Multiline shebang

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Multiline shebang is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Simple shebangs can help with scripting, e.g. #!/usr/bin/env python at the top of a Python script will allow it to be run in a terminal as "./".

Occasionally, a more complex shebang line is needed. For example, some languages do not include the program name in ARGV; a multiline shebang can reorder the arguments so that the program name is included in ARGV.

The syntax for a multiline shebang is complicated. The shebang lines must be simultaneously commented away from the main language and revealed to some shell (perhaps Bash) so that they can be executed. In other words, Polyglots.

Warning: Using a multiline shebang of the form #!/bin/sh ... exec ... !# will set the code's mimetype to text/x-shellscript, which creates problems such as Emacs treating the file as a shell script, no matter which language and file extension it really uses.

See Also:

  • Native shebang - where the "program loaded" is of the actual native task language.


[edit] C

sed -n -e '7,$p' < "$0" | /usr/bin/gcc -x c -o "$0.$$.out" -
$0.$$.out "$0" "$@"
rm $0.$$.out
exit $STATUS
#include <stdio.h>

int main(int argc, char **argv)
  int i;
  for (i = 0; i < argc; i++)
    printf("argv[%d] -> %s\n", i, argv[i]);
  return 0;

Test runs:

$ ./cmulshbang.c
argv[0] -> ./cmulshbang.c.4062.out
argv[1] -> ./cmulshbang.c
$ ./cmulshbang.c 1
argv[0] -> ./cmulshbang.c.4071.out
argv[1] -> ./cmulshbang.c
argv[2] -> 1
$ ./cmulshbang.c 1 2
argv[0] -> ./cmulshbang.c.4080.out
argv[1] -> ./cmulshbang.c
argv[2] -> 1
argv[3] -> 2

Student exercise: use a stable filename for the executable, e.g. "$0.out". Do not remove it, and only recompile it if the script's timestamp is newer than that of the executable.

[edit] Clojure

The namespace = basename = filename minus the extension must be passed as a value to Clojure's -m flag.

":";exec clj -m `basename $0 .clj` $0 ${1+"$@"}

[edit] Common Lisp

Works with: CLISP

Here, the script name is passed once to CLISP and once to ext:*args*, which normally omits it.

exec clisp -q -q $0 $0 ${1+"$@"}

[edit] E

E uses only “#” for line comments, like the shell, so there is no straightforward answer. We can abuse the fact that “>” is also a line comment to achieve this effect. Note that a “>” line comment should ordinarily only occur as part of Updoc (test/documentation) text, so this is not good practice.

In this example, we are including the command name itself in the argument list, which would ordinarily not include it.

>/dev/null; exec rune $0 $0 ${1+"$@"}
println(`I was called as ${interp.getArgs()[0]}.`)

[edit] Emacs Lisp

:;exec emacs -batch -l $0 -f main $*

[edit] Erlang


#!/usr/bin/env escript
main(_) -> io:format("Hello World!~n", []).

This works fine when the module is run by itself with dot slash:

$ ./hello.erl 
Hello World!

But when another Erlang module tries to import the code, or you try to compile manually in erl, you get a syntax error.

$ erl
Erlang R14B03 (erts-5.8.4) [source] [64-bit] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]
Eshell V5.8.4 (abort with ^G)
1> c(hello).
./hello.erl:1: syntax error before: '#'
./hello.erl:4: no module definition

[edit] F#

#light (*
exec fsharpi --exec $0 --quiet
let main = printfn "Hello World"

[edit] Factor

Factor no longer requires a space after #! as of v0.95.

#!/usr/bin/env factor -script

[edit] Gforth

We can use Gforth's (non-ANS standard) support for shebangs and the '#' number prefix to make Gforth skip over the shebang without interfering with shell script interpretation.

#! /bin/sh
#0 [IF] \ lines below read by shell but ignored by Gforth
exec gforth \
-m 256M \
-d 16M \
"$0" "$@"
.( hello world) CR BYE

[edit] Go

sed -n -e '12,$p' < "$0" > ttmmpp.go
go build ttmmpp.go
rm ttmmpp.go
mv ttmmpp $binfile
$binfile "$@"
rm $binfile
exit $STATUS
######## Go Code start on line 12
package main
import (
func main() {
for i, x := range os.Args {
if i == 0 {
fmt.Printf("This program is named %s.\n", x)
} else {
fmt.Printf("the argument #%d is %s\n", i, x)

[edit] J

Assuming this task is asking for a mix of unix shell commands and J, and also that the J binary directory is listed in $PATH

# 0 :0
echo unix shell commands go here
echo presumably this will condition the environment
echo for example:
cd working-directory
echo or maybe you want to modify $PATH, ... whatever...
echo then start up J:
exec jconsole "$0" "$@"
9!:29]1[9!:27'2!:55]1' NB. exit on error
NB. and then the rest of the file is J
echo 'hi!'
echo 'your command line arguments were:'
echo ARGV
echo p:i. 3 4
exit 0


The #!/bin/sh line is interpreted by J as a verb train with no arguments - in other words, it is ignored.

The # 0 :0 line is interpreted by shell as a comment and by J as the beginning of a multiline "hereis" script which basically ignores everything up to the lone right parenthesis.

So then it's just regular shell script up until the line where we turn control over to J. On that line, we use exec (so that the shell process does not hang around, waiting for J to finish - J takes over the current process). And we pass any shell script command line arguments on to J.

On the J side of the fence, we presumably want this code to behave like a normal unix module, so we need to override J's default behavior (which is to provide the J command line). 9!:29]1[9!:27'2!:55]1 is a bit of magic that accomplishes that: it stacks a command to exit with exit code 1 to be executed when we reach the command line. So any errors will terminate the program.

Next, we run the system J profile so that we have all of the standard stuff that that provides. (Or leave this out if that's what you want.)

Finally we do some J stuff and then exit. If everything goes right, the command line exit we stacked earlier just gets ignored.

Here's a variant where the shell script tests J's exit code and does something different based on success or failure.

# 0 :0
echo unix shell commands go here
echo presumably this will condition the environment
echo for example:
cd working-directory
echo or maybe you want to modify $PATH, ... whatever...
echo then start up J:
if jconsole -jprofile "$0" "$@"; then
echo success
echo failure
exit $?
9!:29]1[9!:27'2!:55]1' NB. exit on error
(3 :'0!:0 y')<BINPATH,'/profile.ijs'
NB. and then the rest of the file is J
echo 'hi!'
echo 'your command line arguments were:'
echo ARGV
echo p:i. 3 4
exit 0

The exit $? line tells the shell interpreter to ignore the J part of the file, and the $? reuses J's exit code as the exit code from the shell instance.

[edit] OCaml

ocamlc hates shebangs, so much trickery is needed. The number of underscores in the dummy kkkk identifier corresponds to the number of bash strings in the shebang. Thus, core library .cma files can be specified this way in interpreted mode, though accessing other OCaml scripts requires compiling them first, and referencing the .cmo's here.

if true then ignore begin let kkkk _ _ _ _ = 0 in kkkk
"exec" "ocaml" "$0" "$@" + let fi = 0 and exit _ _ = 0 in if false
then exit
true else 0
let main = print_endline "Hello World!"


$ head -n 2
if true then ignore begin let kkkk _ _ _ _ _ _ = 0 in kkkk
"exec" "ocaml" "$0" "unix.cma" "graphics.cma" "$@" + let fi = 0 and exit _ _ = 0 in if false
$ ocaml
Hello World!
$ /bin/bash
Hello World!
$ ocamlc -o she.byte
$ ./she.byte
Hello World!
$ ocamlopt -o she.opt
$ ./she.opt
Hello World!

[edit] PARI/GP

The PARI equivalent to a multiline shebang is a collection of GP; lines:

GP;addhelp(GP_name, "GP_name(n): Computes the foo of bar(n).");

These commands are passed to GP when invoked by gp2c.

[edit] Perl

From perldoc perlrun, the following is supposed to find perl one way or another under sh, csh or perl.

eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -wS $0 $argv:q'
if $running_under_some_shell;

[edit] Perl 6

eval '(exit $?0)' && eval 'exec perl6 $0 ${1+"$@"}'
& eval 'exec perl6 $0 $argv:q'
if 0;

[edit] PicoLisp

We can use a multi-line comment #{ ... }# to hide the shell commands from Lisp. The opening #{ in turn is a coment for the shell.

exec pil $0 foo bar
# }#
# Lisp code
(println (cadr (file)) (opt) (opt))


$ ./myScript
"myScript" "foo" "bar"

[edit] Pike

we use a multiline comment to hide the shell command from pike, and we can use a preprocessor directive to hide the comment begin from the shell.

#define foo foo /*
exec pike $0 hello world

int main(int argc, array argv)
write("%O\n", argv);


({ /* 3 elements */

[edit] Python

We can use multiple strings to make the shell commands do nothing from Python (actually they become the module docstring.).

"exec" "python" "$0"
print "Hello World"


$ ./myScript
Hello World

Control structures (if/for/etc.) can't be quoted, but one can use the following to embed any script:

"true" '''\'
if [ -L $0 ]; then
exec "$interpreter" "$@"
exit 127

__doc__ = """module docstring"""
print "Hello World"

Here we use a) the code '''\' translates to \ in shell, but opens a multi-line string in Python; b) the true command ignores its argument, c) we always exit before the ending ''' so that the shell interpreter never reads it. Also, remember to set any docstrings by assigning to __doc__ since the docstring is already used for the shell script.

[edit] Racket

#| -*- scheme -*-
# this is sh code
echo running "$0", passing it into itself as an argument
exec racket -tm "$0" "$0"
#lang racket
(provide main)
(define (main arg)
(printf "argument: ~a\nexecuted as: ~a\n"
arg (find-system-path 'exec-file)))

[edit] Ruby

One can use a single-line shebang, like #!/usr/bin/env ruby, and use Kernel#system or `backquotes` to run any extra shell commands. A multi-line shebang is possible, but not necessary.

This script works both ways: either /bin/sh script.rb or ruby script.rb would run multiple lines of shell commands, and then start Ruby.

# Insert shell code here!
printf '%s\n' "Shell running $0"
for arg do
printf '  %s\n' "\${$i}: $arg"
i=`expr $i + 1`
# Switch from shell to Ruby.
exec ${RUBY-ruby} -x "$0" --coming-from-sh "$@"
ARGV[0] == "--coming-from-sh" or exec "/bin/sh", $0, *ARGV
# Insert Ruby code here!
puts "Ruby running #$0"
ARGV.each_with_index do |arg, i|
puts " ARGV[#{i}]: #{arg}"

When running /bin/sh scratch.rb, the shell:

  1. ignores #!/bin/sh, because it is a comment.
  2. runs multiple lines of shell code.
  3. executes ruby -x; user can set RUBY environment variable to pick different Ruby, like RUBY=ruby19 or RUBY=jruby.

ruby -x skips every line until the first Ruby shebang. This line must start with "#!" and must contain "ruby". (So "#!ruby" is the shortest shebang to work.)

When running ruby scratch.rb (without -x option), Ruby notices that the first line "#!/bin/sh" is a foreign shebang.

  • Ruby 1.8 then interprets this shebang and executes /bin/sh.
  • Ruby 1.9 then assumes -x option and skips to the first Ruby shebang. The script is not --coming-from-sh, so it executes /bin/sh.

[edit] Scala

The Scala interpreter (scala) supports shebangs, but the compiler (scalac) does not.

#!/usr/bin/env scala

[edit] Scheme

Works with: Chicken Scheme

#| ... |# provides just the right environment for the multiline shebang. Here, the script name is passed once to the Chicken Scheme Interpreter and once to be picked up in args.

exec csi -ss $0 ${1+"$@"}

[edit] Smalltalk

"exec" "gst" "-f" "$0" "$0" "$@"

[edit] Tcl

It is normal to use a line like this:

#!/usr/bin/env tclsh

But in cases where that is not enough perhaps because it needs some logic to locate the Tcl interpreter to use the differences in the way Tcl and the Bourne shell interpret end-of-line backslashes in comments can be used:

# Next line is comment in Tcl, but not in sh... \
exec tclsh "$0" ${1+"$@"}

Additional complexity can be added so long as the lines for the shell are commented in a Tcl sense.

[edit] TXR

sed -n -e '4,$p' < "$0" | /usr/bin/txr - "$0" "$@"
exit $?
@(next :args)

Test run:

$ ./multilineshebang.txr
$ ./multilineshebang.txr 1
$ ./multilineshebang.txr 1 2 3
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