19.2 Scripts

Racket files can be turned into executable scripts under Unix and Mac OS X. Under Windows, a compatibility layer like Cygwin support the same kind of scripts, or scripts can be implemented as batch files.

19.2.1 Unix Scripts

In a Unix environment (including Linux and Mac OS X), a Racket file can be turned into an executable script using the shell’s #! convention. The first two characters of the file must be #!; the next character must be either a space or /, and the remainder of the first line must be a command to execute the script. For some platforms, the total length of the first line is restricted to 32 characters, and sometimes the space is required.

The simplest script format uses an absolute path to a racket executable followed by a module declaration. For example, if racket is installed in "/usr/local/bin", then a file containing the following text acts as a “hello world” script:

#! /usr/local/bin/racket

#lang racket/base

"Hello, world!"

In particular, if the above is put into a file "hello" and the file is made executable (e.g., with chmod a+x hello), then typing ./hello at the shell prompt produces the output "Hello, world!".

The above script works because the operating system automatically puts the path to the script as the argument to the program started by the #! line, and because racket treats a single non-flag argument as a file containing a module to run.

Instead of specifying a complete path to the racket executable, a popular alternative is to require that racket is in the user’s command path, and then “trampoline” using /usr/bin/env:

#! /usr/bin/env racket

#lang racket/base

"Hello, world!"

In either case, command-line arguments to a script are available via current-command-line-arguments:

#! /usr/bin/env racket

#lang racket/base

(printf "Given arguments: ~s\n"

(current-command-line-arguments))

If the name of the script is needed, it is available via (find-system-path 'run-file), instead of via (current-command-line-arguments).

Usually, then best way to handle command-line arguments is to parse them using the command-line form provided by racket. The command-line form extracts command-line arguments from (current-command-line-arguments) by default:

#! /usr/bin/env racket

#lang racket

(define verbose? (make-parameter #f))

(define greeting

(command-line

#:once-each

[("-v") "Verbose mode" (verbose? #t)]

#:args

(str) str))

(printf "~a~a\n"

greeting

(if (verbose?) " to you, too!" ""))

Try running the above script with the --help flag to see what command-line arguments are allowed by the script.

An even more general trampoline uses /bin/sh plus some lines that are comments in one language and expressions in the other. This trampoline is more complicated, but it provides more control over command-line arguments to racket:

#! /bin/sh

#|

exec racket -cu "$0" ${1+"$@"}

|#

#lang racket/base

(printf "This script started slowly, because the use of\n")

(printf "bytecode files has been disabled via -c.\n")

(printf "Given arguments: ~s\n"

(current-command-line-arguments))

Note that #! starts a line comment in Racket, and #|...|# forms a block comment. Meanwhile, # also starts a shell-script comment, while exec racket aborts the shell script to start racket. That way, the script file turns out to be valid input to both /bin/sh and racket.

19.2.2 Windows Batch Files

A similar trick can be used to write Racket code in Windows .bat batch files:

; @echo off

; Racket.exe "%~f0" %*

; exit /b

#lang racket/base

"Hello, world!"