GNU awk (gawk) Skill

All output assumes gawk (awk --version shows GNU Awk). POSIX awk compatibility is not a goal; gawk extensions are used freely where they help.

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Core Principles

1. Prefer one-liners. Multi-line awk scripts only when a one-liner becomes unreadable.
2. Use -F to set the field separator explicitly. Never assume space.
3. Dry-run friendly by default. awk reads and prints; it never edits files in-place. Redirect explicitly (> out && mv out in) when the user wants in-place behavior.
4. Use BEGIN/END blocks for setup and summary output.
5. Chain with pipes when awk alone isn't the cleanest tool.
6. Never use awk to parse JSON or XML. Redirect to jq / xmllint and say why.

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Mental Model

awk 'BEGIN { setup } /pattern/ { action } END { summary }' file

• awk reads input line by line (records, split by RS, default \n).
• Each record is split into fields by FS (default: any whitespace).
• $0 = entire record. $1, $2, ... $NF = individual fields. NF = number of fields. NR = current record number. FNR = record number within current file.

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Flags

Flag	Meaning
-F SEP	Set field separator (string or regex)
-v VAR=VAL	Set a variable before execution
-f FILE	Read awk program from a file
--sandbox	Disable system(), getline, pipe (safe)

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Command Quick Reference

Print specific fields

# Print fields 1 and 3
awk '{print $1, $3}' file.txt

# Print last field
awk '{print $NF}' file.txt

# Print second-to-last
awk '{print $(NF-1)}' file.txt

# Custom output separator
awk '{print $1 "," $2}' file.txt

Filter lines

# Lines where field 3 > 100
awk '$3 > 100' file.txt

# Lines matching a regex
awk '/error/' file.txt

# Lines where field 2 matches a pattern
awk '$2 ~ /timeout/' file.txt

# Negate match
awk '$2 !~ /debug/' file.txt

# Multiple conditions
awk '$1 == "GET" && $9 >= 500' access.log

Aggregation

# Sum a column
awk '{sum += $3} END {print sum}' file.txt

# Count matching lines
awk '/error/ {count++} END {print count}' file.txt

# Average
awk '{sum += $2; n++} END {print sum/n}' file.txt

# Min / max
awk 'NR==1 || $3 < min {min=$3} END {print min}' file.txt
awk 'NR==1 || $3 > max {max=$3} END {print max}' file.txt

Frequency / histogram

# Count occurrences of each value in field 1
awk '{count[$1]++} END {for (k in count) print count[k], k}' file.txt | sort -rn

# Top 10 IPs in nginx log
awk '{print $1}' access.log | sort | uniq -c | sort -rn | head -10
# Pure awk alternative:
awk '{c[$1]++} END {for (ip in c) print c[ip], ip}' access.log | sort -rn | head -10

Field reformat / transform

# Swap fields 1 and 2
awk '{print $2, $1}' file.txt

# Prepend a string to field 1
awk '{$1 = "prefix_" $1; print}' file.txt

# Change field separator in output (OFS)
awk 'BEGIN{OFS=","} {print $1,$2,$3}' file.txt

# Remove a field (shift left from field 3)
awk '{out=""; for (i=1;i<=NF;i++) if (i!=2) out = out (out?OFS:"") $i; print out}' file.txt

Line ranges

# Print lines 5 to 15
awk 'NR>=5 && NR<=15' file.txt

# Print between two patterns (inclusive)
awk '/START/,/END/' file.txt

# Skip the header line
awk 'NR > 1' file.txt

# Print only the header
awk 'NR == 1' file.txt

CSV / TSV processing

# TSV: print columns 1 and 4
awk -F'\t' '{print $1, $4}' data.tsv

# CSV (simple, no quoted commas): sum column 3
awk -F',' 'NR>1 {sum+=$3} END {print sum}' data.csv

# Add a computed column to CSV
awk -F',' 'BEGIN{OFS=","} NR>1 {$4=$2*$3; print}' data.csv

# Filter CSV rows where column 2 > 50
awk -F',' '$2 > 50' data.csv

# Convert TSV to CSV
awk 'BEGIN{FS="\t"; OFS=","} {$1=$1; print}' data.tsv

For CSV with quoted fields containing commas, use gawk with a proper CSV library or switch to miller (mlr) / csvkit.

Log processing

# Extract HTTP 5xx lines from nginx access log
awk '$9 >= 500 && $9 < 600' access.log

# Sum bytes transferred (field 10) per status code (field 9)
awk '{bytes[$9]+=$10} END {for (s in bytes) print s, bytes[s]}' access.log | sort

# Requests per minute (timestamp in field 4, format [DD/Mon/YYYY:HH:MM:SS])
awk '{match($4, /[0-9]{2}\/[A-Za-z]+\/[0-9]{4}:[0-9]{2}:[0-9]{2}/, m); c[m[0]]++}
     END {for (t in c) print t, c[t]}' access.log | sort

# Strip log prefix, print only message
awk '{$1=$2=$3=""; print substr($0,4)}' app.log

Multi-file processing

# Print filename alongside each line
awk '{print FILENAME, $0}' *.log

# Process file 1 as a lookup table, file 2 as data
awk 'NR==FNR {map[$1]=$2; next} {print $0, map[$1]}' lookup.txt data.txt

In-place editing

# Portable awk has no in-place edit; prefer a temp file
awk '{gsub(/foo/, "bar"); print}' file.txt > file.tmp && mv file.tmp file.txt

# gawk-only in-place edit
gawk -i inplace '{gsub(/foo/, "bar"); print}' file.txt

# All .rb files
for f in src/**/*.rb; do
  awk '{gsub(/OldClass/, "NewClass"); print}' "$f" > "$f.tmp" && mv "$f.tmp" "$f"
done

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Built-in Functions

String

Function	Description
length(s)	String length
substr(s, start, len)	Substring (1-indexed)
index(s, t)	Position of t in s (0 = not found)
split(s, arr, sep)	Split s into arr by sep, returns count
gsub(re, rep, s)	Global replace in s (default: $0)
sub(re, rep, s)	First replace in s
match(s, re, arr)	Match re in s; fills arr with groups
sprintf(fmt, ...)	Format string (like printf)
tolower(s) / toupper(s)	Case conversion
gensub(re, rep, how, s)	gawk: replace with back-references (\1)

Math

Function	Description
int(x)	Truncate to integer
sqrt(x)	Square root
log(x) / exp(x)	Natural log / e^x
sin(x) / cos(x)	Trig (radians)
rand()	Random [0,1)
srand(seed)	Seed RNG

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gawk Extensions Worth Knowing

# OFMT: control float output precision
awk 'BEGIN{OFMT="%.2f"} {print $1 * 1.15}' prices.txt

# Pipe to a command per line
awk '{print $0 | "mail -s alert ops@example.com"}' alerts.txt

# Time functions
awk 'BEGIN{print strftime("%Y-%m-%d", systime())}'

# Two-way pipe (gawk only): close the writer before reading from sort
awk 'BEGIN{cmd="sort"} {print $0 |& cmd} END{close(cmd, "to"); while ((cmd |& getline line) > 0) print line}' file.txt

# Built-in CSV parsing (gawk 5.3+)
gawk --csv '{print $2}' data.csv

# Legacy/simple CSV only: FPAT misses empty fields and escaped quotes
awk 'BEGIN{FPAT="([^,]+)|(\"[^\"]+\")"} {print $2}' data.csv

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Combining with Other Tools

# awk + sort: frequency table
awk '{print $1}' file.txt | sort | uniq -c | sort -rn

# awk + sed: awk extracts, sed cleans
awk '/error/ {print $5}' app.log | sed 's/[^a-z0-9]//g'

# awk + xargs: act on extracted values
awk '$3 > 1000 {print $1}' jobs.txt | xargs kill

# awk as a replacement for cut + grep pipeline
# Instead of: grep 'GET' access.log | cut -d' ' -f7
awk '$6 ~ /GET/ {print $7}' access.log

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Output Format

When responding to an awk request:

1. Show the one-liner in a code block, ready to copy-paste.
2. Add a one-line comment per non-obvious part.
3. If the task fits a multi-step pipeline better, show that pipeline.
4. If the task is better served by jq, mlr (miller), python, or perl, say so briefly and optionally show that alternative too.

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Gotchas

• Portable awk has no in-place editing. Use > tmp && mv tmp original unless you intentionally depend on gawk's -i inplace extension.
• Field assignment triggers rebuild of $0 using OFS. Set OFS in BEGIN if you rely on this.
• Arrays are associative only (hash maps). There are no integer-indexed arrays with guaranteed order; iterate with for (k in arr) (order is undefined). Use a separate key list + asort()/asorti() (gawk) when order matters.
• Rebuilding vs re-splitting. Assigning to a field rebuilds $0 from fields using OFS ($1=$1 is the common idiom). Assigning to $0, or sub()/gsub() on $0, re-splits fields using the current FS.
• Regex constants vs strings. /foo/ is a regex literal; "foo" is a string. $1 ~ /foo/ is correct; $1 ~ "foo" also works but avoids the literal syntax.
• Integer division. 5/2 returns 2.5 in awk (unlike shell). Use int(5/2) for truncation.
• macOS ships BSD/one-true-awk, not gawk. gawk must be installed via Homebrew. FPAT, gensub(), strftime(), and |& are gawk-only.