Shebang (Unix)
In computing, a shebang (also called a hashbang, hashpling, or pound bang) refers to the characters "#!" when they are the first two characters in a script file. Unix-like operating systems take the presence of these two characters as an indication that the file is a script, and try to execute that script using the interpreter specified by the rest of the first line in the file. For instance, shell scripts for the Bourne shell start with the first line:
#!/bin/sh
More precisely, a shebang line consists of a number sign and an exclamation point character ("#!"), followed by the (full) path to the interpreter program that will provide the interpretation. The shebang is looked for and used when a script is invoked directly (as with a regular executable), and largely to the end of making scripts look and act similarly to regular executables, to the operating system and to the user.
Because the "#" character is often used as the comment marker in scripting languages, the contents of the shebang line will be automatically ignored by the interpreter itself; the shebang line only exists to specify the correct interpreter to be used.
Etymology
The name shebang comes from an inexact contraction of SHArp bang or haSH bang [citation needed], 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. Another theory on sh in shebang's name is from default shell sh, usually invoked with shebang.[1]
History
The shebang was introduced by Dennis Ritchie between Version 7 Unix and 8 at Bell Labs. It was then also added to the BSD line at Berkeley (present in 4BSD and activated by default in 4.2BSD)[2]. As Version 8 and later versions were not released to the public, the first widely known appearance of this feature was on BSD.
Portability
Shebangs specify absolute paths to system executables; this can cause problems on systems which have non-standard filesystem layouts (such as GoboLinux or NixOS). Even when systems have fairly standard paths, it is quite possible for variants of the same operating system to have different locations for the desired interpreter.
In the absence of rigidly standardised locations for each interpreter, the shebang would on some systems try to execute something that doesn't exist where the shebang says it is. Therefore shebangs can limit the portability of the file.
Because of this 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, depending on the consistency in past convention of placement of the interpreter. For this and other reasons, POSIX does not standardize the feature.
Often, the program /usr/bin/env can be used to circumvent this limitation.
Another portability problem is the interpretation of the command arguments. Some systems do not split up the arguments; for example, when running the script with the first line like,
#!/usr/bin/env perl -w
it will be invoked as,
/usr/bin/env "perl -w"
That is, "perl -w" will be passed as one argument to /usr/bin/env, rather than two arguments, "perl" and "-w". On Linux, this will lead to the error message,
/usr/bin/env: perl -w: No such file or directory
Cygwin also behaves this way. Some other systems handle the arguments differently.
Plan 9 from Bell Labs demands a shebang when trying to execute a text file because it needs to know how to run the file. For example, a shell script could have the line
#!/bin/rc
at the top. If one is forgotten, the system would say "exec header invalid."
In many Linux systems, /bin/sh is a symlink to /bin/bash, since bash implements all the commands available in the Bourne shell. Some distributions link this to other shells, however. Ubuntu (and future Debian) uses /bin/dash, for instance, for better performance. When script developers erroneously use the #!/bin/sh in scripts that contain bash-specific commands, they will fail on systems that use a different default shell. The problem can be fixed by using only sh-compatible commands or by explicitly stating #!/bin/bash as the shebang.
As magic number
The shebang is actually a human-readable instance of a magic number in the executable file, the magic byte string being 0x23 0x21, the two characters' encoding in ASCII. (Executable files that do not require an interpreter program start with other magic combinations. See File format for more details of magic numbers.)
There have been rumours that some old versions of UNIX look for the normal shebang followed by a space and a slash ("#! /"), but this appears to be untrue[3].
Examples
Some typical interpreters for shebang lines:
#!/bin/bash— Execute using the Bourne-again shell#!/bin/bash -c '/bin/bash'— Execute using bash in the/bin/directory, and callsbashinside the/bin/#!/bin/csh— Execute usingcsh, the C shell#!/bin/ksh— Execute using the Korn shell#!/bin/awk -f— Execute using awk program in the/bin/directory; the -f is necessary; otherwise awk will think its argument is the awk program itself rather than a file that contains it#!/bin/sh— On some systems, such as Solaris, this is the Bourne shell. On Linux systems there is usually no Bourne shell and this is a link to another shell, such as bash. According to the Single UNIX Specification's requirements for /bin/sh, such a shell will usually mimic the Bourne shell's behaviour, but be aware that using #!/bin/sh in shell scripts may invoke different Bourne-compatible shells on different systems.#!/bin/zsh— Execute usingzsh, the Z shell#!/usr/bin/perl— Execute using Perl#!/usr/bin/php— Execute using PHP. Note: since in PHP the shebang line falls outside the "code block" (<?php ?>), it doesn't act as a comment, so PHP must explicitly ignore it.#!/usr/bin/python— Execute using Python#!/usr/bin/lua— Execute using Lua#!/usr/bin/env— Invocation of some other program using env program in/usr/bindirectory#!/usr/bin/ruby— Execute using Ruby#!/usr/bin/tclsh- Execute using Tcl
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.
This file is a shell script, the Unix equivalent of a DOS batch file:
#!/bin/sh
echo "Hi there.";
# The rest of the shell script.
# ...
This file is an AWK script:
#!/bin/awk -f
BEGIN { print "Hello, world!" }
# The rest of the AWK script.
# ...
This file is a Perl script, to be run with 'warnings' enabled (-w):
#!/usr/bin/perl -w
print "Hello world!";
# 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
print "Hello again!";
# The rest of the Perl script.
#...
This is a different way of invoking a bash script, telling it to print each command before it executes it:
#!/bin/bash -x
dosomeveryimportantprocess
# The rest of a perhaps long-running or importantly-timed Bash script.
...
This file is a quine (though it cheats by reading its own source code):
#!/bin/cat Any text can be placed below the shebang.
Solving shebang problems with the env program
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 ...
...and invoking a program which is itself a #! script:
#!/usr/bin/env /usr/local/bin/compileAndGo "$@" ...
Note that the above shows passing of arguments which won't work if you want "compileAndGo" itself to be found by search path (case 1 above) and be aware that while this method does increase portability, it is not foolproof. While env is almost always /usr/bin/env a small number of platforms have it at /bin/env[4].