We often need to replace strings in a file or multiple files in a Unix system. This can be done easily using the sed command. we can easily replace the strings in multiples in a very short span of time
sed replace a string in a file
We can use the below command
sed 's/<string 1>/<string 2>' < file name>
The above will not touch the file, it will provide the output with replacement strings. Also, It would do the first replacement on each line
Example $cat test.txt aaaaa bbbb CCCCC ddddd $sed 's/aaaaa/mmm' test.txt mmm bbbb CCCCC ddddd
Now if we want to replace the string from the file itself
sed -i 's/aaaaa/mmm' < file name>
The above command will change the file itself
Example $cat test.txt aaaaa bbbb CCCCC ddddd $sed -i 's/aaaaa/mmm' test.txt $cat test.txt mmm bbbb CCCCC ddddd
We can change multiple files also using the same command.
sed -i 's/aaaaa/mmm' *txt
If it is required to do the second replacement on each line, the command would be
sed 's/<string 1>/<string 2>/2' < file name>
If it is required to do all the replacements on each line, the command would be
sed 's/<string 1>/<string 2>/g' < file name>
How to backup the files before the replacement
This can be done using the command
sed -i.bak 's/<string 1>/<string 2>' *txt
- This will make a backup of all the files with the “.bak” extension before changing it
- Remember there is no space after i
sed replace a string in a file when another string is not present
We can be using below command
sed '/<string 3>/!s/<string 1>/<string 2>/g'
Here string 3 is the string that should not be present when doing a replacement
sed '/watch/!s/bat/cat/g'
sed replace string with variable
We can use variables in the above command also, but you need to put “” instead of single quotes
for i inls *html
do
x=cat
sed -i "s/$x/bat/g" $i
done
If you have / in the variable, use pipe in sed
for i inls *html
do
x=cat/jet
sed -i "s|$x|bat|g" $i
done
Hope you like this article
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