Built-in Variables

Print line numbers with content (NR)

cat <<'EOF' > /tmp/demo.txt
10.50.1.20  ise-01   active
10.50.1.50  ad-dc    active
10.50.1.1   pfsense  standby
10.50.1.90  bind     active
EOF
awk '{print NR, $0}' /tmp/demo.txt

Show field count per line (NF)

cat <<'EOF' > /tmp/demo.txt
10.50.1.20  ise-01   active
10.50.1.50  ad-dc
10.50.1.1   pfsense  standby  primary
EOF
awk '{print NR": "NF" fields"}' /tmp/demo.txt

Print only line 5 (NR selection)

awk 'NR==5' /etc/passwd

Print lines 5 through 10

awk 'NR>=5 && NR<=10' /etc/passwd

Skip header line

cat <<'EOF' > /tmp/demo.txt
hostname,ip,status,role
ise-01,10.50.1.20,active,pan
ad-dc,10.50.1.50,active,dc
pfsense,10.50.1.1,active,firewall
EOF
awk 'NR>1' /tmp/demo.txt

Count total lines — equivalent to wc -l

awk 'END{print NR}' /etc/passwd

Last field of each line ($NF)

ss -tlnp 2>/dev/null | awk '{print $NF}'

Set input field separator in BEGIN block

awk 'BEGIN{FS=":"} {print $1, $3}' /etc/passwd

Change output separator — semicolons between records

cat <<'EOF' > /tmp/demo.txt
ise-01
ad-dc
pfsense
bind
vyos
EOF
awk 'BEGIN{ORS="; "} {print $1}' /tmp/demo.txt

Multi-character field separator

cat <<'EOF' > /tmp/demo.txt
ise-01::10.50.1.20::active
ad-dc::10.50.1.50::active
pfsense::10.50.1.1::standby
EOF
awk 'BEGIN{FS="::"} {print $1, $2}' /tmp/demo.txt

Track filename when processing multiple files

echo -e "alpha\nbravo" > /tmp/demo1.txt
echo -e "charlie\ndelta" > /tmp/demo2.txt
awk 'FNR==1 {print "=== " FILENAME " ==="} {print FNR, $0}' /tmp/demo1.txt /tmp/demo2.txt

Format /etc/passwd as CSV using BEGIN separators

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