Why is #! the first line in shell scripting ?

Table of content

• What is a shell ?
• Sequence of operation
• Major types of Shells

What is a Shell ?

• A Unix shell is both a command interpreter and a programming language.
  • As a command interpreter, the shell provides the user interface to the rich set of GNU utilities.
  • As a programming language it allows these utilities to be combined.
• Shells may be used interactively or non-interactively.
  • In interactive mode, they accept input typed from the keyboard.
  • In non-interactive mode, shells execute commands read from a file

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Consider a shell program without the first line and study it's execution by the shell.

$ cat > lists
ls
^D                                            Ctrl-D is end-of-file
$ chmod +x lists
$ ./lists

the ls command outputs the current directory content.

Sequence of Operation

• The shell asks the kernel to run the script .

• The program is not compiled rather than it is interpreted, so kernel returns a error not executable format file.

• After receiving the error the shell understands that the program is a script.

• Now the shell creates a child shell of /bin/sh to run the program.

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The use of /bin/shas default shell to run program is fine when only one shell is preset but the current Unix systems contains more than one shell .

So to invoke any particular shell we use #!.

To see the list of shells available in system:-

$ cat /etc/shells
# Pathnames of valid login shells.
# See shells(5) for details.

/bin/sh
/bin/bash
/bin/rbash
/usr/bin/git-shell
/usr/bin/fish
/bin/fish
/bin/zsh
/usr/bin/zsh

When the first line starts with #! the kernel scans the rest of the line for full pathname of the interpreter to run the program.

Example:-

$ cat > lists
#! /bin/fish
ls
^D                                Ctrl-D is end of file
$ chmod +x lists
$ ./lists

Caution

Carefully choose the pathname to prevent cross vendor portability since different vendors puts different things in different places

Example:-

/bin/fish vs /usr/bin/fish

Major types of Shells

• Bourne Shell (sh) :- The Bourne shell was the default shell for Version 7 Unix and Unix-like systems.

• C Shell (csh) :- The C shell was created to look more like the C programming language and that it should be better for interactive use. An improved version was tcsh.

• Bash Shell (bash) :- bash is an acronym for ‘Bourne-Again SHell’. It incorporates useful features from the Korn shell ksh and the C shell csh.

• Z Shell (zsh) :- Z Shell is an extended Bourne shell with many improvements, including some features of bash, ksh, and tcsh.