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I am learning file comparison using awk.

I found syntax like below,

awk '
(NR == FNR) { 
    s[$0]
    next 
} 
{ 
    for (i=1; i<=NF; i++) 
        if ($i in s) 
            delete s[$i] 
} 
END { 
    for (i in s) 
        print i 
}' tests tests2

I couldn't understand what is the Syntax ...Can you please explain in detail?

What exactly does it do?

James Brown
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ironman junior
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    `NR`, `FNR`, `NF` are *Built-in variables*, if you want to know more about them I suggest reading [8 Powerful Awk Built-in Variables – FS, OFS, RS, ORS, NR, NF, FILENAME, FNR](https://www.thegeekstuff.com/2010/01/8-powerful-awk-built-in-variables-fs-ofs-rs-ors-nr-nf-filename-fnr/) – Daweo Feb 22 '22 at 09:00
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    One of the mhe most important unix commands is `man` for _manual_. Open a terminal, type `man awk` and you'll find the answers to most of your questions. If tomorrow you start learning `find` try `man find`. In some cases the shown manual is a short version; the long version is frequently available with the `info` command (e.g. `info sed`). – Renaud Pacalet Feb 22 '22 at 09:15
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    This post can help you: https://stackoverflow.com/questions/32481877/what-are-nr-and-fnr-and-what-does-nr-fnr-imply – Carlos Pascual Feb 22 '22 at 12:02
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    *I found syntax like below,* where did you find it? Was not there any corresponding explanation? – Daweo Feb 22 '22 at 12:34
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    The others aren't lecturing, they're teaching. They're teaching you how to learn the fundamentals and find answers for yourself so you don't have to ask questions about fundamentals. To learn awk get the book Effective AWK Programming, 5th Edition, by Arnold Robbins. – Ed Morton Feb 22 '22 at 14:48
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    I’m voting to close this question because the site exists to help people solve problems with their code, not to explain other peoples code. – Ed Morton Feb 22 '22 at 14:50

1 Answers1

3
awk '                      # use awk
(NR == FNR) {              # process first file
    s[$0]                  # hash the whole record to array s
    next                   # process the next record of the first file
} 
{                          # process the second file
    for (i=1; i<=NF; i++)  # for each field in record
        if ($i in s)       # if field value found in the hash
            delete s[$i]   # delete the value from the hash
} 
END {                      # after processing both files 
    for (i in s)           # all leftover values in s
        print i            # are output
}' tests tests2

For example, for files:

tests:
1
2
3

tests2:
1 2
4 5

program would output:

3
James Brown
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  • Thanks a lot @James ...can you explain lil bit more line 3 and 4. – ironman junior Feb 22 '22 at 11:12
  • `s[$0]` stores a record as a key into an array for a quick lookup when searching values from the second file. `next` breaks the processing of the program for the current record, fetches the next record from the file and starts processing it. The point of `next` here is to stop the program from processing first file records in the program block meant for the records of the second file. – James Brown Feb 22 '22 at 11:31