Info Catalog bash: Executing Commands bash: Basic Shell Features

bash: Shell Scripts

 
 3.8 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 (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
 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.
 

Info Catalog bash: Executing Commands bash: Basic Shell Features