How to use sed to find and replace text in files in Linux / Unix shell

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I am a new Linux user. I wanted to find the text called “foo” and replaced to “bar” in the file named “hosts.txt.” How do I use the sed command to find and replace on Linux or UNIX-like system?

sed find and replace examples for unix and linux
The sed stands for stream editor. It reads the given file, modifying the input as specified by a list of sed commands. By default, the input is written to the screen, but you can force to update file.

Syntax

The syntax is:
sed 's/word1/word2/g' input.file
sed 's/word1/word2/g' input.file > output.file
sed -i 's/word1/word2/g' input.file
sed -i -e 's/word1/word2/g' -e 's/xx/yy/g' input.file

The above replace all occurrences of characters in word1 in the pattern space with the corresponding characters from word2.

Examples that use sed to find and replace

Let us create a text file called hello.txt as follows:
$ cat hello.txt
The is a test file created by nixCrft for demo purpose.
foo is good.
Foo is nice.
I love FOO.

I am going to use s/ for substitute the found expression foo with bar as follows:
sed 's/foo/bar/g' hello.txt
Sample outputs:

The is a test file created by nixCrft for demo purpose.
bar is good.
Foo is nice.
I love FOO.

To update file pass the -i option:
sed -i 's/foo/bar/g' hello.txt
cat hello.txt

The g/ means global replace i.e. find all occurrences of foo and replace with bar using sed. If you removed the /g only first occurrence is changed:
sed -i 's/foo/bar/' hello.txt
The / act as delimiter characters. To match all cases of foo (foo, FOO, Foo, FoO) add I (capitalized I) option as follows:
sed -i 's/foo/bar/gI' hello.txt
cat hello.txt

Sample outputs:

The is a test file created by nixCrft for demo purpose.
bar is good.
bar is nice.
I love bar.

Please note that the BSD implementation of sed (FreeBSD/MacOS and co) does NOT support case-insensitive matching. You need to install gnu sed. Run the following command on Apple Mac OS:
$ brew install gnu-sed
######################################
### now use gsed command as follows ##
######################################
$ sed -i 's/foo/bar/gI' hello.txt
$ cat hello.txt

sed command problems

Consider the following text file:
$ cat input.txt
http:// is outdate.
Consider using https:// for all your needs.

Find word ‘http://’ and replace with ‘https://www.cyberciti.biz’:
sed 's/http:///https://www.cyberciti.biz/g' input.txt
You will get an error that read as follows:

sed: 1: "s/http:///https://www.c ...": bad flag in substitute command: '/'

Our syntax is correct but the / delimiter character is also part of word1 and word2 in above example. Sed command allows you to change the delimiter / to something else. So I am going to use +:
sed 's+http://+https://www.cyberciti.biz+g' input.txt
Sample outputs:

https://www.cyberciti.biz is outdate.
Consider using https:// for all your needs.

Recap: Using sed to find and replace

The general syntax is:
## find word1 and replace with word2 using sed ##
sed -i 's/word1/word2/g' input
## you can change the delimiter to keep syntax simple ##
sed -i 's+word1+word2+g' input
sed -i 's_word1_word2_g' input
## you can add I option to GNU sed to case insensitive search ##
sed -i 's/word1/word2/gI' input
sed -i 's_word1_word2_gI' input

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

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