When a Scheme program file is given to
user module as the current module,
binds a global variable
*argv* to the list of the remaining
command-line arguments, and then loads the Scheme program.
If the first line of scheme-file begins with two character
#!”, the entire line is ignored by
This is useful to write a Scheme program that works as an executable
script in unix-like systems.
Typical Gauche script has the first line like these
#!/usr/local/bin/gosh or, #!/usr/bin/env gosh or, #!/bin/sh :; exec gosh -- $0 "$@"
The second and third form uses a “shell trampoline” technique
so that the script works as far as
gosh is in the PATH.
The third form is useful when you want to pass extra arguments
gosh, for typically
#!-magic of executable scripts
has limitations for the number of arguments to pass the interpreter.
After the file is successfully loaded,
gosh calls a
procedure named ‘
main’ if it is defined in the user module.
Main receives a single argument, a list of command line
arguments. Its first element is the script name itself.
main returns, and its value is an integer,
uses it for exit code of the program.
gosh exits with exit code 70 (
This behavior is compatible with the SRFI-22.
main procedure is not defined,
after loading the script file.
Although you can still write the program
main body as toplevel expressions, like shell scripts or Perl scripts,
it is much convenient to use this ‘
main’ convention, for
you can load the script file interactively to debug.
-m command-line option, you can make
main procedure defined in a module other than the
module. It is sometimes handy to write a Scheme module that can
also be executed as a script.
For example, you write a Scheme module
and within it, you define the
You don’t need to export it. If the file is loaded as a module,
main procedure doesn’t do anything. But if you
-m foo option and give the file as a Scheme script
main procedure is invoked after loading the script.
You can code tests or small example application in such an
alternate main procedure.
Note on R7RS Scripts: If the script is written in R7RS Scheme
(which can be distinguished by the first
see Three forms of import), it is read into
module and its
main isn’t called.
You can give
-mr7rs.main command-line argument to call
main function in R7RS script.
Alternatively, as specified in SRFI-22, if the script interpreter’s
basename is scheme-r7rs, we assume the script is R7RS SRFI-22 script
r7rs.user module rather than
module. We don’t install such an alias, but you can manually
make symbolic link or just copy gosh binary as scheme-r7rs.
Although the argument of the
main procedure is the standard way
to receive the command-line arguments, there are a couple of other
ways to access to the info. See Command-line arguments, for the
Now I show several simple examples below.
First, this script works like
cat(1), without any command-line
option processing and error handling.
#!/usr/bin/env gosh (define (main args) ;entry point (if (null? (cdr args)) (copy-port (current-input-port) (current-output-port)) (for-each (lambda (file) (call-with-input-file file (lambda (in) (copy-port in (current-output-port))))) (cdr args))) 0)
The following script is a simple grep command.
#!/usr/bin/env gosh (define (usage program-name) (format (current-error-port) "Usage: ~a regexp file ...\n" program-name) (exit 2)) (define (grep rx port) (with-input-from-port port (lambda () (port-for-each (lambda (line) (when (rxmatch rx line) (format #t "~a:~a: ~a\n" (port-name port) (- (port-current-line port) 1) line))) read-line)))) (define (main args) (if (null? (cdr args)) (usage (car args)) (let ((rx (string->regexp (cadr args)))) (if (null? (cddr args)) (grep rx (current-input-port)) (for-each (lambda (f) (call-with-input-file f (lambda (p) (grep rx p)))) (cddr args))))) 0)
See also Parsing command-line options, for a convenient way to parse command-line options.