Programming Forums > Script Programming > Bash / Shell Scripting

Jimbo · Nov 16th, 2006, 2:47 AM

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:

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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...

Similarly with cat:

(Toggle Plain Text)

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:

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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?

Nov 16th, 2006, 2:47 AM	#1
Jimbo Battle Programmer Join Date: Feb 2006 Location: Bellevue, WA, USA Posts: 763 Rep Power: 3	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: (Toggle Plain Text) jimmy@vera ~/code/bash $ cat blah.sh #!/bin/echo Hello echo world jimmy@vera ~/code/bash $ ./blah.sh Hello ./blah.sh 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... Similarly with cat: (Toggle Plain Text) 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 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: (Toggle Plain Text) 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 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? __________________ <insert disclaimer here> <insert shameless plug for Visual Studio here>

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Programming Forums > Script Programming > Bash / Shell Scripting

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