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(25) The login shell

The login shell

When you log in to a UNIX machine with a command line session, or open a terminal window in a graphical session, you get a shell. A shell takes commands that you type in decides what to do with them.
There are many different shells available. Csh(C-shell), sh (Bourne shell), tcsh, kcsh, bash (bourne again shell) --- you can even write your own shell.
Your login files (.cshrc .profile) set up some defaults when you log in. These defaults create an environment for you. This environment includes setting your search path so that common commands will work, setting the man page search path, setting the default printer, terminal type and more. You can customize these log in files for your environment.
Shells have some nice features that every user can take advantage of: history, aliases etc.
The shell is also a programming language. The simplest of shell scripts is just a file containing commands as typed.
Some commands are interpreted directly by the shell (internal commands), while for most of them, the shell will create a new process. All the internal commands are specific to the particular shell you are using.
The login "shell" may be set to a custom application, not a general purpose shell (e.g. Webster, or the "newuser" login on some systems)
Unattended background operations, e.g. cron or at jobs still have a shell process to interpret the commands, even though there is no user interface.

slide26.src last modified Jan 31, 2011

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