Shell Scripting — Advanced SED: Fundamentals

# SED cycle (repeated for each input line):
# 1. Read line into pattern space
# 2. Apply commands in sequence to pattern space
# 3. Print pattern space (unless -n)
# 4. Clear pattern space and go to next line

# ── Basic invocation ──────────────────────────────────────
sed 'COMMAND' file.txt          # apply to file
echo "text" | sed 'COMMAND'     # apply to stdin
sed -n 'COMMAND' file.txt       # -n: suppress auto-print
sed -i 'COMMAND' file.txt       # -i: edit file in place (GNU sed)
sed -i.bak 'COMMAND' file.txt   # -i.bak: in-place with backup
sed -E 'COMMAND' file.txt       # -E: extended regex (no backslash)

# ── Multiple commands ─────────────────────────────────────
sed '/ERROR/d; s/WARN/WARNING/' file.txt   # semicolon separator
sed -e '/ERROR/d' -e 's/WARN/WARNING/'   # multiple -e flags
sed '
/ERROR/d
s/WARN/WARNING/
' file.txt                                 # multiline string

# ── GNU sed vs BSD sed (macOS) ─────────────────────────────
# GNU sed: sed -i 's/old/new/' file        (no space needed)
# BSD sed: sed -i '' 's/old/new/' file     (empty string required)
# Safe cross-platform in-place edit:
if [[ "$(uname)" == "Darwin" ]]; then
  sed -i '' 's/old/new/g' file.txt
else
  sed -i 's/old/new/g' file.txt
fi

# ── s — substitute (most used) ────────────────────────────
sed 's/old/new/'        # replace first occurrence per line
sed 's/old/new/g'       # replace all occurrences (global)
sed 's/old/new/2'       # replace second occurrence only
sed 's/old/new/i'       # case-insensitive (GNU sed)
sed 's/old/new/gp'      # replace all + print if changed

# ── d — delete line ────────────────────────────────────────
sed '/pattern/d'        # delete lines matching pattern
sed '/^#/d'             # delete comment lines
sed '/^[[:space:]]*$/d' # delete blank lines
sed '1d'                # delete first line
sed '$d'                # delete last line
sed '2,5d'              # delete lines 2 through 5

# ── p — print ─────────────────────────────────────────────
sed -n '/ERROR/p'       # print only lines matching ERROR
sed -n '5p'             # print only line 5
sed -n '5,10p'          # print lines 5-10
sed -n '$p'             # print last line

# ── q — quit ──────────────────────────────────────────────
sed '5q'                # print first 5 lines and quit (like head -5)
sed '/ERROR/q'          # print up to and including first ERROR

# ── a, i, c — append, insert, change ─────────────────────
sed '/pattern/a\Added after'    # append line after match
sed '/pattern/i\Added before'   # insert line before match
sed '/pattern/c\Replaced line'  # replace entire line

# ── y — transliterate ─────────────────────────────────────
sed 'y/abc/ABC/'         # a→A, b→B, c→C (like tr)
sed 'y/abcdefghijklmnopqrstuvwxyz/ABCDEFGHIJKLMNOPQRSTUVWXYZ/'  # uppercase

# SED address: [address] command
# Address specifies WHICH lines the command applies to

# ── Numeric addresses ─────────────────────────────────────
sed '1s/old/new/'        # only first line
sed '$s/old/new/'        # only last line
sed '2,5s/old/new/'      # lines 2 through 5
sed '2,~5s/old/new/'     # line 2 through next mult of 5 (GNU)
sed '0~2s/old/new/'      # every 2nd line starting at 0 (GNU)
sed '1~2s/old/new/'      # every odd line (GNU)

# ── Regex addresses ───────────────────────────────────────
sed '/ERROR/s/old/new/'  # only on ERROR lines
sed '/^#/d'              # delete comment lines

# ── Range addresses ───────────────────────────────────────
sed '/START/,/END/s/old/new/'   # between START and END
sed '/START/,/END/d'            # delete block between START and END
sed '1,/PATTERN/d'              # delete from line 1 to first PATTERN
sed '/PATTERN/,$d'              # delete from PATTERN to end of file

# ── Negated address (!) ────────────────────────────────────
sed '/ERROR/!d'          # delete lines NOT matching ERROR
sed '1!s/old/new/'       # substitute on all but first line