A compilation of documentation   { en , fr }

Loop line fields in awk

Created on:
Last edited on:
Xavier Béguin

The principe

In awk, whenever you want to loop on a variable number of fields in a record (which usually represents a text line, the fields being words separated by spaces), you have to keep in mind that:

  • the number of fields in the currently processed record is stored in the variable NF;
  • the value of a field can be retrieved by prefixing the character $ to an numerical variable: if the variable i holds 3, then $i is the same as $3 and represents the value of the third field in the current record.


Sample data

To illustrate these functionalities of awk, we'll try to process a text file having a variable number of fields on each line. The first field of each line represents a name, followed by figures associated with the person.

Here is an example of such a file, that we'll designate as the file loto.txt:

# Name list_of_figures
Philip 2 65 12 96
Turanga 55 34 82
Hubert 23
Amy 44 12 9 24
Hermes 32 99

Minimaliste program

In the following awk program that aims to process the above text file, the first line ignores text lines beginning by a hash mark (as it represents a comment). The following block, that applies to all other lines, prints on a new line each number associated with the person, which are the fields of the line starting with the second one (the first one being the name of the person):

  /^#/ { next }
      for (i=2; i<=NF; i++) {
          printf "%s received the number %d\n", $1, $i

Runngin the program on the sample data

Our little awk program can be used from a shell interpreter, as shown in the example session below.

It will scan the file logo.txt, ignoring its first line as it begins with a hash mark, then, for each following line, it will print a new line on its output for each figure associated with the person:

$ awk '
  /^#/ { next }
      for (i=2; i<=NF; i++) {
          printf "%s received the number %d\n", $1, $i
  ' loto.txt
Philip received the number 65
Philip received the number 12
Philip received the number 96
Turanga received the number 55
Turanga received the number 34
Turanga received the number 82
Hubert received the number 23