Reading Input Files

Record number

NR The total number of input records seen so far.

w | awk 'NR > 2 { print }'

Record splitting with standard awk

RS The record separator, by default a newline.

awk 'BEGIN { RS = ":" } { print $0 }' /etc/passwd

Record splitting with gawk

RT

RS is a single character, RT contains the same single character.

RS is a regular expression, RT contains the actual input text that matched the regular expression.

echo record 1 AAAA record 2 BBBB record 3 |
awk 'BEGIN { RS = "\n|( *[[:upper:]]+ *)" }
{ print "Record =", $0,"and RT = [" RT "]" }'
Record = record 1 and RT = [ AAAA ]
Record = record 2 and RT = [ BBBB ]
Record = record 3 and RT = [
]

Fields

NF The number of fields in the current input record.

The input record is automatically separated into chunks called fields.

$0	All fields.
$1	First field.
$NF	Last field.

awk '/li/ { print $1, $NF }' mail-list

Contents of a field

Change fields content.

awk '{ nboxes = $3 ; $3 = $3 - 10 ; print nboxes, $3 }' inventory-shipped

Create new field.

awk '{ $6 = ($5 + $4 + $3 + $2) ; print $6 }' inventory-shipped

How fields are separated

FS The input field separator, a space by default.

• Single character

awk 'BEGIN { FS = ":" ; OFS = ":"} ;$3 > 999 { print $1,$6,$7 }' /etc/passwd

• Regexp

echo ' a b c d e  ' | awk 'BEGIN { FS = "[ \t\n]+" } {print $2}'

Each character a separate field

echo ab cd | awk 'BEGIN { FS = "" }
{ for (i = 1; i <= NF; i++) print "Field", i, "is", $i }'

FS from the command line

awk -F: '$5 == ""' /etc/passwd
# same as
awk 'BEGIN { FS = ":" } $5 == "" { print }' /etc/passwd

Field-splitting summary

• FS == ” “

  Fields are separated by runs of whitespace. Leading and trailing whitespace are ignored.

  This is the default.
• FS == any other single character

  Fields are separated by each occurrence of the character. Multiple successive occurrences delimit empty fields, as do leading and trailing occurrences.

  The character can even be a regexp metacharacter; it does not need to be escaped.
• FS == regexp

  Fields are separated by occurrences of characters that match regexp .

  Leading and trailing matches of regexp delimit empty fields.
• FS == “”

  Each individual character in the record becomes a separate field.

  (This is a common extension; it is not specified by the POSIX standard.)
• FIELDWIDTHS == list of columns

  Based on character position.
• FPAT == regexp

  On the text surrounding text matching the regexp.

Record-splitting summary

• RS == “\n”

  Records are separated by the newline character (\n).

  In effect, every line in the datafile is a separate record, including blank lines.

  This is the default.
• RS == any single character

  Records are separated by each occurrence of the character. Multiple successive occurrences delimit empty records.
• RS == “”

  Records are separated by runs of blank lines. When FS is a single character, then the newline character always serves as a field separator, in addition to whatever value FS may have. Leading and trailing newlines in a file are ignored.
• RS == regexp

  Records are separated by occurrences of characters that match regexp. Leading and trailing matches of regexp delimit empty records.

Multiple-line records

cat addresses
Jane Doe
123 Main Street
Anywhere, SE 12345-6789

John Smith
456 Tree-lined Avenue
Smallville, MW 98765-4321

awk 'BEGIN { RS = "" ; FS = "\n" } {
print "Name is:", $1
print "Address is:", $2
print "City and state are:", $3
print "#######"
}' addresses

Explicit input with getline

awk 'BEGIN {
"date \"+%F %T\"" | getline current_time
close("date")
print "Report printed on " current_time
}'
Report printed on 2020-01-23 00:01:31

Getline summary

Variant	Effect	awk/gawk
getline	Sets $0 , NF , FNR , NR , and RT	awk
getline var	Sets var , FNR , NR , and RT	awk
getline < file	Sets $0 , NF , and RT	awk
getline var < file	Sets var and RT	awk
command | getline	Sets $0 , NF , and RT	awk
command | getline var	Sets var and RT	awk
command |& getline	Sets $0 , NF , and RT	gawk
command |& getline var	Sets var and RT	gawk

Input with a timeout

PROCINFO The elements of this array provide access to information about the running AWK program.

PROCINFO[“input_name”, “READ_TIMEOUT”] = timeout in milliseconds

awk 'BEGIN {PROCINFO["/dev/stdin", "READ_TIMEOUT"] = 5000}
{while ((getline < "/dev/stdin") > 0)
print $0}'

awk 'BEGIN { PROCINFO["-", "READ_TIMEOUT"] = 5000 }
{ print "You entered: " $0 }'