Exec
1. When referring to a command line such as Linux or Unix, exec is a BOURNE and POSIX shell command that replaces the current shell process with the command specified after exec and does not create a new PID. For example, if you were to run exec <command>, the shell would be replaced by that command. When that command is exited the shell will exit.
Tip: If you're trying to execute a script or program use type ./ in front of the script or program, don't use exec.
2. When referring to a programming or scripting language such as C, Perl, or PHP, exec is a function that executes a program from within a program. For example, in Perl you could use the below line to print the files in the current directory on a Linux system.
exec "ls"
3. With a SSI, the exec command is used to execute a command on the server and display the output to the web page. For example, if we wanted to display the same output used in the above example on a webpage using SSI you could use a command similar to the below example.
<!--#exec cmd="ls"-->
Also see: Execute, Fork, PID, Programming definitions