What is .bashrc file?

Question

Unix shells when starting read .bashrc file and execute commands contained in them. What is this file and what do they execute?

Answer 1

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

The man page is the authoritive reference:

   When an interactive shell that is not a login shell  is  started,  bash
   reads  and  executes  commands  from /etc/bash.bashrc and ~/.bashrc, if
   these files exist. 

   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 com‐
   mand 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.

Answer 2

When Bash starts, it executes the commands in a variety of different scripts.

When Bash is invoked as an interactive login shell, 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.

When a login shell exits, 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.

http://en.wikipedia.org/wiki/Bash

Here are some tricks and tips:

http://www.cyberciti.biz/tips/howto-linux-unix-bash-shell-setup-prompt.html

Let us try to set the prompt so that it can display today’d date and hostname:

PS1="\d \h $ "

Answer 3

It should contain various "initialization" commands for your shell, e.g.:

• Creating useful aliases (for example alias ll='ls -l').
• Adding more directories to PATH.
• Setting new environment variables.