Programming Forums - What does the ! do in #! at the start of a .sh?

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What does the ! do in #! at the start of a .sh?

Someone mentioned to me recently that the ! part of the #! to start a shell script had some special meaning. He mentioned something along the lines of how the # denotes a comment, but the ! causes the line to still be interpreted somehow. So I was playing around with it and noticed some odd things. It seems like the script calls itself, but with itself as a parameter? Or does the #! line just prepend itself to the list of parameters actually passed in (hence how a script gets itself as $0)?

First some oddities with echo:

jimmy@vera ~/code/bash $ cat blah.sh 

#!/bin/echo Hello 

echo world

jimmy@vera ~/code/bash $ ./blah.sh 

Hello ./blah.sh

I can understand if it were to execute either the second line (with the first ignored as a comment), or the second (and stopping after running echo). But it echo's both Hello and the filename... :confused:

Similarly with cat:

jimmy@vera ~/code/bash $ cat me

this file is called me

jimmy@vera ~/code/bash $ cat blah.sh 

#! /bin/cat me 

echo Hello world

jimmy@vera ~/code/bash $ ./blah.sh 

this file is called me

#! /bin/cat me 

echo Hello world

Again, it executes the first line (with the #!), but not the second, and again it adds itself as a parameter to the first line.

and to be neat and tidy, I tried rm as well:

jimmy@vera ~/code/bash $ cat blah.sh 

#! /bin/rm me 

echo hello world

jimmy@vera ~/code/bash $ ls

blah.sh  filenamesWithSpaces.sh  me

jimmy@vera ~/code/bash $ ./blah.sh 

jimmy@vera ~/code/bash $ ls

filenamesWithSpaces.sh

The second guess I came up with seems almost logical, except that at this point I'd modify it to say it somehow passes the file and its arguments to whatever is cited after the #!. But I'm still making guesses, so does anyone know how it actually works?

It's called a shebang. It's the *nix way of determining what interpreter to use when executing a shell script. The operating system looks for the shebang at the start of the script its executing, and then passes the entire contents of the file to the program specified by the shebang. Any additional arguments are appended to the end of the program.

For instance, say I have a file called hello.fb:

#! /usr/bin/foobar -y

All of this text as well as the line above

is passed to "foobar -y" as STDIN

Then this line:

./hello.fb world

Is the same as:

/usr/bin/foobar -y world < hello.fb

Now for the useless information: shebang is a corruption of hash bang, the # being the hash mark and the ! being the bang mark.

It constitutes a magic number. UNIX can interpret the magic number for a lot of files, and then behave accordingly.

#!/bin/ksh
loads the ksh shell for the duration of the script - the script file becomes stdin.

look for /usr/include/magic.h and /usr/include/sys/magic.h for more information. man magic or info magic may also help.

I've heard a few UK-based Unix hackers call it the "Whiz bang." Not really very informative, I know, but I find it quite amusing. Maybe I should get out more...