Shebang

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) causing Unix-like operating systems to execute the commands in the text file using a specific interpreter (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 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 "#" 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 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:

• #!/bin/bash

  -- Execute using

  [[bash]]

  program in the <pre/bin/</pre> directory

• #!/bin/csh

  -- Execute using

  csh

  , the C shell instead

• #!/usr/bin/perl

  -- Execute using Perl

• #!/bin/sh

  -- Execute using

  sh

  program in the

  /bin/

  directory (On some systems, such as 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 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
...

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

  #!

  script.

#!/usr/bin/env /usr/local/bin/compileAndGo
...

References

• Wikipedia.org

External links

• Details about the shebang mechanism on various Unix flavours
• FOLDOC shebang article
• unix faq (search for #! there)