SED
Sed is a Stream Editor used for modifying the files in unix (or linux)
Features of sed
Consider the below file for as example for our practice
$ cat file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. unix is free os unix linux learn os open source
Replacing or substituting string
$ sed 's/unix/linux/' file.txt learn about sed in linux or linux linux commands stream editor for linux or linux. unix is free os linux linux learn os open source
Here the s specifies the substitution operation. The / are delimiters. The unix is the search pattern and the linux is the replacement string.
By default, the sed command replaces the first occurrence of the pattern in each line and it won’t replace the second, third…occurrence in the line.
Replacing the nth occurrence of a pattern in a line
Use the /1, /2 etc flags to replace the first, second occurrence of a pattern in a line. The below command replaces the second occurrence of the word unix with linux in a line
$ sed 's/unix/linux/2' file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. linux is free os unix linux learn os open source
$ sed 's/unix/linux/1' file.txt learn about sed in linux or linux linux commands stream editor for linux or linux. unix is free os linux linux learn os open source
Replacing all the occurrence of the pattern in a line.
The substitute flag /g (global replacement) specifies the sed command to replace all the occurrences of the string in the line.
$ sed 's/unix/linux/g' file.txt learn about sed in linux or linux linux commands stream editor for linux or linux. linux is free os linux linux learn os open source
Replacing from nth occurrence to all occurrences in a line.
Use the combination of /1, /2 etc and /g to replace all the patterns from the nth occurrence of a pattern in a line. The following sed command replaces the third, fourth, fifth… “unix” word with “linux” word in a line.
$ sed 's/unix/linux/3g' file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. unix is free os unix linux learn os open source
Changing the slash (/) delimiter
You can use any delimiter other than the slash. As an example if you want to change the web url to another url as
$ cat file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. unix is free os unix linux learn os open source https:// www http:// https://www
$ sed 's/http:\/\//www/' file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. unix is free os unix linux learn os open source https:// www www https://www
In this case the url consists the delimiter character which we used. In that case you have to escape the slash with backslash character, otherwise the substitution won’t work.
Using too many backslashes makes the sed command look awkward. In this case we can change the delimiter to another character as shown in the below example.
$ sed 's|http://|www|' file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. unix is free os unix linux learn os open source https:// www www https://www
$ sed 's_http://_www_' file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. unix is free os unix linux learn os open source https:// www www https://www
Using & as the matched string
There might be cases where you want to search for the pattern and replace that pattern by adding some extra characters to it. In such cases & comes in handy. The & represents the matched string.
$ sed 's/unix/{&}/' file.txt
learn about sed in {unix} or linux
{unix} commands
stream editor for linux or {unix}. unix is free os
{unix}
linux
learn os
open source
https://
www
http://
https://www
$ sed 's/unix/{&&}/' file.txt
learn about sed in {unixunix} or linux
{unixunix} commands
stream editor for linux or {unixunix}. unix is free os
{unixunix}
linux
learn os
open source
https://
www
http://
https://www
Using \1,\2 and so on to \9
The first pair of parenthesis specified in the pattern represents the \1, the second represents the \2 and so on. The \1,\2 can be used in the replacement string to make changes to the source string. As an example, if you want to replace the word “unix” in a line with twice as the word like “unixunix” use the sed command as below.
$ sed 's/\(unix\)/\1\1/' file.txt learn about sed in unixunix or linux unixunix commands stream editor for linux or unixunix. unix is free os unixunix linux learn os open source https:// www http:// https://www
The parenthesis needs to be escaped with the backslash character. Another example is if you want to switch the words “unixlinux” as “linuxunix”, the sed command is
$ sed 's/\(unix\)\(linux\)/\2\1/' file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. unix is free os unix linux learn os open source https:// www http:// https://www
Another example is switching the first three characters in a line
$ sed 's/^\(.\)\(.\)\(.\)/\3\2\1/' file.txt aelrn about sed in unix or linux inux commands rtseam editor for linux or unix. unix is free os inux nilux aelrn os epon source tthps:// www tthp:// tthps://www
Duplicating the replaced line with /p flag
The /p print flag prints the replaced line twice on the terminal. If a line does not have the search pattern and is not replaced, then the /p prints that line only once.
$ sed 's/unix/linux/p' file.txt learn about sed in linux or linux learn about sed in linux or linux linux commands linux commands stream editor for linux or linux. unix is free os stream editor for linux or linux. unix is free os linux linux linux learn os open source https:// www http:// https://www
Printing only the replaced lines
Use the -n option along with the /p print flag to display only the replaced lines. Here the -n option suppresses the duplicate rows generated by the /p flag and prints the replaced lines only one time.
$ sed -n 's/unix/linux/p' file.txt learn about sed in linux or linux linux commands stream editor for linux or linux. unix is free os linux
Note : If you use -n alone without /p, then the sed does not print anything.
Running multiple sed commands.
You can run multiple sed commands by piping the output of one sed command as input to another sed command.
$ sed 's/unix/linux/' file.txt| sed 's/os/system/' learn about sed in linux or linux linux commands stream editor for linux or linux. unix is free system linux linux learn system open source https:// www http:// https://www
Sed provides -e option to run multiple sed commands in a single sed command. The above output can be achieved in a single sed command as shown below.
$ sed -e 's/unix/linux/' -e 's/os/system/' file.txt learn about sed in linux or linux linux commands stream editor for linux or linux. unix is free system linux linux learn system open source https:// www http:// https://www
Replacing string on a specific line number.
You can restrict the sed command to replace the string on a specific line number. An example is
$ sed '4 s/unix/linux/' file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. unix is free os linux linux learn os open source https:// www http:// https://www
Replacing string on a range of lines.
You can specify a range of line numbers to the sed command for replacing a string.
$ sed '1,3 s/unix/linux/' file.txt learn about sed in linux or linux linux commands stream editor for linux or linux. unix is free os unix linux learn os open source https:// www http:// https://www
Here the sed command replaces the lines with range from 1 to 3. Another example is
$ sed '2,$ s/unix/linux/' file.txt learn about sed in unix or linux linux commands stream editor for linux or linux. unix is free os linux linux learn os open source https:// www http:// https://www
Here $ indicates the last line in the file. So the sed command replaces the text from second line to last line in the file.
Replace on a lines which matches a pattern.
You can specify a pattern to the sed command to match in a line. If the pattern match occurs, then only the sed command looks for the string to be replaced and if it finds, then the sed command replaces the string.
$ sed '/linux/ s/unix/centos/' file.txt learn about sed in centos or linux unix commands stream editor for linux or centos. unix is free os unix linux learn os open source https:// www http:// https://www
Deleting lines.
You can delete the lines a file by specifying the line number or a range or numbers.
$ sed '2 d' file.txt learn about sed in unix or linux stream editor for linux or unix. unix is free os unix linux learn os open source https:// www http:// https://www
$ sed '5,$ d' file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. unix is free os unix
Duplicating lines
You can make the sed command to print each line of a file two times.
$ sed 'p' file.txt learn about sed in unix or linux learn about sed in unix or linux unix commands unix commands stream editor for linux or unix. unix is free os stream editor for linux or unix. unix is free os unix unix linux linux learn os learn os open source open source https:// https:// www www http:// http:// https://www https://www
Sed as grep command
You can make sed command to work as similar to grep command.
$ sed -n '/unix/ p' file.txt learn about sed in unix or linux unix commands stream editor for linux or unix. unix is free os unix
Here the sed command looks for the pattern “unix” in each line of a file and prints those lines that has the pattern.
You can also make the sed command to work as grep -v, just by using the reversing the sed with NOT (!).
$ grep -v 'unix' file.txt
$ sed -n '/unix/ !p' file.txt linux learn os open source https:// www http:// https://www
The ! here inverts the pattern match
Add a line after a match.
sed '/unix/ a "Add a new line"' file.txt
Add a line before a match
sed '/unix/ i "Add a new line"' file.txt
Add a line after a match.
sed '/unix/ a "Add a new line"' file.txt
Change a line
sed '/unix/ c "Change line"' file.txt
Transform like tr command
sed 'y/ul/UL/' file.txt
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