How do you run a script on login in *nix?

At login, most shells execute a login script, which you can use to execute your custom script. The login script the shell executes depends, of course, upon the shell:

• bash: .bash_profile, .bash_login, .profile (for backwards compabitibility)
• sh: .profile
• tcsh and csh: .login
• zsh: .zshrc

You can probably find out what shell you're using by doing

echo $SHELL

from the prompt.

For a slightly wider definition of 'login', it's useful to know that on most distros when X is launched, your .xsessionrc will be executed when your X session is started.

From wikipedia Bash

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.