sed is a program that was developed for the original UNIX operating systems. As a result, there’s some differences between the Berkeley Software Distribution (BSD) edition used by Apple’s MacOS computers, and the GNU edition used by Linux operating systems.

Getting Started

I’ve found the one on MacOS to not only be frustrating, but to be incompatible with most of the advice you’ll be reading about on StackOverflow. And let’s face it, if you can’t copy other people’s code, you’re doomed (why else would you be here?)

Type this below to install the GNU sed known as sed onto your computer. And add /usr/local/opt/gnu-sed/libexec/gnubin/sed to your ${PATH}

brew install gnu-sed

You can run the commands below by typing sed in place of sed but I’d recommend adding a bash alias.

Using Sed

Making changes to a file

The -i flag is used to tell sed to directly overwrite the file. It requires a string placed after it, which specifies the filename appended to the old version.

sed -i '.old' 's/original/replacement/' myfile.txt
# myfile.txt.old will contain the original string

Note: When it comes to substitution commands, both the input and the replacement must be nested inside of / chars s/like/this/

Replacing text in a file

sed -i 's/before/after' file.txt

Replacing multiple matches at once

By default, sed’s s will not replace every single instance. To run it globally, use the /g flag at the end of the command:

# [removing damn and replacing it with darn]
sed -i 's/damn/darn/g' swearing.txt

Specifying multiple instructions

# [Method 1: using '-e']
sed -i -e 's/local/remote/g' -e 's/real/virtual/g' file.txt
# [Method 2: using ';']
sed -i 's/local/remote/g;s/real/virtual/g' file.txt

Note: There can be some compatibility problems with using ; and it’s recommended to use the -e flag when attempting to put multiple commands in a single sed execution.

Extended Regular Expressions

You can use extended regular expression syntax with the -E flag.

sed -E 's_[0-9]{3,4}_###_g' <<< "(650)941-8758"
# => (###)###-###

Prepending/Appending to Files

You can prepend a line using 1i and append using \$a

# Prepend to the beginning of a file
sed -i "1i My name is $(whoami)." file.txt
# Append to the end of a file
sed -i "\$a Nice to meet you, ${USER}." file.txt