In one our earlier articles, we saw how to insert a line or append a line to an existing file using sed. In this article, we will see how we can do data manipulation or substitution in files using sed.
Let us consider a sample file, sample1.txt, as shown below:
apple orange banana pappaya
1. To add something to the beginning of a every line in a file, say to add a word Fruit:
$ sed 's/^/Fruit: /' sample1.txt Fruit: apple Fruit: orange Fruit: banana Fruit: pappayaThe character 's' stands for substitution. What follows 's' is the character, word or regular expression to replace followed by character, word or regular expression to replace with. '/' is used to separate the substitution character 's', the content to replace and the content to replace with. The '^' character tells replace in the beginning and hence everyline gets added the phrase 'Fruit: ' in the beginning of the line.
2. Similarly, to add something to the end of the file:
$ sed 's/$/ Fruit/' sample1.txt apple Fruit orange Fruit banana Fruit pappaya FruitThe character '$' is used to denote the end of the line. And hence this means, replace the end of the line with 'Fruit' which effectively means to add the word 'Fruit' to the end of the line.
3. To replace or substitute a particular character, say to replace 'a' with 'A'.
$ sed 's/a/A/' sample1.txt Apple orAnge bAnana pAppayaPlease note in every line only the first occurrence of 'a' is being replaed, not all. The example shown here is just for a single character replacement, which can be easily be done for a word as well.
4. To replace or substitute all occurrences of 'a' with 'A'
$ sed 's/a/A/g' sample1.txt Apple orAnge bAnAnA pAppAyA5. Replace the first occurrence or all occurrences is fine. What if we want to replace the second occurrence or third occurrence or in other words nth occurrence.
To replace only the 2nd occurrence of a character :
$ sed 's/a/A/2' sample1.txt apple orange banAna pappAyaPlease note above. The 'a' in apple has not changed, and so is in orange since there is no 2nd occurrence of 'a' in this. However, the changes have happened appropriately in banana and pappaya
6. Now, say to replace all occurrences from 2nd occurrence onwards:
$ sed 's/a/A/2g' sample1.txt apple orange banAnA pappAyA7. Say, you want to replace 'a' only in a specific line say 3rd line, not in the entire file:
$ sed '3s/a/A/g' sample1.txt apple orange bAnAnA pappaya'3s' denotes the substitution to be done is only for the 3rd line.
8. To replace or substitute 'a' on a range of lines, say from 1st to 3rd line:
$ sed '1,3s/a/A/g' sample1.txt Apple orAnge bAnAnA pappaya9. To replace the entire line with something. For example, to replace 'apple' with 'apple is a Fruit'.
$ sed 's/.*/& is a Fruit/' sample1.txt apple is a Fruit orange is a Fruit banana is a Fruit pappaya is a FruitThe '&' symbol denotes the entire pattern matched. In this case, since we are using '.*' which means matching the entire line, '&' contains the entire line. This type of matching will be really useful when you a file containing list of file names and you want to say rename them as we have shown in one of our earlier articles: Rename group of files
10. Using sed, we can also do multiple substitution. For example, say to replace all 'a' to 'A', and 'p' to 'P':
$ sed 's/a/A/g; s/p/P/g' sample1.txt APPle orAnge bAnAnA PAPPAyAOR This can also be done as:
$ sed -e 's/a/A/g' -e 's/p/P/g' sample1.txt APPle orAnge bAnAnA PAPPAyAThe option '-e' is used when you have more than one set of substitutions to be done.
OR The multiple substitution can also be done as shown below spanning multiple lines:
$ sed -e 's/a/A/g' \ > -e 's/p/P/g' sample1.txt APPle orAnge bAnAnA PAPPAyA