unix - What is the .bashrc file?

Unix shells when starting read the .bashrc file and execute commands written in it. What is this file and what does it execute?

Answer

Actually, it's bash specifically that reads .bashrc (and /etc/bash.bashrc). There are lots of different shells.

The bash man page (by Brian Fox and Chet Ramey; also info page "Bash Startup Files") is the authoritative reference:

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 file name.

The file is just shell commands. It is typically used to change prompts, set environment variables, and define shell procedures. Traditionally, the file .profile is used for this purpose, but bash has so many extensions that it needs its own startup file for users that want to put bashisms in startup files.

"Not a login shell" means things like script launches and usually terminal windows started by window managers. Sometimes I set up *nix systems to have .bashrc and BASH_ENV just source .profile. As long as you don't stray outside of POSIX shell commands then you will get the same initialization in any shell.

It's particularly valuable when sh is really bash, which sometimes happens. To do this use:

. .profile

One reason this is all so complex is because sometimes people put things that produce output into shell startup files, or they unconditionally set prompts. This causes lots of problems when running shell programs and backtick commands within languages, not to mention system(3) from C programs. The way bash starts up is designed, I think, to have one file where output and prompt setting is OK and one file where it isn't. Traditionally, a run-time test would be done to distinguish interactivity, for example, checking to see if the prompt is set.