If you heard about SED in linux… and you want to know more about it, here is a good place to be.

SED – stream editor

sed is the best bash option to modify text files without GUI. This is very useful when you have hundreds of texts to modify, or if you want to automate a task (because your server cannot use GUI).

In case you don’t know, when you type man sed in your terminal, the whole definition will show up. We will cover some common use case, but if you want to go deeper, the manual is the reference.

Replace

from echo

for instance, let’s say you have this text :
“Hello, this is Mike. Everyone loves Mike”
If you want to switch Mike for John then you can type

echo "Hello, this is Mike. Everyone loves Mike" | sed 's/Mike/John/g'

And this will show up in you terminal :

# Hello, this is John. Everyone loves John

Note that here, sed uses regex to replace Mike by John. If you only type sed 's/Mike/John/' then only the first occurence would be modified. More about regex here

from file

Similarly, you can modify a file. let’s create a file

nano mike.txt

And write inside Hello, this is Mike. Everyone loves Mike

Now, let’s use sed to change Mike into John, without opening nano

sed -i 's/Mike/John/g' mike.txt

If you see an error, it’s because you’re not on a Linux system. try this instead

sed -i '' 's/Mike/John/g' mike.txt

Now let’s see our file:

cat mike.txt
# Hello, this is John. Everyone loves John

The -i option stands for “in-place extension”. It has two options:

• create a backup file if you specify a new extension. for instance to create a mike.txt.backup, you would have to type sed -i .backup 's/Mike/John/g' mike.txt
• overwrite our mike.txt file without backup. That’s why you may need to force a 0 extension backup typing -i ''

Note : If you don’t know how to use nano, check this

Delete

sed can delete lines, if you tell it the line number followed by d. In our case, we can remove the line we have by typing :

sed '1d' mike.txt

Mulitple lines

There are two options :

• '1d;4d' deletes line 1 and line 4
• '1,4d' deletes from line 1 till line 4

Regex mathing

To delete lines matching a specific regex

• '/^Hello/d' deletes lines begining with “Hello”
• '/^Hello/d;/^Bye/d' deletes every lines between the first line begining with “Hello”, and the last one begining with “Bye”

Note you can even mix the two, like so '/^Hello/d',3d

Filter

Quite the same format as deleting lines, but use p instead of d

Use case

Replace some text in many files

Let’s say you are in a folder, and there are hundreds of text files. In every text file, you need to change the word “Microsoft windows” to “Ubuntu”.

Here is what you can do:

for f in *.txt; do sed -i -e 's/Microsoft windows/Ubuntu/' "$f"; done

And that’s it.

Note: if you want to keep the old files, you can put the new text files in another directory:

for f in *.txt; do sed 's/Microsoft windows/Ubuntu/' "$f" > ../new_directory/"$f" ; done

Dig Further

I would recommend to look at tr for deleting or replacing characters in text files.

Also for Json manupulation, I would recommend jq. It can be useful for text replacing of just one property. Good complement for many json manipulation, like this:

for file in *.json; do jq -c '.prop = (.prop | sub("Microsoft";"Ubuntu"; "g") )' file

Reference

A French blog with good examples : buzut.net
Regex manual regexr
Find & replace in mulitple files grep -RiIl --exclude-dir=tests 'search' | xargs sed 's/search/replace/g'
tr linux-france.org
How to remove line breaks /n