sed

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. Next, add /usr/local/opt/gnu-sed/libexec/gnubin/sed to your ${PATH}

# Install GNU sed
brew install gnu-sed

# Add GNU sed to the front of the path variable
cat <<< 'path=(/usr/local/opt/gnu-sed/libexec/gnubin/sed ${path})' >> ~/.zshrc

Making changes to a file

Include the -i flag have sed perform the substitution in-place

• In-place replacement, with a backup of file.txt written to file.txt.old

sed -i '.old' 's/old/new/' file.txt

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

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

• Prepending a line to the start of file.txt:

  # The whitespace after '1i' is not interpreted
  sed -i '1i New last line' file.txt
  

• Appending a line to the end of file.txt:

  # The whitespace after '$a' is not interpreted
  sed -i '$a New last line' file.txt
  

Changing Case

• Convert each file in the directory from snake_case.pdf to Title Case.pdf

  for file in *; {
      new=$(<<< ${file} sed \
          -e 's/.*/\L&/' \
          -e 's/[a-z]*/\u&/g' \
          -e 's/_/ /g' \
          -e 's/\.Pdf/.pdf/'
      )
      print mv ${file} ${new}
  }