SED is most commonly used command in UNIX it is known as stream editor. This command can be used to perform following different types of operations:
- searching
- find and replace
- insertion
- deletion
SED command in linux supports regular expression which allows you to perform complex pattern matching.
sed <options> <script> <input-file>
In order to learn this command better let's take following text as an example. Create a file called test.txt and add following contents:
Then he waddled away, waddle waddle Then he waddled away, waddle waddle Then he waddled away, waddle waddle
Using sed command to search and replace a string in a file
Let's replace word We in above example to Our using sed command as shown below:
# search waddle in test.txt and replace with toddle sed 's/waddle/toddle/' test.txt
Output:
Then he toddled away, waddle waddle Then he toddled away, waddle waddle Then he toddled away, waddle waddle
Replacing the nth occurrence of a pattern in a line
Let say that in above example we only wanted to replace nth occurance in a line. Use following syntax:
sed 's/[search]/[replace]/[nth]' <file-name>
You can use following code to replace nth occurance in a line.
# search waddle in test.txt and replace with toddle sed 's/waddle/toddle/2' test.txt
Output:
Then he waddled away, toddle waddle Then he waddled away, toddle waddle Then he waddled away, toddle waddle
Replacing from the nth occurrence till end of a pattern in a line
You can use following code to replace from nth occurance until the end of a line.
# search waddle in test.txt and replace with toddle # this will start searching from 2nd occurance until end of the line sed 's/waddle/toddle/2g' test.txt
Output:
Then he waddled away, toddle toddle Then he waddled away, toddle toddle Then he waddled away, toddle toddle
Replacing from the nth occurrence till end of a pattern on a specific line
You can use following code to replace from nth occurance until the end of a line for line number 3.
# search waddle in test.txt and replace with toddle # this will start searching from 2nd occurance until end of the line sed '3 s/waddle/toddle/2g' test.txt
Output:
Then he waddled away, waddle waddle Then he waddled away, waddle waddle Then he waddled away, toddle toddle
Followings are some more advanced examples:
| Command | Summary |
|---|---|
| sed 's/waddle/toddle/p' test.txt | Duplicate the replaced lines |
| sed -n 's/waddle/toddle/p' test.txt | Printing only the replaced lines |
| sed '1,2 s/waddle/toddle/' test.txt | Replacing string for specific lines |
| sed '2,$ s/waddle/toddle/' test.txt | Replace string from line 2 till end of the file |
| sed '2d' test.txt | Delete the 2nd line |
| sed '$d' test.txt | Delete the last line |
| sed '2,$d' test.txt | Delete from 2nd till end of the file |
| sed '/^$/d' test.txt | Delete all blank lines in a file |
| sed 's/^/. /' test.txt | Insert 5 spaces to the left of every lines |
| sed = test.txt | sed 'N;s/\n/ /' | add line number in front of each line |
| sed '/./=' test.txt | sed '/./N; s/\n/ /' | add line number in front of each line and avoid blank line number |
| sed '/waddle/d' test.txt | delete all the line that contains word waddle |
| sed '3~2d' test.txt | start from line 3 and delete every 2nd line |
| sed ‘/pattern/,+2d’ test.txt | Delete the lines which matches the pattern and 2 lines after to that |
| sed -n 'x,yp' test.txt | Viewing a file from x to y range |
| sed 'x,yd' test.txt | View entire file except range x to y |
| sed -n '4'p test.txt | Print nth line of the file |
| sed -n 'n,$'p test.txt | Print from nth line until the end of the file |
| sed -n /pattern/p test.txt | Print only lines that matches the pattern |
| sed -n ‘/pattern/,+xp’ test.txt | Print the lines which matches the pattern up-to the next xth lines |
| sed G test.txt | Double space line in a file |
| sed '/^$/d;G' test.txt | Double space lines in a file there should not be more then one blank line |
| sed 'G;G' test.txt | Triple space a file |
| sed 'n;d' test.txt | Undo double-spacing (assumes even-numbered lines are always blank) |
| sed -n '$=' test.txt | Count total lines in a file |
| sed 's/^[ \t]*//' test.txt | Delete leading whitespace (spaces, tabs) from front of each line |
| sed 's/[ \t]*$//' test.txt | Delete trailing whitespace (spaces, tabs) from end of each line |
| sed 's/^[ \t]*//;s/[ \t]*$//' test.txt | Delete BOTH leading and trailing whitespace from each line |
| sed '/baz/s/foo/bar/g' test.txt | Substitute "foo" with "bar" ONLY for lines which contain "baz" |
| sed '/baz/!s/foo/bar/g' test.txt | Substitute "foo" with "bar" EXCEPT for lines which contain "baz" |
| sed '$!N;s/\n/ /' test.txt | Join pair of lines together by removing newline |
| sed 2q test.txt | Print first two lines of the file just like head command |
| sed -e :a -e '$q;N;11,$D;ba' test.txt | Print last 10 lines of the file just like tail command |
| sed '/AAA/!d; /BBB/!d; /CCC/!d' test.txt | Grep for AAA and BBB and CCC (in any order) |
| sed '/AAA.*BBB.*CCC/!d' test.txt | Grep for AAA and BBB and CCC (in that order) |
| sed -n '/^.\{65\}/p' | Print only lines of 65 characters or longer |
| sed -n '/Iowa/,/Montana/p' test.txt | Print section of file between two regular expressions (inclusive), case sensitive |
| sed '/Iowa/,/Montana/d' test.txt | Print all of file EXCEPT section between 2 regular expressions |
| sed '$!N; /^\(.*\)\n\1$/!P; D' test.txt | Delete duplicate, consecutive lines from a file, keep the first line as is |