GNU Info

(bashref.info)Shell Scripts

---

Prev: Executing Commands Up: Basic Shell Features

---

Enter node , (file) or (file)node

Shell Scripts
=============

   A shell script is a text file containing shell commands.  When such
a file is used as the first non-option argument when invoking Bash, and
neither the `-c' nor `-s' option is supplied (Note: Invoking Bash),
Bash reads and executes commands from the file, then exits.  This mode
of operation creates a non-interactive shell.  The shell first searches
for the file in the current directory, and looks in the directories in
`$PATH' if not found there.

   When Bash runs a shell script, it sets the special parameter `0' to
the name of the file, rather than the name of the shell, and the
positional parameters are set to the remaining arguments, if any are
given.  If no additional arguments are supplied, the positional
parameters are unset.

   A shell script may be made executable by using the `chmod' command
to turn on the execute bit.  When Bash finds such a file while
searching the `$PATH' for a command, it spawns a subshell to execute
it.  In other words, executing
     filename ARGUMENTS

is equivalent to executing
     bash filename ARGUMENTS

if `filename' is an executable shell script.  This subshell
reinitializes itself, so that the effect is as if a new shell had been
invoked to interpret the script, with the exception that the locations
of commands remembered by the parent (see the description of `hash' in
Note: Bourne Shell Builtins) are retained by the child.

   Most versions of Unix make this a part of the operating system's
command execution mechanism.  If the first line of a script begins with
the two characters `#!', the remainder of the line specifies an
interpreter for the program.  Thus, you can specify Bash, `awk', Perl,
or some other interpreter and write the rest of the script file in that
language.

   The arguments to the interpreter consist of a single optional
argument following the interpreter name on the first line of the script
file, followed by the name of the script file, followed by the rest of
the arguments.  Bash will perform this action on operating systems that
do not handle it themselves.  Note that some older versions of Unix
limit the interpreter name and argument to a maximum of 32 characters.

   Bash scripts often begin with `#! /bin/bash' (assuming that Bash has
been installed in `/bin'), since this ensures that Bash will be used to
interpret the script, even if it is executed under another shell.