In UNIX and Linux, a shell is the basic program that is used in the command line. Some believe the name is a metaphor for a snail - the shell wraps around the kernel. Like most things in the Linux world, there are many different variations and versions, often building off each other and fixing some weakness of a previous shell. Examples include ZSH, FISH, and many others.

** Note: Technically, shells can exist in graphical form too - GNOME and KDE being popular examples. These are rarely (if ever) used on servers.

In Enterprise Linux, only a few command-line shells are in widespread use:

  1. BASH (Borne-Again Shell)

    • This is far-and-away the most commonly used shell in Enterprise Linux. All Linux distributions have BASH available, even if it is not the default. Not all UNIX distributions include BASH.
    • Standard file location: /bin/bash or (/usr/bin/bash)
    • #!/bin/bash is usually included at the beginning of each shell script so the kernel knows which interpreter to use

  2. KSH (Korn Shell)

    • Popular among the remaining UNIX distributions such as AIX.
    • Lacks many features that are standard in BASH, most notably tab-completion and “up-arrow” history

  3. SH (Shell)

    • The original shell interface that was used in UNIX
    • Still exists in modern Linux distrubutions
    • Standard file location: /bin/sh (or /usr/bin/sh)