Find and replace with sed command in Linux

Find and replace feature is always handy. It can turn into a torture when it comes to change or delete a simple constant string in a text file. There is a handy tool in linux for doing these kind of tihngs. Actually sed is not a text editor but it is used outside of the text file to make changes.

How sed works?

Sed tool reads given text file line by line, does the defined operation then displays the results unless you give a target text file. We can clarify with some examples.

The syntax of the sed command is like: sed [options] '{command}' [file name]

Substitute 

's/{old value}/{new value}/'

$ echo could you smile a little smile for me | sed 's/little/big/'

$ could you smile a big smile for me

little is replaced by big.

Multiple change

it is used with -e option.

$ echo could you smile a little smile for me | sed -e 's/little/big/' -e 's/smile/laugh/'

$ could you laugh a big smile for me

In this example we can use ; rather than -e they are same.

$ echo could you smile a little smile for me | sed -e 's/little/big/; s/smile/laugh/'

$ could you laugh a big smile for me

Attention that the second smile remained same. To change all occurences, g option must be used.

Global changes

g option must be used.

$ echo could you smile a little smile for me | sed 's/smile/laugh/g'

$ could you laugh a little laugh for me

Some examples with text files

$ cat sed.txt

ankara 00

istanbul 00

izmir 00

ankara 00

izmir 00

istanbul 00

$ sed '/ankara/ s/00/06/' sed.txt

ankara 06

istanbul 00

izmir 00

ankara 06

izmir 00

istanbul 00

$ sed '/ankara/ s/00/06/; /istanbul/ s/00/34/; /izmir/ s/00/35/' sed.txt

ankara 06

istanbul 34

izmir 35

ankara 06

izmir 35

istanbul 34

change second and third lines.

$ sed '2,3 s/00/99/' sed.txt

ankara 00

istanbul 99

izmir 99

ankara 00

izmir 00

istanbul 00

delete lines contains izmir.

$ sed '/izmir/ d' sed.txt

ankara 00

istanbul 00

ankara 00

istanbul 00

write the changed text to another file.

$ sed '/ankara/ s/00/06/; /istanbul/ s/00/34/; /izmir/ s/00/35/' sed.txt > sednew.txt