# sed Quick Reference

*Stream editing, substitution, addresses, hold space, in-place transforms*

> Source: GNU sed Manual (gnu.org/software/sed) · MIT

## Basics

### Running sed

```
sed 's/old/new/' file.txt          # substitute first match per line
sed 's/old/new/g' file.txt         # substitute all matches per line
sed -n '5p' file.txt               # print only line 5
sed '3d' file.txt                  # delete line 3
echo "hello" | sed 's/hello/hi/'   # pipe input
```

### Command-Line Flags

| Command | Description |
|---------|-------------|
| `-n` | Suppress automatic printing; only print with `p` |
| `-e 'cmd'` | Execute a sed command (chain multiple with `-e`) |
| `-f script.sed` | Read commands from a file |
| `-i[suffix]` | Edit file in-place (optional backup suffix) |
| `-E / -r` | Use extended regular expressions |

## Substitution

### Substitution Syntax

```
sed 's/foo/bar/' f                 # first occurrence per line
sed 's/foo/bar/g' f                # all occurrences
sed 's/foo/bar/3' f                # 3rd occurrence only
sed 's/foo/bar/gi' f               # all, case-insensitive
sed 's|/usr/bin|/opt/bin|g' f      # alternate delimiter
```

### Substitution Flags

| Command | Description |
|---------|-------------|
| `g` | Replace all occurrences on the line |
| `N (number)` | Replace only the Nth occurrence |
| `p` | Print line if substitution was made |
| `w file` | Write substituted lines to file |
| `i / I` | Case-insensitive match (GNU) |

## Addresses

### Address Examples

```
sed '3s/a/b/' f                    # only on line 3
sed '2,5s/a/b/' f                  # lines 2 through 5
sed '/^#/d' f                      # delete lines starting with #
sed '/start/,/end/d' f             # delete range between patterns
sed '1~2d' f                       # delete odd-numbered lines (GNU)
```

### Address Types

| Command | Description |
|---------|-------------|
| `N` | Line number N |
| `$` | Last line |
| `N,M` | Line range from N to M |
| `/regex/` | Lines matching regex |
| `/regex1/,/regex2/` | Range from first match to second match |
| `N~step` | Starting at N, every step-th line (GNU) |
| `addr!` | Negate — apply to non-matching lines |

## Delete & Print

### Delete & Print Commands

```
sed '5d' f                         # delete line 5
sed '/^$/d' f                      # delete blank lines
sed -n '10,20p' f                  # print lines 10–20
sed -n '/error/p' f                # print lines matching pattern
sed '/debug/!d' f                  # keep only matching lines
```

### Command Reference

| Command | Description |
|---------|-------------|
| `d` | Delete pattern space, start next cycle |
| `D` | Delete up to first newline in pattern space |
| `p` | Print pattern space |
| `P` | Print up to first newline in pattern space |
| `q` | Quit after printing current pattern space |
| `Q` | Quit without printing (GNU) |

## Insert & Append

### Insert, Append, Change

```
sed '3i\inserted line' f           # insert before line 3
sed '3a\appended line' f           # append after line 3
sed '3c\replaced line' f           # replace line 3
sed '/marker/a\new line' f         # append after pattern match
```

### Commands

| Command | Description |
|---------|-------------|
| `i\text` | Insert text before current line |
| `a\text` | Append text after current line |
| `c\text` | Replace current line with text |
| `r file` | Read and append contents of file |
| `R file` | Read and append one line from file (GNU) |
| `w file` | Write pattern space to file |

## Hold Space

### Hold Space Commands

| Command | Description |
|---------|-------------|
| `h` | Copy pattern space to hold space |
| `H` | Append pattern space to hold space |
| `g` | Copy hold space to pattern space |
| `G` | Append hold space to pattern space |
| `x` | Exchange pattern and hold spaces |

### Hold Space Examples

```
sed -n '1!G;h;$p' f               # reverse lines (tac)
sed '/^$/{ x; s/\n//; x; }' f     # collapse hold on blank
sed -n 'H;${x;s/\n/ /g;p;}' f     # join all lines with space
```

## Multiple Commands

### Chaining Commands

```
sed -e 's/foo/bar/g' -e 's/baz/qux/g' f
sed 's/foo/bar/g; s/baz/qux/g' f
sed '/header/{ s/old/new/; s/foo/bar/; }' f
sed -f commands.sed input.txt
```

### Grouping & Branching

| Command | Description |
|---------|-------------|
| `{ cmd1; cmd2; }` | Group commands for same address |
| `:label` | Define a branch label |
| `b label` | Branch (jump) to label |
| `t label` | Branch if last `s///` succeeded |
| `T label` | Branch if last `s///` failed (GNU) |

## In-place Editing

### In-place Examples

```
sed -i 's/old/new/g' file.txt            # edit in place (GNU)
sed -i.bak 's/old/new/g' file.txt        # backup as file.txt.bak
sed -i '' 's/old/new/g' file.txt         # macOS in-place (no backup)
sed -i '/^#/d' config.txt                # remove comments in place
```

### Platform Notes

| Command | Description |
|---------|-------------|
| `GNU sed -i` | Suffix is optional; `-i` alone edits without backup |
| `BSD/macOS sed -i` | Requires suffix argument; use `-i ''` for no backup |
| `-i.bak` | Creates backup file with `.bak` extension |
| `Multiple files` | `sed -i 's/a/b/g' *.txt` edits all matching files |

## Regex

### Regex in sed

| Command | Description |
|---------|-------------|
| `.` | Any single character |
| `*` | Zero or more of preceding |
| `\+` | One or more (BRE) — `+` in ERE |
| `\?` | Zero or one (BRE) — `?` in ERE |
| `^` | Start of line |
| `$` | End of line |
| `[abc]` | Character class |
| `\( \)` | Capture group (BRE) — `()` in ERE |
| `\1, \2` | Back-reference to capture group |
| `&` | Entire matched string (in replacement) |

### Regex Examples

```
sed 's/[0-9]\+/NUM/g' f               # replace numbers
sed -E 's/(foo)(bar)/\2\1/g' f         # swap groups (ERE)
sed 's/.*/(&)/' f                      # wrap line in parens
sed 's/[ \t]*$//' f                    # strip trailing whitespace
```

## Common Patterns

### One-Liners

```
sed -n '1p' f                          # first line (head -1)
sed '$!d' f                            # last line (tail -1)
sed '/^$/d' f                          # remove blank lines
sed 's/^[ \t]*//' f                    # strip leading whitespace
sed '=' f | sed 'N;s/\n/\t/' f         # number lines
```

### Recipes

| Command | Description |
|---------|-------------|
| `Double-space` | `sed G` — append blank line after each line |
| `Remove HTML tags` | `sed 's/<[^>]*>//g'` |
| `Extract emails` | `sed -nE 's/.*([a-z]+@[a-z.]+).*/\1/p'` |
| `Comment lines` | `sed 's/^/# /'` — prefix each line with `# ` |
| `Trim blank lines at EOF` | `sed -e :a -e '/^\n*$/{$d;N;ba' -e '}'` |
| `Replace nth line` | `sed 'Nc\new text'` — replace line N |