Sed in Linux

1. Introduction


• sed (Stream Editor) is a powerful Unix tool used for:
  • searching
  • filtering
  • substituting
  • modifying text
  • batch editing files


• sed works on a stream of text line-by-line.
• It is especially powerful for automation, scripting, and command-line text processing.



2. Basic Syntax

sed 'command' filename

• Commands are placed inside single quotes.
• sed supports many editing commands; the most common is s for substitution.



3. Print File Contents (Default Behavior)

sed '' file.txt

• sed always prints each processed line unless told otherwise.
• '' means "no modifications."



4. Substitution Command (s)

sed 's/old/new/' file.txt

• Replaces the first occurrence of old with new in each line.



5. Global Replacement (g)

sed 's/foo/bar/g' file.txt

• g means "replace all occurrences per line."
• Without g, only the first match is replaced.



6. Using Different Delimiters

sed 's|/usr/local|/opt/apps|' file.txt

• | or # can be used as delimiters when the text contains slashes.
• Makes commands much easier to read.



7. Edit File In-Place (-i)

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

• Modifies the file directly.
• Use cautiously: changes cannot be undone unless you specify backup.

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

• Creates file.txt.bak before modifying.



8. Print Specific Lines

sed -n '3p' file.txt

• -n disables automatic printing.
• 3p prints line 3.

sed -n '5,8p' file.txt   # print lines 5 to 8




9. Delete Lines

sed '3d' file.txt

• Deletes line 3.

sed '2,5d' file.txt      # delete lines 2 to 5

• Useful for cleaning text files or logs.



10. Insert and Append Lines

sed '3i This is an inserted line' file.txt

• i inserts before a line.

sed '3a This is an appended line' file.txt

• a appends after a line.



11. Replace a Whole Line

sed '4c This replaces line 4 completely' file.txt

• c replaces an entire line with new content.



12. Using Patterns Instead of Line Numbers

sed '/error/d' file.log

• Deletes all lines containing the word error.

sed '/BEGIN/,/END/p' config.txt

• Works with ranges defined by patterns.



13. Multiple Commands

sed -e 's/foo/bar/' -e 's/x/y/' file.txt

• Apply multiple operations in order.

sed '
    s/foo/bar/g
    s/apple/orange/g
' file.txt

• Commands can also be written on multiple lines.



14. Advanced Example: Remove Empty Lines

sed '/^$/d' file.txt

• ^$ matches empty lines.



15. Advanced Example: Remove Trailing Spaces

sed 's/[ ]*$//' file.txt

• Replaces trailing spaces with nothing.



16. Advanced Example: Replace Only on Matching Lines

sed '/error/ s/failed/FAILED/' file.txt

• Only lines containing error will have failed replaced.



17. Hold Space and Pattern Space (For Advanced Users)


• sed maintains two memory areas:
  • pattern space — the current line
  • hold space — temporary buffer


• Commands like h, H, g, G allow advanced line manipulation.

sed 'h; $!d; x' file.txt

• This swaps first and last lines (example of advanced usage).



18. Summary of Common sed Commands



Command	Description	Example
s/a/b/	Substitute first occurrence	s/foo/bar/
s/a/b/g	Substitute all occurrences	s/old/new/g
p	Print	-n '3p'
d	Delete	'/error/d'
i	Insert before	'3i text'
a	Append after	'3a text'
c	Replace line	'4c new text'
-i	Edit file in-place	-i 's/a/b/' file