Awk

Taken from Tutorials Point See also Geeks for Geeks

'awk' is a interpreted programming language designed for text processing. An awk program consists of commands that are executed sequentially on lines of text. Optional BEGIN and END blocks can be used for setup and tear-down. Use single-quotes to mark the start and end of the awk script.

In its simplest form awk can be used to print the contents of a file:

awk '{print}' <filename>

The '-v' option can be used to assign variables before the awk commands are executed:

awk -v name="World!" 'BEGIN{printf "Hello, %s\n", name}'

'awk' treats input lines and output lines as space-separated fields. Columns from each line can be printed using their indexes (starting from 1):

awk '{print $3,$4}' <filename

Or you can insert a tab separator between the output fields:

awk '{print $3 "\t" $4}' <filename>

Filters can be added to select which lines to print:

awk '/<pattern>/ {print $3 "\t" $4}' <filename>

In the absence of a body block, awks default action is to print the whole record. $0 can be used to refer to the whole input record. These two programs therefore produce the same output:

awk '/<pattern>/ {print $0}' <filename>
awk '/<pattern>/' <filename>

Variables can be used without prior declaration:

awk '/<pattern>/{++counter} END{print "Lines matched = ", counter}' <filename>

'length' is a built-in function to return the length of its operand. To print all lines longer than 82 characters use:

awk 'length($0) > 82' <filename>    # using default action of {print}

Awk comes with a number of built-in variables:

Variable	Meaning
ARGC	number of arguments provided at the command line
ARGV	an array of the command line arguments
CONVFMT	the conversion format for numbers
ENVIRON	hash of environment variables
FILENAME	the current filename
FS	current field separator (default is space)
RS	current record separator (default is newline)
NF	number of fields in current record
NR	number of the current record
FNR	number of current record in current file
OFMT	the output format for numbers
OFS	the output field separator (default space)
ORS	the output record separtor (default newline)
RLENGTH	the length of the string matched in match function
RSTART	the first position in the string matched in match function
SUBSEP	the separator character for array subscripts

awk 'BEGIN {print "Arguments = ", ARGC}' one two three four

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

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

awk 'NF < 10' <filename>                # print lines with less than 10 fields
awk 'NF < 10 {print NF}' <filename>     # print number of fields in lines with less than 10 fields

awk 'NR < 11' <filename>                # prints first 10 records

awk 'BEGIN { if (match("Hello, World!", "or")) {print RLENGTH}'

Additional variables are available with GNU AWK (gawk):

Variable	Meaning
ARGIND	Index in ARGV of the current file being processed
BINMODE	used to set binmode for file I/O
ERRNO	error number if getline or close fails
FIELDWIDTHS	use a space-separated list of fieldwidths instead of a delimiter
IGNORECASE	used to make awk case-insensitive
LINT	used to dynamically set lint options
PROCINFO	used to access process information
TEXTDOMAIN	text domain of the AWK program