awk — Quoting

WRONG — shell expands $1 inside double quotes

cat <<'EOF' > /tmp/demo.txt
10.50.1.1  gateway   UP
10.50.1.20 ise-01    UP
10.50.1.50 ad-dc     DOWN
EOF

# WRONG — shell eats $1 before awk sees it
awk "{print $1}" /tmp/demo.txt

# CORRECT — single quotes protect awk variables
awk '{print $1}' /tmp/demo.txt

Pass shell variables safely with -v

pattern="DOWN"
awk -v pat="$pattern" '$0 ~ pat {print}' /tmp/demo.txt

WRONG — variable inside single quotes is literal

cat <<'EOF' > /tmp/demo.txt
2026-04-11 ERROR auth failure for user evan
2026-04-11 INFO  session started
2026-04-11 ERROR certificate expired on ise-01
EOF

# WRONG — $var is literal text, not shell variable
var="ERROR"
awk '/$var/ {print}' /tmp/demo.txt

# CORRECT — use -v flag
awk -v pat="$var" '$0 ~ pat' /tmp/demo.txt

Regex in variable with slashes — use index() not ~

echo '/var/log/messages:auth failure' | \
  awk -v pat="/var/log" 'index($0, pat)'

# index() returns position (truthy) for literal substring match
# ~ would choke on the unescaped slashes in the pattern

Pass multiple shell variables

cat <<'EOF' > /tmp/demo.txt
eth0  10.50.1.90   53   1200
eth0  10.50.1.20   443  85
eth1  10.50.1.1    80   3500
eth0  10.50.1.50   88   450
EOF

min=100
max=2000
awk -v lo="$min" -v hi="$max" '$4+0 >= lo && $4+0 <= hi' /tmp/demo.txt