Question

Shell Programming

Asked by: amintushar23


hi

i am newbie to linux shell programming
i have seen many shell programs in which
its starts

#!bin/sh

can u tell me what exactly this statement means & why it has to be used

Tushar

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Asked On
2004-06-07 at 08:24:06ID21016090
Topic

Linux Programming

Participating Experts
3
Points
500
Comments
3

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Answers

 

by: brunomsilvaPosted on 2004-06-07 at 08:46:25ID: 11250494

hi,

the first line of the shell script is used to say where the interpreter of that script will be, for example
#!/usr/bin/perl says that this file will be interpreted by perl which is in /usr/bin

 

by: ahoffmannPosted on 2004-06-07 at 15:26:24ID: 11254032

> #!bin/sh
might be a typo, is probably:
   #!/bin/sh

 

by: NovaDenizenPosted on 2004-06-08 at 07:18:13ID: 11260155

When you run a command the OS doesn't immediately know whether the file being run is a script or a binary file.  The OS only knows that it is an executable file.  The first thing the OS does is read the first few bytes of the file.  If it starts with the four bytes "\177ELF", then the OS knows that this executable file is an ELF binary file, and begins the process of executing it.  If it starts with the two bytes "#!" then the OS knows that it is dealing with an interpreted script file, and that the rest of the first line contains the name of the program it should use to interpret the program.  

So, if you have a /bin/sh script file called myScript, and you try to run it:
1.  The OS reads the first few bytes of the file.
2.  The OS recogizes them as #!, the script indicator.
3.  The OS reads the rest of the first line, namely "/bin/sh"
4.  The OS runs the command "/bin/sh myScript"
5.  The bourne shell interpreter "/bin/sh" starts up
6.  /bin/sh realizes that its first argument is "myScript"
7.  /bin/sh opens up and begins to read "myScript"
8.  /bin/sh runs the commands found in "myScript"

You can take advantage of this and do some weird things.  If you create a script file containing this line:
#!/bin/ls -l

Then when you run it, it performs an "ls -l" on itself.

If you create a script with:
#!/bin/cat

Then you have created a minimal script which outputs itself!

20120131-EE-VQP-002

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