Take the 2-minute tour ×
Super User is a question and answer site for computer enthusiasts and power users. It's 100% free, no registration required.

I got an awk script named my_awk:

#!usr/bin/awk -f

{ 
if ($1 == "#START") { FS=":";} 
else if ($1 == "#STOP") { FS = " ";}
else { print $3} 
}

And I invoked it through:

cat my_file | awk -f my_awk

I'm new to awk so I'm ignorant about awk's mechanism. Will this my_awk script execute its command to the whole my_file or to each line of the file?

share|improve this question

1 Answer 1

up vote 2 down vote accepted

When you pipe something to a script, the script will only be executed once. So, for example if you do:

foo | bar

then bar is called only once, with its STDIN being whatever foo wrote to STDOUT.

You have a useless use of cat there, since you could just do:

awk -f my_awk < my_file

Or, since awk can directly work with file name arguments:

awk -f my_awk my_file

If you go further, awk itself is a tool that works on line-by-line basis, but it's really called only once by the shell. From this tutorial:

Like most UNIX utilities, AWK is line oriented. That is, the pattern specifies a test that is performed with each line read as input.

share|improve this answer
    
but how awk works of the file? The script I wrote above has successfully changed the FS separator for 2 different lines! –  Zen Jun 18 at 2:29
    
See my updated answer. –  slhck Jun 18 at 5:52

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.