WYNTK: UNIX System Admininistrator

When there are two versions of a program, the one executed is the one earliest in your search path.

On our system, there are two versions of the ls command: /usr/bin/ls and /usr/5bin/ls. The one that is executed when a user types ls depends on the order that /usr/5bin and /usr/bin are listed in the user’s search path.

For example, if a search path reads:

/bin:/usr/bin:/usr/ucb:/usr/5bin:/usr/local/bin

Then the version of ls that is installed in /usr/bin is the one that is encountered first and is therefore the one that is executed. If you wanted to use the other version of ls, one way to do it is to edit your search path to put /usr/5bin earlier in your path:

/bin:/usr/5bin:/usr/bin:/usr/ucb:/usr/local/bin

Beware, however, that this means you also get other 5bin versions of commands, which you may not want. A better way of using your preferred ls is to define an alias .

The shell, configuration files, and environment variables

The program that gives you your UNIX prompt and processes the commands you type there is called a shell. There are many different shells that run on UNIX systems, but the two basic families are the Bourne shell and the C shell . The shell is technically just another program on the system— for example, the Korn shell is /bin/ksh, and the C shell is /bin/csh.

When you are given a login account, you are also assigned a shell. The shell you are given determines what commands you can run, what features you can use with the commands (such as aliasing, filename completion, ...