Configuring bash in Docker

Published: Aug 19, 2020 by Holger Hoefling

This is the first of a series of posts dealing with questions around using docker containers in data science.

How to use bash in docker using configuration files

Abstract

When using Docker for data science, a particular problem is to ensure that the correct environments are loaded at all times, e.g. conda.

In order to ensure that the environment is loaded correctly at all times, it has to be initalized when starting an interactive shell as well as when starting a program using a run command.

In the following we will show that this can best be achieved in 2 ways: a) Always start each interactive shell and each run command using a login shell. b) For interactive shells define a bashrc-file and set the environemnt variable $BASH_ENV to the same file for non-interactive shells (e.g. run commands).

Type of shells

Before we see all this in detail, we first review how the bash shell configuration works. For shells, there are two properties that each shell has. It is either a login or a non-login shell as well as an interactive or non-interactive shell.

Login shells

A login shell is typically started when logging into a computer. In it, certain startup scripts are sourced that can be used to set the initial values for environment varialbles, e.g. PATH. A new bash shell can be explicitly turned into a login shell by using the -l or –login options.

Interactive shells

An interactive shell is a shell that has its input, output and error streams connected to a terminal. This is typically the case when you start a shell inside another shell or when starting a shell in a docker container. The typical case of a non-interactive shell is a shell that is started in order to run a script. The option -i can be used to explicitly turn a shell into an interactive shell.

In addition to these option there are other switches that can be used to customize the behaviour which startup scripts get run and we will go over them and their effects later.

Configuration files

There are a number of different configuration files that are sourced in different situations. Here an overview of the ones relevant for bash:

  • /etc/profile: This system-wide script is sourced by login-shells at startup before any other files are sourced
  • /etc/profile.d: A system-wide directory from which additional scripts are sourced by login shells. While not formally listed in the GNU manual linked above, most distributions also read all scripts in this directory.
  • ~/.bash_profile, ~/.bash_login, ~/.profile: These are scripts for individual users that are read by login shells. Only the first of these scripts that exists and is readable is used. If the option –noprofile is used, none of these scripts is sourced.
  • /etc/bashrc or /etc/bash.bashrc: A system-wide script that is sourced by interactive shells. CentOS uses /etc/bashrc whereas Debian-based systems use /etc/bash.bashrc.
  • ~/.bashrc: This user-specific script is sourced for all interactive shells. If the option –norc is used, this file is not being sourced. If the option –rcfile file is being used, file is sourced instead of ~/.bashrc.
  • $BASH_ENV: If a non-interactive shell is started and the environment variable BASH_ENV exists, then the script file referenced in BASH_ENV will be sourced.

Behaviour for sh

When bash is invoked with the name sh, then its behviour changes.

  • login: This behviour occurs when sh is started with the –login option. It sources /etc/profile and ~/.profile in this order. The –noprofile prevents this (clarify if it prevents reading of both files or only one of them).
  • interactive: It looks for the environment variable ENV and sources the file referenced here. The option –rcfile has no effect.
  • non-interactive:: No startup files are being sourced.

POSIX mode:

When started in POSIX mode, only the file referenced in the variable ENV is sourced. No other files are sourced.

The docker setup

The rules for when which configuration files are executed for which shell can be quite challenging to remember - at least for users that don’t use this functionality every day. The setup becomes even more challenging when used together with Docker, where it is a priori less clear which type of shell is in use at which point.

In order to make this easier to remember we create a small docker container that shows which configuration files are run in which order under different conditions.

In the container, echo commands specifying the name of the file being sourced are either - added at the end of the configuration file - replace the configuration file with the echo command

The reason for these two different conditions is that by default some configuration files by default source other files and with this setup we want to highlight the exact connections between files.

Bash as an interactive shell

We can of course also regularly start bash as an interactive shell in the docker container usign the -it option for the docker-run command. We also specify to docker to log us in as userA.

In this case, /etc/bash.bashrc and /home/userA/.bashrc are being sourced (please note that it doesn’t occur as an explicit code-block here as the interactive part does not work in a Jupyter notebook).

docker run -it --user userA hhoeflin/shell_test:bash-append /bin/bash

## Source /etc/bash.bashrc
## Source /home/userA/.bashrc
## userA@55411b7d527f:/$ exit

Another option to start an interactive shell is the -i option to bash. This cannot be combined with the bash -c option to run a command passed as a string, but we can use it when executing a script. However this results in an error

docker run --user userA hhoeflin/shell_test:bash-append /bin/bash -i /home/userA/script.sh

## bash: cannot set terminal process group (-1): Inappropriate ioctl for device
## bash: no job control in this shell
## Source /etc/bash.bashrc
## Source /home/userA/.bashrc
## /home/userA

In this case, instead of /etc/bash.bashrc and /home/userA/.bashrc, we can force a specific file to be used with the –rcfile option. We can also ensure that for interactive shells, no configuration script is loaded with –norc.

The non-interactive bash shell

Instead of the interactive shell, we however usually when running a container are being dropped into a non-interactive, non-login shell.

docker run hhoeflin/shell_test:bash-append /bin/bash

As we can see, this sources no files at all. But when we now set the BASH_ENV variable to /etc/profile, we see that the script gets loaded - together with the script file in /etc/profile.d.

docker run -e BASH_ENV=/etc/profile hhoeflin/shell_test:bash-append /bin/bash

## Source /etc/profile.d/test.sh
## Source /etc/profile

Strictly speaking the script in /etc/profile.d should not have been loaded - at least we did not explicitly ask for it. The reason is that all script in /etc/profile.d get sourced by the default /etc/profile. We confirm this by running the same command, but this time using the version of the configuration scripts that got replaced with the echo commands - not appended to.

docker run -e BASH_ENV=/etc/profile hhoeflin/shell_test:bash-replace /bin/bash

## Source /etc/profile

Bash as a login shell

When running bash, we can ask for it to be a login shell using the -l or –login option.

docker run hhoeflin/shell_test:bash-append /bin/bash --login

## Source /etc/profile.d/test.sh
## Source /etc/profile

and all the profile-related scripts get sourced. When at the same time we pass the –noprofile option

docker run hhoeflin/shell_test:bash-append /bin/bash --login --noprofile

it prevents any profile scripts from being loaded.

Now this was all so far for the root user. We can also do this for any other user

docker run --user userA hhoeflin/shell_test:bash-append /bin/bash --login

## Source /etc/profile.d/test.sh
## Source /etc/profile
## Source /home/userA/.bash_profile

in which case ~/.bash_profile gets loaded, as this is the first user-specific configuration file. For userB and userC, we can see similar results, just with their user-specific config files. For userD that has all 3 profile-configuration files, ony the first, ~/.bash_profile gets loaded.

Summary

We have seen how to use login shell and interactive/non-interactive shells in a Docker container. If the goal is to have a specific script run in interactive shells and non-interactive shells executing a command, then there are basically 2 choices to make this happen.

The first choice is to make interactive shells as well as non-interactive shells both login shells and put the script that should be executed into either /etc/profile.d if it should be run for all users or into ~/.profile if it is intended for a specific user. This however requires to set the -l option on all shells that are run.

The second option is to set the script as /etc/bash.bashrc (or /etc/bashrc on CentOS). In this case, this will automatically be run for all interactive shells. For non-interactive shells, the BASH_ENV variable pointing to this file would ensure that it is sourced in this case as well.

Overall, requiring users to always request a login shell and not include any bashrc files is overall a bit more consistent in my opinion.

References

In order to compile this Dockerfile and writeup I used various sources on the internet. A good introduction to the subject is GNU - Bash Startup Files. Another very nice post is Bash interactive, login shell types.

I hope you enjoyed this post. Please let me know if there are any comments below.

docker shell bash

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