AWK provides several built-in variables.

They play an important role while writing AWK scripts.

Here is a list of some of the built-in variables supported by AWK:

It implies the number of arguments provided at the command line.

awk 'BEGIN {print "Arguments =", ARGC}' One Two Three Four

returns:

Arguments = 5

It represents the index in ARGV of the current file being processed.

awk '{ 
   print "ARGIND   = ", ARGIND; print "Filename = ", ARGV[ARGIND] 
}' junk1 junk2 junk3

returns:

ARGIND   =  1
Filename =  junk1
ARGIND   =  2
Filename =  junk2
ARGIND   =  3
Filename =  junk3

It is an array that stores the command-line arguments.

The array's valid index ranges from 0 to ARGC-1.

awk 'BEGIN { 
   for (i = 0; i < ARGC - 1; ++i) { 
      printf "ARGV[%d] = %s\n", i, ARGV[i] 
   } 
}' one two three four

returns:

ARGV[0] = awk
ARGV[1] = one
ARGV[2] = two
ARGV[3] = three

It is used to specify binary mode for all file I/O on non-POSIX systems.

Numeric values of 1, 2, or 3 specify that input files, output files, or all files, respectively, should use binary I/O.

String values of r or w specify that input files or output files, respectively, should use binary I/O.

String values of rw or wr specify that all files should use binary I/O.

It represents the conversion format for numbers.

Its default value is %.6g.

awk 'BEGIN { print "Conversion Format =", CONVFMT }'

returns:

Conversion Format = %.6g

It is an associative array of environment variables.

awk 'BEGIN { print ENVIRON["USER"] }'

returns:

peter

A string indicates an error when a redirection fails for getline or if a close call fails.

awk 'BEGIN { ret = getline < "junk.txt"; if (ret == -1) print "Error:", ERRNO }'

returns:

Error: No such file or directory

A space separated list of field widths variable is set, GAWK parses the input into fields of fixed width, instead of using the value of the FS variable as the field separator.

It represents the current file name.

awk 'END {print FILENAME}' test.txt

returns:

test.txt

It is similar to NR, but relative to the current file.

It is useful when AWK is operating on multiple files.

It represents the number of the current record in the current file.

echo -e "One Two\nOne Two Three\nOne Two Three Four" | awk 'FNR < 3'

returns:

One Two
One Two Three

It represents the (input) field separator and its default value is space.

You can also change this by using -F command line option.

awk 'BEGIN {print "FS = " FS}' | cat -vte

returns:

FS =  $

When this variable is set, GAWK becomes case-insensitive.

awk 'BEGIN{IGNORECASE = 1} /peter/' test.txt

returns:

10   Peter     Terence    Roux        45

awk 'tolower($0) ~ /peter/' test.txt

It provides dynamic control of the –lint option from the GAWK program.

When this variable is set, GAWK prints lint warnings.

When assigned the string value fatal, lint warnings become fatal errors, exactly like –lint=fatal.

awk 'BEGIN {LINT = 1; a}'

returns:

awk: cmd. line:1: warning: reference to uninitialized variable `a'
awk: cmd. line:1: warning: statement has no effect

It represents the number of fields in the current record.

For instance, the following example prints only those lines that contain more than two fields.

echo -e "One Two\nOne Two Three\nOne Two Three Four" | awk 'NF > 2'

returns:

One Two Three
One Two Three Four

It represents the number of the current record.

For instance, the following example prints the record if the current record number is less than three.

echo -e "One Two\nOne Two Three\nOne Two Three Four" | awk 'NR < 3'

returns:

One Two
One Two Three

It represents the output format number and its default value is %.6g.

awk 'BEGIN {print "OFMT = " OFMT}'

returns:

OFMT = %.6g

It represents the output field separator and its default value is space.

awk 'BEGIN {print "OFS = " OFS}' | cat -vte

returns:

OFS =  $

It represents the output record separator and its default value is newline.

awk 'BEGIN {print "ORS = " ORS}' | cat -vte

returns:

ORS = $
$

This is an associative array containing information about the process, such as real and effective UID numbers, process ID number, and so on.

awk 'BEGIN { print PROCINFO["pid"] }'

returns:

30510

It represents the length of the string matched by match function.

AWK's match function searches for a given string in the input-string.

awk 'BEGIN { if (match("One Two Three", "re")) { print RLENGTH } }'

returns:

2

It represents (input) record separator and its default value is newline.

awk 'BEGIN {print "RS = " RS}' | cat -vte

returns:

RS = $
$

It represents the first position in the string matched by match function.

awk 'BEGIN { if (match("One Two Three", "Thre")) { print RSTART } }'

returns:

9

It represents the separator character for array subscripts and its default value is \034.

awk 'BEGIN { print "SUBSEP = " SUBSEP }' | cat -vte

returns:

SUBSEP = ^\$

It represents the entire input record.

awk '{print $0}' test.txt

returns:

10   Peter     Terence    Roux        45
11   Virginia  Genevieve  Roux        45
12   Felix     Devon      Roux         5
13   David     Bruce      Stevenson   48
14   Bob       James      Smith       16
48   Adam      Winter     Ridley      23

It represents the text domain of the AWK program.

It is used to find the localized translations for the program's strings.

awk 'BEGIN { print TEXTDOMAIN }'

returns:

messages

It represents the nth field in the current record where the fields are separated by FS.

awk '{print $2 "\t" $5}' test.txt

returns:

Peter   45
Virginia     45
Felix   5
David   48
Bob   16
Adam   23