Bash replace string where specific condition is true

Question

I am trying to replace a string where a specific condition is true in the same line

I have a file with some lines. I to replace a word (word1) with another (word2) in every line that starts with another word (word3)

For example:

foo moo see
kaa haa qee
foo dee see
uuu ooo rrr
foo dee laa

I want to replace the word "see" with "raa" in every line which begins with "foo". So doing that will result in the lines being like:

foo moo raa
kaa haa qee
foo dee raa
uuu ooo rrr
foo dee laa

I was told that I can use sed to do that but I didn't figure out how to do that.

Please help! (with sed only)

fedorqui · Accepted Answer · 2013-12-18 18:33:44Z

8

This is very basic sed:

$ sed '/^foo/s/see/raa/g' file
foo moo raa
kaa haa qee
foo dee raa
uuu ooo rrr
foo dee laa

Explanation

sed 's/something/replacement/g' replaces something strings with replacement. The g indicates that it is to be done every time it can.
Adding /^foo/ makes it just take into consideration the lines starting with foo.
If you want the file to be edited in place, that is, to have the file content updated with the substitution, add -i: sed -i.bak '/^foo/s/see/raa/g' file. file.bak will contain the previous file.

Update

How to use variables instead of foo, see, raa ? – Khaleal

Like this and with double quotes:

$ d="foo"
$ e="see"
$ sed "/^$d/s/$e/raa/g" file
foo moo raa
kaa haa qee
foo dee raa
uuu ooo rrr
foo dee laa

and so on.

edited Dec 18, 2013 at 18:33

answered Dec 18, 2013 at 17:22

fedorqui

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3 Comments

Khaleal Over a year ago

How to use variables instead of foo, see, raa ?

fedorqui Over a year ago

See my updated answer, @Khaleal If it's done, remember you can accept the answer.

NeronLeVelu Over a year ago

just be carefull with special char in word like . * \ & ? + that have sed meaning in s. Your sample does not show any of them but this is maybe not the case in all your file

Jotne · Accepted Answer · 2013-12-18 19:08:42Z

4

I know its asked for sed, but posting an awk for other to read, if they like.

awk '/^foo/ {sub(/see/,"raa")}1' file

answered Dec 18, 2013 at 19:08

Jotne

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