What is the difference between an internal and an external command in Unix?
Some commands in Unix are internal, built into the shell. For example, the cd command is built-in. That is, the shell interprets the command and changes the current directory. On the other hand, cat command is an external program stored in the file /bin/cat. The shell doesn’t start a separate process to run internal commands. External commands require the shell to run a new subprocess; this takes some time, especially if the system is busy. Internal commands are functions that are built into the command interpreter, External commands are those not included in the interpreter, and are instead invoked by calling an external binary. Whether or not a particular command is internal or external varies by system. For example, echo is an internal command in MS-DOS (it is built into COMMAND.COM), 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 beginning with /, like /bin/ls, there
