell will affect the loading process of environment variables, which can be divided into the following categories.

  • Interactive (Interactive)
    • Login: Shell instances started in login mode, such as SSH login.
    • Non-login:
      • Creating a new Shell instance by typing Bash in the Shell.
      • Opening a new terminal session in GNOME Terminal.
  • Non-Interactive (Non-Interactive)
    • Shell instances started in scripts.

Interactive Shell

What is an Interactive Shell

Official Documentation

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

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

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

Interactive Login

In Ubuntu, the file loading process is roughly as follows when using SSH to login.

  1. /etc/profile: Entry File - A, executed by all users.
  2. /etc/bash.bashrc: Global environment configuration file, A loads this file.
  3. /etc/profile.d: Global environment configuration directory, A loads all files in this directory.
  4. ~/.profile: User environment configuration entry file - B.
  5. ~/.bashrc: User environment configuration file, B loads this file.

rc(Run Commands), originating from Unix tradition.

/etc/profile

system-wide: System-wide configuration file.

# /etc/profile: system-wide .profile file for the Bourne shell (sh(1))
# and Bourne compatible shells (bash(1), ksh(1), ash(1), ...).

# Check if the prompt variable PS1 is set
if [ "${PS1-}" ]; then
  # Check if the BASH variable is set and its value is not /bin/sh
  if [ "${BASH-}" ] && [ "$BASH" != "/bin/sh" ]; then
    # The file bash.bashrc already sets the default PS1.
    # PS1='\h:\w\$ '
    # If the bash.bashrc file exists, load it
    if [ -f /etc/bash.bashrc ]; then
      . /etc/bash.bashrc
    fi
  else
    # Is the current user's id equal to 0 (root)
    if [ "$(id -u)" -eq 0 ]; then
      # Set the prompt for root to #
      PS1='# '
    else
      # Set the prompt for ordinary users to $
      PS1='$ '
    fi
  fi
fi

# Check if the profile.d directory exists
if [ -d /etc/profile.d ]; then
  # Iterate through all sh files in the directory
  for i in /etc/profile.d/*.sh; do
    # If the file is readable, load it
    if [ -r $i ]; then
      . $i
    fi
  done
  unset i
fi

/etc/bash.bashrc

The file content is a bit long, just look at a few lines of comments.

# System-wide .bashrc file for interactive bash(1) shells.

# To enable the settings / commands in this file for login shells as well,
# this file has to be sourced in /etc/profile.

# If not running interactively, don't do anything
[ -z "$PS1" ] && return

If not interactive (PS1 variable not set), exit directly.

/etc/profile.d

Just take a look at the files in the directory.

ls -l /etc/profile.d
total 24
-rw-r--r-- 1 root root   96 Oct 15  2021 01-locale-fix.sh
-rw-r--r-- 1 root root  835 Apr  8  2022 apps-bin-path.sh
-rw-r--r-- 1 root root  726 Nov 16  2021 bash_completion.sh
-rw-r--r-- 1 root root 1107 Mar 23  2022 gawk.csh
-rw-r--r-- 1 root root  757 Mar 23  2022 gawk.sh
-rw-r--r-- 1 root root 1557 Feb 17  2020 Z97-byobu.sh

~/.profile

This file will load the ~/.bashrc file we commonly use.

# ~/.profile: executed by the command interpreter for login shells.
# This file is not read by bash(1), if ~/.bash_profile or ~/.bash_login
# exists.
# see /usr/share/doc/bash/examples/startup-files for examples.
# the files are located in the bash-doc package.

# the default umask is set in /etc/profile; for setting the umask
# for ssh logins, install and configure the libpam-umask package.
#umask 022

# if running bash
if [ -n "$BASH_VERSION" ]; then
    # include .bashrc if it exists
    if [ -f "$HOME/.bashrc" ]; then
      . "$HOME/.bashrc"
    fi
fi

# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/bin" ] ; then
    PATH="$HOME/bin:$PATH"
fi

# set PATH so it includes user's private bin if it exists
if [ -d "$HOME/.local/bin" ] ; then
    PATH="$HOME/.local/bin:$PATH"
fi

Looking at the header comments, you can know that if ~/.bash_profile and ~/.bash_login exist, ~/.profile will not be loaded, and the file loading order is as follows.

bash_profile > bash_login > profile

~/.bashrc

Let’s analyze the header just below.

# ~/.bashrc: executed by bash(1) for non-login shells.
# see /usr/share/doc/bash/examples/startup-files (in the package bash-doc)
# for examples

# If not running interactively, don't do anything
case $- in
    *i*) ;;
      *) return;;
esac

Looking at the first line of comments, this file is executed by non-login Shell. As mentioned earlier, ~/.profile also loads this file, which means both login and non-login Shell use this file.

Let’s analyze these magical codes (it’s really a genius design🤪):

# If not running interactively, don't do anything
case $- in
    *i*) ;;
      *) return;;
esac

$- is a special variable that represents the options enabled when the Shell runs, which can be printed out.

echo $-
himBHs
  • *i* is a matching condition: whether $- contains i.
  • i option indicates that the current Shell is interactive.
  • If the match is successful, it will exit the case.
  • If the match fails, it will execute return, exiting the script.

In summary: if not running interactively, exit directly, do nothing.

Interactive Non-Login

Here we discuss non-login cases. For example:

  • Creating a new Shell instance by typing Bash in the Shell.
  • Subshell generated by executing command groups using ().
  • Opening a new terminal session in GNOME Terminal.

One sentence summary: does not load /etc/profile, only loads ~/.bashrc.

Non-Interactive

This method does not have a command line prompt and does not load any configuration files, even if you manually load ~/.bashrc, it will not take effect. Because the analysis above mentions that in non-interactive mode, it will directly exit, do nothing.

# If not running interactively, don't do anything
case $- in
    *i*) ;;
      *) return;;
esac

Of course, you can use the BASH_ENV variable to set the file to load, as explained below.

If this variable is set when Bash is invoked to execute a shell script, its value is expanded and used as the name of a startup file to read before executing the script. See Bash Startup Files.

Persistence of Env Vars

You only need to write the environment variables in the corresponding files, such as ~/.bashrc.

echo 'export MY_VARIABLE="my_value"' >> ~/.bashrc
source ~/.bashrc
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