Linux startup scripts

When it comes to perform customization to startup scripts in order to initialize or tune something, it’s not obvious to pick the right startup file. The way the system is reading startup files depends on the context and mainly if the shell is

• Interactive / non-interactive shell
• Login / non-login shell

Some definitions

• Login: Means that the shell is run as part of the login of the user to the system. Typically used to do any configuration that a user needs / wants to establish his work-environment.
• Non-login: Any other shell run by the user after logging on, or which is run by any automated process which is not coupled to a logged in user.
• Interactive: As the term implies: Interactive means that the commands are run with user-interaction from keyboard. E.g. the shell can > prompt the user to enter input.
• Non-interactive: the shell is probably run from an automated process so it can’t assume if can request input or that someone will see the > output. E.g Maybe it is best to write output to a log-file. ¹

Setting up test image

The best way to figure out what happens is to perform a simple test. In this test I build a simple docker image exporting different environment variables each storing the current number of milliseconds since epoch.

• TEST_PROFILE is set by /etc/profile
• TEST_BASHRC is set by /etc/bash.bashrc

FROM ubuntu:focal

# number of ms since epoch
RUN echo 'export TEST_PROFILE="$(date +%s%3N)"' >> /etc/profile && \
    echo 'export TEST_BASHRC="$(date +%s%3N)"' >> /etc/bash.bashrc

CMD echo "PROFILE -> $TEST_PROFILE, BASHRC -> $TEST_BASHRC"

We can now build the startup image docker build --rm -t startup.

Login shell

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.²

Interactive

In this case both startup scripts are invoked, both variables are set.

$ docker run -it --rm startup bash -l

$ echo "PROFILE -> $TEST_PROFILE, BASHRC -> $TEST_BASHRC"

# PROFILE -> 1603348756283, BASHRC -> 1603348756277

Non-interactive

In this case only /etc/profile is invoked, only TEST_PROFILE is set. Login shell is obtained by using the -l (for login) option.

$ docker run --rm startup bash -lc 'echo "PROFILE -> $TEST_PROFILE, BASHRC -> $TEST_BASHRC"' 

# PROFILE -> 1603372796585, BASHRC -> 

Non-login shell

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.²

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.²

Interactive

In this case only /etc/bash.bashrc is invoked, only TEST_BASHRC is set.

$ docker run -it --rm startup bash

$ echo "PROFILE -> $TEST_PROFILE, BASHRC -> $TEST_BASHRC"

# PROFILE -> , BASHRC -> 1603348681395

Non-interactive

In this case nothing happen, both variables are empty.

$ docker run --rm startup

# PROFILE -> , BASHRC -> 

Wrap up

What we’ve learned in short.

• Login shell: /etc/profile is invoked
• Interactive shell: /etc/bash.bashrc is invoked

See also my previous article on a very close topic Skeletal profiles (/etc/skel).

References / Further reading

• What do you mean by interactive shell?
• Why is /etc/profile not invoked for non-login shells?
• What’s the difference between .bashrc and /etc/bash.bashrc?