How to “SED” Options and its usage



Sed – The Stream Editor

A stream editor is used to perform basic text transformations on an input file. Sed command is mainly used to replace the text in a file. But it is a powerful text processing tool.

Some uses of sed command are explained below with examples:

Consider the text file “test” as an example

1) Replace all the occurrence of the pattern in a line

Here in the above example, sed is used to replace all occurrences of  “system” with “software” within the file test.

Let us have a look into the switches used for the task

   Denotes
 s  Substitution operation
 /  Delimiter
 system  Search pattern
 software  Replacement string
 g  Global Replacement flag

Sed command by default replaces only the first occurrence of a string in each line. The /g flag specifies the sed command to replace all the occurrences of the pattern in each line.

The table below shows some more options:

 Command  Purpose
 sed ‘s/system/software/’ test  Replace the first occurrence of a string in each line
 sed ‘s/system/software/n’ test  Replace the nth occurrence of a pattern in a line
 sed ‘s/system/software/ng’ test  Replace from nth occurrence to all occurrences in a line
 sed ‘n s/system/software/’ test  Replace string on a specific line number (nth line)
 sed ‘n,$ s/system/software/’ test  Replace string on a range of lines (from nth line till last)
 sed ‘/as/ s/system/software/’ test  Replace on a line which matches a pattern (here pattern is “as”)
sed ‘s/system/software/w test2’ test Copy replaced the line to another file
 sed ‘/your/ c Replaced line’ test  Replace the lines which contain a pattern (In this case, the entire line with the pattern “your” will be replaced with “Replaced line”.)
 sed ‘/your/ a Replaced line’ test  Add a new line after a match
  sed ‘/your/ i Replaced line’ test   Add a new line before a match

 

2) Use a different delimiter

sed 's|system|software|' test

Instead of ‘|’ we can also use ‘_’ or ‘@’ as the delimiters. This is mainly used when the search pattern or the replacement string is url.

 

3) Use & as the replacement string

sed 's/system/& and &' test

Here in the above example ‘&’ represents the replacement string and hence replace ‘system’ with ‘system and system’.

4) Duplicate the replaced line

sed 's/system/software/p' test

The /p(print) flag prints the replaced line twice in the terminal. In this case, the lines which are not replaced will be printed only once.

The following command generates the duplicate of each line in this file:

sed 'p' test

 

5) Print only the replaced lines

sed -n 's/system/software/p' test

Here the -n option suppresses the duplicate of the replaced lines generated by the /p flag.

If we use the option -n alone without /p, then the sed does not print anything.

 

6) Delete lines

sed '2 d' test

In this case, it deletes the second line from the file ‘test’.

sed '2,$ d' test

Here, it deletes a range of lines i.e, from the second line till the last line.

7) Run multiple sed commands

sed -e 's/An/The' -e 's/is/was/' test

In order to run multiple sed commands either we can pipe the output of one sed command to the other or we can use the ‘-e’ option.

8) Sed as grep command

Case 1:
cat test
An operating system is a vital component of the system.
The operating system controls your computer's tasks and system resources to optimize performance.
An operating system is a collection of softwares.
An operating system is abbreviated as OS.

Case 2:
sed -n '/vital/ p' test
An operating system is a vital component of the system.


Case 3:
sed -n '/vital/ !p' test
The operating system controls your computer's tasks and system resources to optimize performance.
An operating system is a collection of softwares.
An operating system is abbreviated as OS.

Here in the above example,

Case 1 shows the contents of the file “test”

Case 2 executes sed command to print the lines which match the pattern “vital”. This is same as grep command.

Case 3 executes sed command to print the lined which do not match with the pattern “vital”. This is same as grep -v.

 

9) Sed as tr command

sed 'y/ATp/atP/' test

The sed command along with y flag acts as tr command. Here in the above example, the sed command use ‘y’ flag to replace ‘A’ with ‘a’ and ‘T’ with ‘t’ which is similar to the action performed by tr command.

 

10) Edit the source file

sed -i 's/system/software/' test

Sed command by default does not edit the source file for our safety but by using ‘-i’ option source file can be edited.

11) Take a backup of source file prior to editing

sed -i.back 's/system/software/' test

In the above sed command, before editing the source file sed command takes the backup of ‘test’ as ‘test.back’.

sed command is case-sensitive. Use /I or /i flag for case-insensitive search.

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