sed - Linux

Overview

The sed command, short for stream editor, is a powerful utility for parsing and transforming text in Unix and Unix-like operating systems. sed is primarily used for filtering and transforming text from files or input streams. Its primary purpose lies in searching, finding, replacing, inserting, and deleting strings and lines. sed is most effective for its ability to efficiently process text without opening an interactive text editor.

Syntax

The basic syntax of the sed command is:

sed [options]... {script-only-if-no-other-script} [input-file]...
  • options: Command-line options that modify the behavior of sed.
  • script: Series of commands that sed will execute.
  • input-file: File(s) that sed reads. If none specified, sed reads from the standard input.

Options/Flags

Here is a list of commonly used options in sed:

  • -e script: Add the script to the commands to be executed.
  • -f script-file: Add the contents of script-file to the commands to be executed.
  • -i[SUFFIX]: Edit files in-place (makes backup if extension supplied).
  • -n: Suppress automatic printing; sed will only output lines explicitly told to print.

Typical use cases:

  • Use -e to execute multiple editing commands.
  • Use -i to directly modify a file instead of outputting the changed version to the standard output.

Examples

Example 1: Simple Search and Replace

Replace all occurrences of ‘hello’ with ‘world’ in a file:

sed 's/hello/world/g' filename.txt

Example 2: In-place Editing

Modify a file in-place and replace ‘foo’ with ‘bar’:

sed -i 's/foo/bar/g' filename.txt

Example 3: Print Specific Lines

Print the first 10 lines of a file:

sed -n '1,10p' filename.txt

Example 4: Delete Lines

Delete line 3 from a file:

sed '3d' filename.txt

Example 5: Advanced Pattern Use

Change “red” to “blue” only for lines containing “color”:

sed '/color/s/red/blue/' filename.txt

Common Issues

  • Modifying Files Directly: When using the -i option without specifying a backup, original files are overwritten, leading to data loss if not used carefully. Always consider backing up files before using -i.

  • Complex Regular Expressions: sed uses by default a basic type of regular expression (BRE), and some expected regex features will not work unless you use extended regular expressions with the -E flag.

Integration

sed can be very powerful when used in combination with other tools like awk for text processing, or within shell scripts to automate batch editing tasks. Here’s an example of sed used with find:

find . -type f -name "*.txt" -exec sed -i 's/old/new/g' {} +

This command finds all .txt files in the current directory and subdirectories, replacing ‘old’ with ‘new’ in each file.

  • awk: A text-processing language that is more full-featured but also more complex.
  • grep: Command-line tool for searching plain-text data sets for lines that match a regular expression.
  • tr: Translates or deletes characters from stdin, used for basic transformations.

For more in-depth tutorials or examples, refer to the GNU sed official documentation.