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Quick sed reference

sed (stream editor) is a command-line utility for parsing and transforming text

Usage

$ sed 'command' filename

Substitution:

  • Replaces the first occurrence on each line

Suppose you have a file named names.txt:

John Smith
Jane Doe
John Johnson

To replace the first occurrence of “John” with “Jonathan” on each line:

$ sed 's/John/Jonathan/' names.txt

Output:

Jonathan Smith
Jane Doe
Jonathan Johnson

Global substitution:

  • Replaces all occurrences of a pattern in each line

Using the same names.txt file:

$ sed 's/John/Jonathan/g' names.txt

Output:

Jonathan Smith
Jane Doe
Jonathan Jonathanson

Delete lines:

Consider a log file app.log:

INFO: Application started
ERROR: Database connection failed
INFO: User logged in
ERROR: Invalid input

To remove all error lines:

$ sed '/ERROR/d' app.log

Output:

INFO: Application started
INFO: User logged in

In-place editing:

  • The -i option allows sed to modify files directly

Let’s say you have a configuration file named `config.txt:

Server Configuration
HOST=localhost
PORT=8080
DEBUG=true
ENVIRONMENT=development

To change the development environment to production, you could use:

$ sed -i 's/development/production/' config.txt

After running this command, `config.txt will be modified directly:

# Server Configuration
HOST=localhost
PORT=8080
DEBUG=true
ENVIRONMENT=production

Using find and exec with sed

The find command can be used together with sed to apply transformations= to multiple files in a directory structure. This is particularly useful wh= en you need to make changes across many files at once.

Syntax:

$ find [path] [expression] -exec sed [sed_options] [sed_script] {} +

Example:

Let’s say you have a project directory with multiple .txt files, and you = want to replace all occurrences of “colour” with “color” in these files:

$ find . -name "*.txt" -exec sed -i 's/colour/color/g' {} +

Breakdown:

  • find .: Start searching in the current directory
  • -name "*.txt": Look for files ending with .txt
  • -exec: Execute a command on each file found
  • sed -i 's/colour/color/g': The sed command to run (in-place edit, globa= l substitution)
  • {} +: {} is replaced with the filename, and + tells find to pass as= many filenames as possible to one invocation of sed

More examples:

  1. Replace “foo” with “bar” in all .c files, creating backups:

    $ find . -name "*.php" -exec sed -i.bak 's/foo/bar/g' {} +

  2. Delete all lines containing “DEBUG” in .log files:

    $ find /var/log -name "*.log" -exec sed -i '/DEBUG/d' {} +

  3. Add a new line after each line containing “START” in .conf files:

    $ find /etc -name "*.conf" -exec sed -i '/START/a\New line here' {} +

  4. Combine multiple sed operations:

    $ find . -type f -exec sed -i -e 's/old/new/g' -e '/pattern/d' {} +