Shebang - Revision history

Christoph: updated article

2006-05-25T23:20:33Z

updated article

Christoph: typo

2005-12-14T22:56:53Z

typo

Christoph: Started article

2005-12-14T22:56:18Z

Started article

New page

The word '''shebang''' has two distinct meanings:

* a word meaning "any matter of present concern; thing; business" (1869)
* the "#!" in the first two columns of the first line of a UNIX script file.

== Computing ==
In [[computing]], a [[shebang]] is a special line that begins an executable [[text file]] (commonly called a [[script (computer programming)|script]]) causing [[Unix-like]] [[operating system]]s to execute the commands in the text file using a specific [[interpreter (computer software)|interpreter]] ([[computer program|program]]).

A shebang consists of a [[number sign]] and an [[exclamation point]] character (so "#!") in the first two columns of the first line, followed by the [[Path (computing)|path]] to the interpreter program that will provide the interpretation. The name ''shebang'' comes from an inexact [[contraction (linguistics)|contraction]] of ''sharp bang'' or ''haSH bang'', referring to the two typical Unix names of the two characters. (Unix jargon uses ''sharp'' or ''hash'' to refer to the number sign character and ''bang'' to refer to the exclamation point, hence ''shebang''.) Because the "#" character is used as the comment marker in these executable text files, the actual contents of the shebang line will be ignored by the interpreter; the shebang line only exists to specify the correct interpreter to be used.

The shebang is actually a human-readable instance of a [[Magic number (programming)#Magic numbers in files|magic number]] in the executable file (with shebang equalling the magic number 0x2321). Executable files that do not require an interpreter program start with other magic combinations. (See [[File format]] for more details of magic numbers.)

Some typical shebang lines:
* <pre>#!/bin/bash</pre> -- Execute using <pre>[[bash]]</pre> program in the <pre/bin/</pre> directory
* <pre>#!/bin/csh</pre> -- Execute using <pre>csh</pre>, the [[C shell]] instead
* <pre>#!/usr/bin/perl</pre> -- Execute using [[Perl]]
* <pre>#!/bin/sh</pre> -- Execute using <pre>sh</pre> program in the <pre>/bin/</pre> directory (On some systems, such as [[Solaris Operating Environment|Solaris]], this is the [[Bourne shell]]. On [[Linux]] systems there is no Bourne shell and this is a link to another shell, such as bash. Using #!/bin/sh in shell scripts will thus invoke different shells on different systems. However, the [[Single UNIX Specification]] specifies certain requirements for /bin/sh, and some shells, such as bash, will alter their behaviour to match them when invoked with the name "sh".)

Shebang lines can also include specific options that will be passed to the interpreter; see the examples below. However, implementations differ widely on how options are parsed.

It is common for different variants of even the same operating system to have different locations for the desired interpreter. In the absence of a rigidly standardised filesystem structure among different Unix systems, this method can also limit the portability of the file. Thus, it is not uncommon to need to edit the shebang line after copying a [[script (computer programming)|script]] from one computer to another because the path that was coded into the script may not apply on a new machine. For this and other reasons, [[POSIX]] does not standardize the feature.

=== Example shebang lines ===
This file is a [[bash]] script, the [[Unix]] equivalent of a [[DOS]] [[batch file]]:

#!/bin/bash
#
# The rest of the bash script
...

This file is a [[Perl]] script, to be run with 'warnings' enabled (-w):

#!/usr/bin/perl -w
#
# The rest of the Perl script
...

This file is also a Perl script but it assumes that the Perl interpreter is in a different place.
Also, the Perl interpreter is run without extra warnings being enabled.

#!/usr/local/bin/perl
#
# The rest of the Perl script
...

<div id="env">
Invocation via the [[env]] program is useful for two purposes:
*searching for a program via the PATH environment variable

#!/usr/bin/env python
#
# The rest of the Python script
...

However, this may result in the ''wrong'' python being invoked, depending on the invoker's PATH, so its use is discouraged.

*invoking a program which is itself a <pre>#!</pre> script.

#!/usr/bin/env /usr/local/bin/[http://Yost.com/computers/compileAndGo compileAndGo]
...

</div>

==References==
*[http://en.wikipedia.org/wiki/Shebang Wikipedia.org]
==External links==
*[http://homepages.cwi.nl/~aeb/std/shebang/ Details about the shebang mechanism on various Unix flavours]
*[[FOLDOC:shebang|FOLDOC shebang article]]
*[http://info.unix-fu.org/General-UNIX/UnixFAQ3of7.txt unix faq] (search for #! there)

[[Category:Technical and Specialised Skills]]

← Older revision		Revision as of 22:56, 14 December 2005
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	*[http://info.unix-fu.org/General-UNIX/UnixFAQ3of7.txt unix faq] (search for #! there)		*[http://info.unix-fu.org/General-UNIX/UnixFAQ3of7.txt unix faq] (search for #! there)

−	[[Category:Technical and ~~Specialised~~ Skills]]	+	[[Category:Technical and Specialized Skills]]

@@ Line 1: / Line 1: @@
-The word '''shebang''' has two distinct meanings:
+{{lowercase|title=shebang}}
-* a word meaning "any matter of present concern; thing; business" (1869)
+In computing, a '''shebang''' is a specific pair of characters used in a special line that begins a text file (commonly called a [[:Category:Scripting languages|script]]) causing Unix-like operating systems to execute the commands in the text file using a specified interpreter (program) when executed.
-* the "#!" in the first two columns of the first line of a UNIX script file.
-== Computing ==
+A shebang consists of a number sign and an exclamation point character ("<tt>#!</tt>") in the first two columns of the first line, followed by the path to the interpreter program that will provide the interpretation. The name ''shebang'' comes from an inexact contraction of ''sharp bang'' or ''haSH bang'', referring to the two typical Unix names of the two characters. (Unix jargon uses ''sharp'' or ''hash'' to refer to the number sign character and ''bang'' to refer to the exclamation point, hence ''shebang''.) Because the "<tt>#</tt>" character is used as the comment marker in these executable text files, the actual contents of the shebang line will be ignored by the interpreter; the shebang line only exists to specify the correct interpreter to be used.
-In [[computing]], a [[shebang]] is a special line that begins an executable [[text file]] (commonly called a [[script (computer programming)|script]]) causing [[Unix-like]] [[operating system]]s to execute the commands in the text file using a specific [[interpreter (computer software)|interpreter]] ([[computer program|program]]).
-A shebang consists of a [[number sign]] and an [[exclamation point]] character (so "#!") in the first two columns of the first line, followed by the [[Path (computing)|path]] to the interpreter program that will provide the interpretation. The name ''shebang'' comes from an inexact [[contraction (linguistics)|contraction]] of ''sharp bang'' or ''haSH bang'', referring to the two typical Unix names of the two characters. (Unix jargon uses ''sharp'' or ''hash'' to refer to the number sign character and ''bang'' to refer to the exclamation point, hence ''shebang''.) Because the "#" character is used as the comment marker in these executable text files, the actual contents of the shebang line will be ignored by the interpreter; the shebang line only exists to specify the correct interpreter to be used.
+The shebang is actually a human-readable instance of a magic number in the executable file (with shebang equalling the magic byte string <code>0x23 0x21</code>). Executable files that do not require an interpreter program start with other magic combinations. (See [[File format]] for more details of magic numbers.)
-−
-The shebang is actually a human-readable instance of a [[Magic number (programming)#Magic numbers in files|magic number]] in the executable file (with shebang equalling the magic number 0x2321). Executable files that do not require an interpreter program start with other magic combinations. (See [[File format]] for more details of magic numbers.)
 Some typical shebang lines:
-* <pre>#!/bin/bash</pre> -- Execute using <pre>[[bash]]</pre> program in the <pre/bin/</pre> directory
+*<code>#!/bin/bash</code> -- Execute using <code>[[bash]]</code> program in the <code>/bin/</code> directory
-* <pre>#!/bin/csh</pre> -- Execute using <pre>csh</pre>, the [[C shell]] instead
+*<code>#!/bin/csh</code> -- Execute using <code>csh</code>, the [[C shell]] instead
-* <pre>#!/usr/bin/perl</pre> -- Execute using [[Perl]]
+*<code>#!/bin/ksh</code> -- Execute using the <code>[[Korn Shell]]</code>
-* <pre>#!/bin/sh</pre> -- Execute using <pre>sh</pre> program in the <pre>/bin/</pre> directory (On some systems, such as [[Solaris Operating Environment|Solaris]], this is the [[Bourne shell]]. On [[Linux]] systems there is no Bourne shell and this is a link to another shell, such as bash.  Using #!/bin/sh in shell scripts will thus invoke different shells on different systems.  However, the [[Single UNIX Specification]] specifies certain requirements for /bin/sh, and some shells, such as bash, will alter their behaviour to match them when invoked with the name "sh".)
+*<code>#!/bin/awk</code> -- Execute using [[AWK_programming_language|awk]] program in the <code>/bin/</code> directory
-Shebang lines can also include specific options that will be passed to the interpreter; see the examples below.  However, implementations differ widely on how options are parsed.
+Shebang lines can also include specific options that will be passed to the interpreter; see the examples below. However, implementations differ widely on how options are parsed.
-It is common for different variants of even the same operating system to have different locations for the desired interpreter. In the absence of a rigidly standardised filesystem structure among different Unix systems, this method can also limit the portability of the file. Thus, it is not uncommon to need to edit the shebang line after copying a [[script (computer programming)|script]] from one computer to another because the path that was coded into the script may not apply on a new machine.  For this and other reasons, [[POSIX]] does not standardize the feature.
+It is common for different variants of even the same operating system to have different locations for the desired interpreter. In the absence of a rigidly standardised filesystem structure among different Unix systems, this method can also limit the portability of the file. Thus, it is not uncommon to need to edit the shebang line after copying a script from one computer to another because the path that was coded into the script may not apply on a new machine. For this and other reasons, [[POSIX]] does not standardize the feature.
-=== Example shebang lines ===
+==Example shebang lines==
-This file is a [[bash]] script, the [[Unix]] equivalent of a [[DOS]] [[batch file]]:
+This file is a [[bash]] script:
   #!/bin/bash
@@ Line 29: / Line 27: @@
   ...
 This file is a [[Perl]] script, to be run with 'warnings' enabled (-w):
@@ Line 37: / Line 41: @@
   ...
+This file is also a Perl script, but it assumes that the Perl interpreter is in a different place. Also, the Perl interpreter is run without extra warnings being enabled.
 This file is also a Perl script but it assumes that the Perl interpreter is in a different place.
 Also, the Perl interpreter is run without extra warnings being enabled.
   #!/usr/local/bin/perl
@@ Line 46: / Line 48: @@
   ...
-<div id="env">
+This file is a quine:
 Invocation via the [[env]] program is useful for two purposes:
-*searching for a program via the PATH environment variable
+ #!/bin/cat
   #!/usr/bin/env python
+  #
   # The rest of the Python script
   ...
-However, this may result in the ''wrong'' python being invoked, depending on the invoker's PATH, so its use is discouraged.
+and invoking a program which is itself a <code>#!</code> script:
   #!/usr/bin/env /usr/local/bin/compileAndGo
-*invoking a program which is itself a <pre>#!</pre> script.
-−
-  #!/usr/bin/env /usr/local/bin/[http://Yost.com/computers/compileAndGo compileAndGo]
   ...
-</div>
+== External links ==
+* [http://www.in-ulm.de/~mascheck/various/shebang/ Details about the shebang mechanism on various Unix flavours]
-==References==
+* [http://homepages.cwi.nl/~aeb/std/hashexclam.html #! - the Unix truth as far as I know it] (a more generic approach)
-*[http://en.wikipedia.org/wiki/Shebang Wikipedia.org]
+* [http://info.unix-fu.org/General-UNIX/UnixFAQ3of7.txt Unix F.A.Q] (search for #! there)
-==External links==
+* [http://en.wikipedia.org/wiki/Shebang_%28Unix%29 Wikipedia article on '''shebang''']
-*[http://homepages.cwi.nl/~aeb/std/shebang/ Details about the shebang mechanism on various Unix flavours]
-*[[FOLDOC:shebang|FOLDOC shebang article]]
-*[http://info.unix-fu.org/General-UNIX/UnixFAQ3of7.txt unix faq] (search for #! there)
-[[Category:Technical and Specialized Skills]]
+[[Category:Scripting languages]]