shell-script headers (#!/bin/sh vs #!/bin/csh)

Question

Why do all script files start with

#!/bin/sh

or with

#!/bin/csh

Is that required? What\'s the purpose o

Answer 1

This defines what shell (command interpreter) you are using for interpreting/running your script. Each shell is slightly different in the way it interacts with the user and executes scripts (programs).

When you type in a command at the Unix prompt, you are interacting with the shell.

E.g., #!/bin/csh refers to the C-shell, /bin/tcsh the t-shell, /bin/bash the bash shell, etc.

You can tell which interactive shell you are using the

 echo $SHELL

command, or alternatively

 env | grep -i shell

You can change your command shell with the chsh command.

Each has a slightly different command set and way of assigning variables and its own set of programming constructs. For instance the if-else statement with bash looks different that the one in the C-shell.

This page might be of interest as it "translates" between bash and tcsh commands/syntax.

Using the directive in the shell script allows you to run programs using a different shell. For instance I use the tcsh shell interactively, but often run bash scripts using /bin/bash in the script file.

Aside:

This concept extends to other scripts too. For instance if you program in Python you'd put

 #!/usr/bin/python

at the top of your Python program