sed Command Guide

Overview

sed (Stream Editor) is a powerful Unix utility for parsing and transforming text using simple commands and regular expressions. It is commonly used for tasks such as search-and-replace, line deletion, and text substitution in scripts or pipelines.

Basic Syntax 

sed [options] 'command' file

Common options: | Option | Description | |———|————-| | -n | Suppress automatic printing of pattern space | | -e | Add script to execute | | -i | Edit file in place | | -r | Use extended regular expressions | | -f | Read sed commands from a file |

Basic Examples 

sed -n '5p' file.txt        # Print line 5
sed -n '1,3p' file.txt      # Print lines 1 to 3
sed -n '/pattern/p' file.txt  # Print lines matching pattern

Replace Text 

sed 's/foo/bar/' file.txt             # Replace first occurrence of 'foo' with 'bar'
sed 's/foo/bar/g' file.txt            # Replace all occurrences on each line
sed -i 's/foo/bar/g' file.txt         # Replace in file directly
sed 's/[0-9]//g' file.txt             # Remove all digits

Delete Lines 

sed '1d' file.txt             # Delete first line
sed '1,5d' file.txt           # Delete lines 1 through 5
sed '/pattern/d' file.txt     # Delete lines matching pattern

Insert / Append Text 

sed '2i\Inserted before line 2' file.txt    # Insert before line 2
sed '3a\Appended after line 3' file.txt     # Append after line 3

Modify Specific Lines 

sed '5s/old/new/' file.txt      # Replace only on line 5
sed '/error/s/fail/pass/g' file.txt  # Replace text only in lines containing 'error'

Using sed with Pipelines

Replace Text from stdin 

echo "hello world" | sed 's/world/sed/'

Remove Blank Lines 

cat file.txt | sed '/^$/d'

Number All Lines 

sed = file.txt | sed 'N;s/\n/\t/'

Regular Expressions in sed

  • . — Matches any character
  • ^ — Start of line
  • $ — End of line
  • * — Zero or more occurrences
  • \{n,m\} — Repetition count
  • [] — Character class
  • () — Grouping (use -r or -E for extended)
  • | — Alternation (OR) in extended regex

Example:

sed -r 's/(cat|dog)/animal/g' file.txt

Advanced Examples

Replace Only on Specific Line Range 

sed '10,20s/error/fixed/g' log.txt

Change Text Between Patterns 

sed '/BEGIN/,/END/s/foo/bar/g' file.txt

sed -n '/pattern/!p' file.txt

Replace Using Variables in Shell 

search="foo"
replace="bar"
sed -i "s/${search}/${replace}/g" file.txt

Remove Comments and Blank Lines 

sed '/^#/d;/^$/d' config.txt

Replace Tabs with Spaces 

sed 's/\t/    /g' file.txt

Tips and Tricks

1. Backup Before Editing In-Place 

sed -i.bak 's/foo/bar/g' file.txt

2. Chain Multiple Commands 

sed -e 's/foo/bar/' -e '/baz/d' file.txt

3. Read from File with Commands 

sed -f script.sed file.txt

4. Use & to Reuse the Matched Pattern 

echo "error" | sed 's/.*/(&)/'     # Output: (error)

5. Debug Complex Expressions 

sed -n 'l' file.txt

6. Replace with Newline Characters 

sed 's/;/\n/g' file.txt

7. Swap Two Words 

echo "foo bar" | sed 's/\(.*\) \(.*\)/\2 \1/'

8. Extract Text Between Delimiters 

echo "<tag>value</tag>" | sed -n 's/.*<tag>\(.*\)<\/tag>.*/\1/p'

9. Replace nth Occurrence in Line 

sed 's/foo/bar/2' file.txt   # Replace 2nd occurrence only

10. Work with Multiline Patterns (GNU sed only) 

sed ':a;N;$!ba;s/old/new/g' file.txt

Common Use Cases

Task Command
Remove leading whitespace sed 's/^\s*//' file.txt
Remove trailing whitespace sed 's/\s*$//' file.txt
Double-space file sed G file.txt
Print only unique lines sed '$!N; /\n.*$/!p; D' file.txt
Convert lowercase to uppercase sed 's/[a-z]/\U&/g' file.txt
Convert uppercase to lowercase sed 's/[A-Z]/\L&/g' file.txt

References