Finding and replacing text inside of files is a pretty common task when you’re refactoring code. GUI editors, especially ones that support “projects” make this task fairly trivial. If your trusty editor doesn’t happen to support this, or if you find yourself needing to do a find and replace out on a remote server, you can do so quite easily with the
sed is short for “stream editor” and came on the scene back in 1974 and was influenced by the
You read that correctly, the
sed command has been super charging the Unix command-line for nearly 50 years.
Depending on your Unix-like system of choice, there’s a solid chance you already have
sed available. If not, you should be able to install it easily with your favorite package manager.
Some of the following commands will explicitly be for the GNU implementation of
sed which differs from the BSD implementation, like the version that ships with macOS.
If you’re on macOS, it’s recommended that you install the GNU version. The package is named
gnu-sed if you’re using Homebrew. For the examples below, simply swap
gsed (or simply alias it) and you’ll be good to go.
You’re welcome to follow along at home using your own files, but just in case, let’s go ahead and create a test file to work with:
$ mkdir /tmp/gator-sed $ cd /tmp/gator-sed $ echo 'The quick brown fox jumped...' >> original.txt $ cat original.txt The quick born fox jumped...
Note that most of the commands we’ll be discussing will be non-destructive in nature. That said, we will also be covering how to edit files in place, which will overwrite the existing file.
It’s always good to make backups of your files before doing any extreme munging.
sed does have a way to automatically make backups as well, which we’ll also touch on.
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Finding and replacing text with a preview
Now I’m not saying I dislike foxes, but they definitely aren’t alligators. To give
sed a whirl, let’s go ahead and replace “fox” with “alligator”.
By default, when you feed
sed a file, it will output the changes to the standard output. This could be piped to a new file (which we’ll discuss later on) but it’s also a great opportunity to preview your changes to ensure they are accurate:
$ sed 's/fox/alligator/g' original.txt The quick brown alligator jumped...
Case in point about previewing, let’s go ahead and change the color too:
$ sed 's/brown fox/green alligator/g' original.txt The quick green alligator jumped...
Finding and replacing text to a new file
Now that we know how to find and replace text, to save it to a new file, we simply pipe the output to a new file:
$ sed 's/brown fox/green alligator/g' original.txt > new.txt $ cat new.txt The quick green alligator jumped...
Finding and replacing text in place
The aforementioned example provides a bit of safety as it doesn’t overwrite the original file, but that sort of safety isn’t always desired.
When you’re ready to live dangerously, we can edit the file in place by passing in the
$ sed 's/quick/fancy/g' new.txt -i $ cat new.txt The fancy green alligator jumped...
-i argument also accepts an optional “suffix” value that is used to create backup files for you automatically:
$ sed 's/jumped/swam/g' new.txt -i.bak $ cat new.txt The fancy green alligator swam... $ cat new.txt.bak The fancy green alligator jumped...
Case-insensitive find and replace
While not immediately apparent, all of the replacements we’ve done thus far have been case sensitive.
If we were to try to replace
the in our file, we wouldn’t be able to:
$ sed 's/the/A/g' new.txt The fancy green alligator swam...
Fortunately, the GNU implementation of
sed supports including an additional modifier flag in our find and replace string:
% sed 's/the/A/gi' new.txt A fancy green alligator swam...
Unfortunately, BSD-based systems, including macOS and FreeBSD, use a different implementation of
sed which doesn’t support the
i modifier flag.
Multiple find and replace
Let’s say we wanted to replace the
The as well as
alligator to capitalize it. We could approach it by typing out everything between
alligator as part of our find and replace text.
I’m lazy and like to save keystrokes, so we’re not going to do that.
Instead, we can chain together multiple find and replace strings:
% sed -e 's/The/A/g' -e 's/alligator/Alligator' new.txt A fancy green Alligator swam...
Despite it’s age, the
sed command packs quite the punch.
To take things a step further, you could combine the
sed command with the power of wild cards
* and other file globs to quickly and efficiently make text changes across multiple files and directories.