I installed Red Hat Enterprise Linux 5 on my PC. When i typed 'emacs' on the console, "command not found" error occurrd. Why.

From people.redhat.com

When someone comes into #redhat and has troubles, often users give that person specific commands to run, to help diagnose the problem. Often users run those programs but get "Command not found" messages so they assume they do not have said program installed. Most times however, that is not the case. The program is just not in the user's $PATH. For example, fuser or modprobe are located in "/sbin" directory. Traditionally /sbin is not in a user's PATH because they contain system binaries which users should not have to use. In order to run these programs, you have to do one of two things. 1) add "/sbin" to your path (not recommended) or 2) add the path to the command (commonly referred to as "absolute paths"). So instead of typing: fuser, type: /sbin/fuser.

Now the problem is solved by this raises another question, how did you know fuser was in /sbin? You may have heard of a command called which that will show you the full path of shell commands. But a more useful command to learn is called whereis. "whereis" will not only locate the binary, but it'll find its man pages and even source code. "whereis fuser" will return /sbin/fuser & fuser's man page location.