Practice Use drills for recall and labs for real operating judgment.

LAB-TEXT-04 - Stream Editing with sed

Use sed to preview replacements, save safe in-place edits, and delete matching lines in a practice file.

TXT Text Processing

Stream Editing with sed

Use sed to preview replacements, save safe in-place edits, and delete matching lines in a practice file.

40 min INTERMEDIATE LINUX Curriculum-reviewed
Prerequisites
Lab LAB-TEXT-02 - The Surgeon's Knife: grep
Success criteria
  • Use sed to preview replacements, save safe in-place edits, and delete matching lines in a practice file.
  • Repeat the workflow without copy-paste or step-by-step prompting.
Safety notes
  • Preview sed changes before using `-i`, and prefer `-i.bak` while you are still learning.

Part A: The Field Guide

What This Lab Is Really About

sed is useful when you need to make a repeatable text change without opening a text editor each time.

The important learning goal is not speed. It is confidence:

  • preview the change
  • understand what matched
  • then save it safely

Command Reference

Safe sed workflow

sed ‘s/debug/info/’ config.txt sed ‘s/debug/info/g’ config.txt sed -i.bak ‘s/debug/info/g’ config.txt sed ‘/DEBUG/d’ app.log

Part B: The Drill Deck

Terminal required: work only on practice files in a disposable folder.

G
Guided Step by step - type exactly this and compare the result
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Exercise G1: Preview a Replacement

  1. Create a practice folder:
mkdir -p "$HOME/sed_lab"
cd "$HOME/sed_lab"
  1. Create a file:
printf "mode=debug\nlog=debug\n" > config.txt
  1. Preview a replacement without saving:
sed 's/debug/info/g' config.txt
  1. Read the original file with cat config.txt and confirm it has not changed yet.

Exercise G2: Save the Edit with a Backup

  1. Run:
sed -i.bak 's/debug/info/g' config.txt
  1. Read the updated file:
cat config.txt
  1. Confirm the backup exists:
ls

Exercise G3: Delete Matching Lines

  1. Create a small log file:
printf "INFO boot\nDEBUG warmup\nINFO ready\nDEBUG cache\n" > app.log
  1. Remove debug lines in the output:
sed '/DEBUG/d' app.log
  1. Confirm that only the non-debug lines are printed.
S
Solo Task described, hints available - figure it out
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Exercise S1: Use a Different Delimiter

  1. Create a file containing a path:
echo "path=/usr/local/bin/python" > path.conf
  1. Replace the path using # as the sed delimiter:
sed 's#/usr/local/bin/python#/opt/python3/bin#g' path.conf
  1. Confirm why a different delimiter is easier to read here than escaping every slash.

Exercise S2: Save the Path Change

  1. Make the previous change permanent with a backup:
sed -i.bak 's#/usr/local/bin/python#/opt/python3/bin#g' path.conf
  1. Verify both the changed file and the backup.
M
Mission Real scenario - no hints, combine multiple skills
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Mission M1: Patch a Practice Config File

Create a file named sshd_practice.conf with this content:

Port 22
X11Forwarding    yes
PermitRootLogin no

Now write one sed command that:

  1. makes a backup of the file
  2. changes X11Forwarding yes to X11Forwarding no
  3. keeps the rest of the file unchanged

Afterward, verify the result with grep X11Forwarding sshd_practice.conf.