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I am writing a simple awk script to read a file holding a single number (single line with a single field), subtract a constant and then write the result to another file. This is a warmup exercise to do a more complex problem. So, if the input file has X, then the output file has X-C

When I write the following in the command line, it works:

 awk '{$1 = $1 - 10; print $0}' test.dat > out.dat

The output looks like this (for X = 30 and C = 10):

20

However, I wrote the following awk script :

#!/bin/awk
C=10 
{$1 = $1 - C; print $0}

Next, when I run the awk script using:

./script.awk test.dat > out.dat

I get an output file with two lines as follows :

X 
X-C

for example, if X=30 and C=10 I get an output file having

30 
20

Why is the result different in both cases? I tried removing "-f" in the shebang but I receive and error when I do this.

dbrane
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  • `awk` can't "see" a shell environment variable. you have to pass it in, `awk -v C=10 '{...code using var C }' file > out`. Good luck. – shellter Aug 10 '22 at 22:22
  • Dont use a shebang to call awk. See https://stackoverflow.com/a/61002754/1745001 – Ed Morton Aug 10 '22 at 22:42

1 Answers1

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This is your awk program:

C=10 
{$1 = $1 - C; print $0}

Recall that awk programs take the form of a list of pattern-action pairs.

Missing action results in the default action being performed (print the input). Missing pattern is considered to return true.

Your program is equivalent to:

C=10 { print $0 }
1 { $1 = $1 -C ; print $0 }

The first pattern C=10 assigns 10 to variable C and because assignments return the value assigned, returns 10. 10 is not false, so the pattern matches, and the default action happens.

The second line has a default pattern that returns true. So the action always happens.

These two pattern-action pairs are invoked for every record that is input. So, with one record input, there will be two copies printed on output.

jhnc
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  • So, how do I make the user pass the value of "C" to the awk script? I do not want the output to be X , X-C. I just want X-C in the output. Thanks – dbrane Aug 10 '22 at 22:54
  • you can set C inside a `BEGIN` action. Or change how you invoke awk and do `awk -v var=value ...` – jhnc Aug 10 '22 at 23:10
  • I solved the problem by just making the user pass C as an argument. Thanks ! – dbrane Aug 10 '22 at 23:21