45.5 A File That Shows Itself... and What #! Does

If your UNIX understands ( 44.4 ) executable files that start with #! , you can use this nice trick to make executable files that display themselves (or part of themselves). I used this to make a program named help on a system that didn't have any online help. A program like cat ( 25.2 ) isn't what you want because it'll display the #! line as well as the message. Watch what happens:

% 

cat help

 #!/bin/cat For help with UNIX, call the ACS Consulting Hotline at 555-1212.  man command               shows the manual for a command    ... % 

chmod +x help

 % 

help

 #!/bin/cat For help with UNIX, call the ACS Consulting Hotline at 555-1212.  man command               shows the manual for a command    ...

The trick is to invoke an interpreter that shows all the lines except the line starting with #! . For example, this file uses sed ( 34.24 ) and its d command to ignore ("delete") the first line:

% 

cat help

 #!/bin/sed 1d For help with UNIX, call the ACS Consulting Hotline at 555-1212.  man command               shows the manual for a command    ... % 

help

 For help with UNIX, call the ACS Consulting Hotline at 555-1212.  man command               shows the manual for a command    ...

For longer files, try using more +2 ( 25.3 ) ; this file will show itself screenful-by-screenful, starting at line 2:

% 

cat help

 #!/usr/ucb/more +2 For help with UNIX, call the ACS Consulting Hotline at 555-1212.  man command               shows the manual for a command    ...

You have to give the absolute pathname to the interpreter because the kernel doesn't use your search path ( 8.7 ) . The kernel can pass just one argument to the interpreter. More than one argument probably won't work. In the next example, I try to pass two arguments to grep -but the kernel passes the whole string -v # as just one argument. That confuses grep , which complains about every character from the space on:

% 

cat help

 #!/bin/grep -v # For help with UNIX, call the ACS Consulting Hotline at 555-1212.  man command               shows the manual for a command    ... % 

help

 grep: illegal option -- grep: illegal option -- ^ grep: illegal option -- # Usage: grep -hblcnsvi pattern file . . .

(Remember, there's no shell interpreting the arguments here. The kernel does it.)