7.2.5.1. What are scripts?
A shell script is, as we saw in the shell configuration
examples, 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,
Bash reads and executes commands
from the file, then exits. This mode of operation creates a
non-interactive shell. 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 (everything following the
name of the script) 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 sub-shell to execute it. In other words, executing
filename ARGUMENTS
is equivalent to executing
bash filename ARGUMENTS
if "filename" is an executable shell
script. This sub-shell 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 hash in the Info pages) 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 or shell 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.
Bash scripts often begin
with
(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.
