sed for Stream Editing

1. sed is a line-oriented stream editor

flowchart LR
  I["input line"] --> P["pattern space"]
  P --> S["sed commands"]
  S --> O["output"]
  O --> I

By default, sed reads a line, applies its program, prints the pattern space, and repeats.

2. s/// performs substitution

printf '%s\n' 'environment=dev' |
sed 's/dev/staging/'

Add the g flag when every matching occurrence on the line should be replaced.

printf '%s\n' 'dev dev dev' |
sed 's/dev/staging/g'

3. Choose readable delimiters

printf '%s\n' '/srv/app/v1' |
sed 's#/srv/app/v1#/srv/app/v2#'

The substitution delimiter does not have to be /. Pick a delimiter that minimizes escaping.

4. Addresses restrict commands to selected lines

sed '/^DEBUG/d' app.log
sed -n '1,5p' app.log

-n suppresses default printing; the p command then explicitly emits selected lines.

5. Capture groups preserve parts of a record

printf '%s\n' 'version=2.14.7' |
sed -E 's/^version=([0-9]+)\..*/major=\1/'

6. Shell variables and sed programs are different languages

old='dev'
new='staging'
sed "s/^environment=$old$/environment=$new/" app.env

7. Prefer explicit temporary-file updates for portability

tmp=$(mktemp)

if sed 's/^enabled=false$/enabled=true/' app.conf >"$tmp"; then
  mv -- "$tmp" app.conf
else
  status=$?
  rm -f -- "$tmp"
  exit "$status"
fi

This avoids implementation differences around sed -i and gives you a place to validate transformed content before replacement.

8. sed is not a structured-data parser

9. Hands-on lab: transform a simple env file

mkdir -p "$HOME/devops-academy/bash/chapter08/lesson03"
cd "$HOME/devops-academy/bash/chapter08/lesson03"

cat > app.env <<'EOF'
SERVICE=api
ENVIRONMENT=dev
REPLICAS=2
DEBUG=true
EOF

tmp=$(mktemp)
if sed -E \
  -e 's/^ENVIRONMENT=.*/ENVIRONMENT=staging/' \
  -e 's/^REPLICAS=.*/REPLICAS=3/' \
  -e '/^DEBUG=/d' \
  app.env >"$tmp"; then
  mv -- "$tmp" app.env
else
  status=$?
  rm -f -- "$tmp"
  exit "$status"
fi

cat app.env

10. Knowledge check

11. Summary

sed excels at line-oriented substitutions, deletions, and addressed edits. Keep variable interpolation boundaries explicit, avoid portability surprises around in-place editing, and use structured parsers for structured data.

12. Further reading

• GNU sed manual.
• POSIX sed utility specification.
• BSD/macOS sed manual pages.
• ShellCheck documentation on shell quoting around sed.