Identity

Property	Value
Binary	`awk`, `sed`, `gawk`, `mawk`
Config	No persistent config — invoked directly
Logs	No persistent logs — output to terminal
Type	CLI tool
Install	`apt install gawk` / `dnf install gawk` (sed is pre-installed)

Key Operations

Task	Command
sed: substitute first match per line	`sed 's/old/new/' file`
sed: substitute all matches per line	`sed 's/old/new/g' file`
sed: in-place edit (GNU)	`sed -i 's/old/new/g' file`
sed: in-place with backup	`sed -i.bak 's/old/new/g' file`
sed: delete lines matching pattern	`sed '/pattern/d' file`
sed: delete blank lines	`sed '/^$/d' file`
sed: print specific line number	`sed -n '5p' file`
sed: print line range	`sed -n '5,10p' file`
sed: address range (between two patterns)	`sed -n '/start/,/end/p' file`
sed: append line after match	`sed '/pattern/a\new line' file`
sed: insert line before match	`sed '/pattern/i\new line' file`
awk: print specific column	`awk '{print $2}' file`
awk: custom field separator	`awk -F: '{print $1}' /etc/passwd`
awk: conditional on column value	`awk '$3 > 100 {print $0}' file`
awk: pattern-action block	`awk '/error/ {print $0}' file`
awk: sum a column	`awk '{sum += $2} END {print sum}' file`
awk: BEGIN/END blocks	`awk 'BEGIN {FS=":"} {print $1} END {print "done"}' file`
awk: print last field	`awk '{print $NF}' file`
awk: print all but first field	`awk '{$1=""; print $0}' file`

Symptom	Cause	Fix
`sed -i` errors on macOS	BSD sed requires empty string arg: `sed -i ''`	Use `sed -i.bak` (works both places) or install GNU sed via brew
sed backreference `$1` not working	sed uses `\1`, not `$1` for capture groups	Change to `sed 's/$pattern$/\1/'` or use extended regex: `sed -E 's/(pattern)/\1/'`
awk `-F` separator not splitting correctly	Multiple spaces or tabs as separator	Use `-F'[ \t]+'` for any whitespace, or default FS (no flag) which splits on any whitespace
`$0` includes old `$1` content after assignment	Assigning `$1=""` modifies `$0` with current OFS	Set `OFS=":"` in BEGIN if separator matters after field modification
`gawk` feature fails in POSIX awk	`gensub`, `match` with array, etc. are gawk extensions	Explicitly invoke `gawk` or rewrite using POSIX-compatible alternatives
Substitution changes too many lines	No address restriction on sed command	Add a line number or pattern address: `sed '3s/old/new/'` or `sed '/pattern/s/old/new/'`

GNU vs BSD sed -i syntax: GNU sed (Linux): sed -i 's/a/b/' file. BSD sed (macOS): sed -i '' 's/a/b/' file. The safest cross-platform form is sed -i.bak which works on both and leaves a backup.
awk backreferences use \1, sed uses \1 too: The confusion comes from people expecting shell-style $1. In both tools, captured groups are \1, \2, etc. With sed -E or awk ERE, the group syntax is () without backslashes, but the reference is still \1.
Default FS splits on any whitespace: When no -F is given, awk treats runs of spaces and tabs as a single separator and ignores leading whitespace. Convenient, but surprising when the input has intentional spaces within fields.
gawk has extensions POSIX awk lacks: gensub(), multi-dimensional arrays, PROCINFO, and third-arg match() are gawk-only. Scripts relying on these will silently fail or error on systems running mawk (Ubuntu default) or nawk.
sed processes one line at a time by default: Multi-line patterns require hold-space tricks (H, G, N) or a tool better suited to the job (awk with RS, perl, or python). Attempting multi-line sed without understanding hold space produces subtle bugs.