What is the difference between NR and FNR?

NR is the cumulative record number across every input file, so it keeps counting as awk moves from one file to the next. FNR resets to 1 at the start of each file, which is useful for detecting headers or per-file logic.

How do I change the field separator in awk?

Set the FS variable, either on the command line with -F',' or inside a BEGIN block with FS=",". For output, set OFS to control the separator that print places between fields.

What is the difference between sub and gsub?

sub replaces only the first match of the regex in the target string and returns 0 or 1. gsub replaces every non-overlapping match and returns the total number of substitutions made.

How do I run code only once before or after processing?

Use the special BEGIN pattern for setup code that runs before any input is read, such as setting FS or printing a header. Use END for code that runs after the last record, ideal for printing totals.

Are awk string positions zero-based or one-based?

awk uses one-based indexing for strings and fields. substr(s,1,3) returns the first three characters, index returns 1 for a match at the start, and $1 is the first field.

What is the awk Reference?

Searchable awk reference covering built-in variables (NR, NF, FS, OFS, RS), string functions (split, gsub, substr, match), numeric and I/O functions, and the BEGIN/END/range patterns — each with a working snippet. It runs free in your browser on Gera Tools, with nothing uploaded.

awk Reference — Gera Tools

Name: awk Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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awk

awk is a line-oriented text-processing language: it reads input record by record (usually line by line), splits each record into fields, and runs pattern-action rules against it. Its power comes from a compact set of built-in variables like NR, NF and FS, a library of string and numeric functions, and the special BEGIN/END patterns. This page is a searchable, offline reference to all of them, each with a working snippet.

How it works

An awk program is a sequence of pattern { action } rules. For every input record:

The record is split into fields $1, $2, … up to $NF using the field separator FS; $0 is the whole record.
Each rule whose pattern matches runs its action. A pattern can be a regex /error/, a boolean expression NR > 1, a range /START/,/END/, or the special BEGIN/END markers.
Built-in variables track state: NR (record number), NF (field count), FNR (per-file record number), FILENAME, and the separators FS, OFS, RS, ORS.

String functions such as split, gsub, sub, substr, match, index and sprintf let you reshape text, while int, sqrt, log and friends handle numbers. print and printf produce output, and getline reads extra input.

Practical examples

Sum a column and print the total at the end:

awk '{ sum += $2 } END { print "total:", sum }' data.tsv

Print the last field of each line regardless of how many fields there are:

awk '{ print $NF }' file

Reformat a colon-delimited file into tab-separated output:

awk 'BEGIN { FS=":"; OFS="\t" } { print $1, $3 }' /etc/passwd

Skip the header row and process from line 2 onward:

awk 'NR > 1 { print $0 }' report.csv

Print only lines where a field matches a value:

awk -F',' '$3 == "ERROR" { print $0 }' logs.csv

Replace every occurrence of a pattern in a field (not the whole line):

awk '{ gsub(/foo/, "bar", $2); print }' file

Key built-in variables at a glance

Variable	Meaning
`$0`	The entire current record
`$1`, `$2`, …	Fields 1, 2, … (1-indexed)
`$NF`	The last field
`NR`	Total record number across all files
`FNR`	Record number within the current file (resets per file)
`NF`	Number of fields in the current record
`FS`	Input field separator (default: whitespace)
`OFS`	Output field separator (default: space)
`RS`	Input record separator (default: newline)
`ORS`	Output record separator (default: newline)
`FILENAME`	Name of the current input file

Things that trip people up

awk indexes from 1, not 0. $1 is the first field; substr(s,1,3) returns the first three characters.
Unset variables are 0 in numeric context and "" in string context — not errors.
Assigning to a field (e.g. $2 = "new") rebuilds $0 using OFS as the separator, which is how you do in-place column edits.
NR keeps counting across multiple input files; use FNR when you need per-file line numbers.
sub replaces only the first match; gsub replaces all matches. Both modify the target in place and return the match count.