A shell is the program that reads the text you type in a terminal window. There are a variety of shells used on the computer systems that support CIAO, the most popular being csh, tcsh, sh (the Bourne shell), ksh (the Korn shell), and bash (the GNU version of the Bourne shell). The shell version you use is reported by the echo $0 command:

unix% echo $0
tcsh

Here we see that we use the tcsh shell.

The syntax for using CIAO tools is the same in all the shells. However, the syntax of the scripting language does change between different shells; for instance, in the Running CIAO section of the Starting CIAO thread, it is necessary to know which shell you are using.