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Some commands that you type are internal, which means they are built into the shell, and it's the shell that performs the action. For example, the cd command is built-in. The ls command, on the other hand, is an external program stored in the file /bin/ls.
The shell doesn't start a separate process to run internal commands. External commands require the shell to fork and exec (Section 27.2) a new subprocess (Section 24.3); this takes some time, especially on a busy system.
When you type the name of a command, the shell first checks to see if it is a built-in command and, if so, executes it. If the command name is an absolute pathname (Section 1.16) beginning with /, like /bin/ls, there is no problem: the command is likewise executed. If the command is neither built-in nor specified with an absolute pathname, most shells (except the original Bourne shell) will check for aliases (Section 29.2) or shell functions (Section 29.11), which may have been defined by the user -- often in a shell setup file (Section 3.3) that was read when the shell started. Most shells also "remember" the location of external commands (Section 27.6); this saves a long hunt down the search path. Finally, all shells look in the search path for an executable program or script with the given name.
The search path is exactly what its name implies: a list of directories that the shell should look through for a command whose name matches what is typed.
The search path isn't built into the shell; it's something you specify in your shell setup files.
By tradition, Unix system programs are kept in directories called /bin and /usr/bin, with additional programs usually used only by system administrators in either /etc and /usr/etc or /sbin and /usr/sbin. Many versions of Unix also have programs stored in /usr/ucb (named after the University of California at Berkeley, where many Unix programs were written). There may be other directories containing programs. For example, the programs that make up the X Window System (Section 1.22) are stored in /usr/bin/X11. Users or sites often also have their own directories where custom commands and scripts are kept, such as /usr/local/bin or /opt.
The search path is stored in an environment variable (Section 35.3) called PATH (Section 35.6). A typical PATH setting might look something like this:
PATH=/bin:/usr/bin:/usr/bin/X11:/usr/ucb:/home/tim/bin:
The path is searched in order, so if there are two commands with the same name, the one that is found first in the path will be executed. For example, your system certainly has the ls command we mentioned earlier -- and it's probably in /bin/ls.
You can add new directories to your search path on the fly, but the path is usually set in shell setup files.
-- TOR
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| 1.8. Anyone Can Program the Shell |  | 1.10. The Kernel and Daemons |
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