AWK is a language for processing files of text. A file is treated as a sequence of records, and by default each line is a record. Each line is broken up into a sequence of fields, so we can think of the first word in a line as the first field, the second word as the second field, and so on. An AWK program is of a sequence of pattern-action statements. AWK reads the input a line at a time. A line is scanned for each pattern in the program, and for each pattern that matches, the associated action is executed.

Basics

1. Print /etc/mtab (awk reads a line at time and prints it) :

   awk '{ print }' /etc/mtab

2. Print /etc/mtab ($0 denotes the whole line) :

   awk '{ print $0 }' /etc/mtab

3. List mounted filesystems ($1 denotes the first element of the line) :

   awk '{ print $1 }' /etc/mtab

4. List groups (-F chooses the Field Separator) :

   awk -F":" '{ print $1 }' /etc/group

5. List groups and id (awk concatenates print() arguments) :

   awk -F":" '{ print $1 " " $3 }' /etc/group

6. List groups and id (Nicer format) :

   awk -F":" '{ print "group: " $1 "\tid: " $3 }' /etc/group

7. Launching external awk scripts :

   # first.awk
   BEGIN {
       FS=":"
   }
   { print $1 }

   awk -f first.awk /etc/mtab

8. List IPv4 addressess :

   ifconfig | awk '/inet / { print $2 }'

9. List processes run by root :

   # psroot.awk
   $1 == "root" { 
       printf("ROOT: "); 
       for (i=11; i<=NF; i++) 
           printf("%s ", $i);
       printf("\n")
   }

   ps au | awk -f psroot.awk

10. Print X warnings :

    awk '$1 ~ /(WW)/ { print }' /var/log/Xorg.0.log

11. Print a file removing comments :

    awk '! /^#/ { print }' /etc/fstab

12. Print number of files/directories :

    ls -lA |  awk 'BEGIN{ x=0 } { x=x+1 } END{ print x-1 }'

13. Awk as calculator :

    echo | awk '{ print ((2*5^2-1)%7) }'

14. Count empty lines :

    # blanklines.awk
    BEGIN { x=0 } 
    /^$/  { x=x+1 } 
    END   { print "I found " x " blank lines :)" }

    awk -f blanklines.awk /etc/profile

15. Use regexp as FS (note the difference though) :

    echo '   a   b c  d '| awk -F"[ \t\n]+" '{ print $2 }'

    echo '   a   b c  d '| awk -F" " '{ print $2 }'

16. Understand NR and NF :

    # nrnf.awk
    BEGIN { x=0 } 
          { print "Words on line " NR ": " NF ; x+=NF } 
    END   { print "Total lines: " NR ; print "Total words: " x }

    awk -f nrnf.awk nrnf.awk