Classic Shell Scripting

Self-Contained Scripts: The #! First Line

When the shell runs a program, it asks the Unix kernel to start a new process and run the given program in that process. The kernel knows how to do this for compiled programs. Our nusers shell script isn't a compiled program; when the shell asks the kernel to run it, the kernel will fail to do so, returning a "not executable format file" error. The shell, upon receiving this error, says "Aha, it's not a compiled program, it must be a shell script," and then proceeds to start a new copy of /bin/sh (the standard shell) to run the program.

The "fall back to /bin/sh" mechanism is great when there's only one shell. However, because current Unix systems have multiple shells, there needs to be a way to tell the Unix kernel which shell to use when running a particular shell script. In fact, it helps to have a general mechanism that makes it possible to directly invoke any programming language interpreter, not just a command shell. This is done via a special first line in the script file—one that begins with the two characters #!.

When the first two characters of a file are #!, the kernel scans the rest of the line for the full pathname of an interpreter to use to run the program. (Any intervening whitespace is skipped.) The kernel also scans for a single option to be passed to that interpreter. The kernel invokes the interpreter with the given option, along with the rest of the command line. For example, assume a csh script^[3] named /usr/ucb/whizprog ...