Create a PEM from a PPK file

Try this to install putty-tools

sudo apt install putty-tools
puttygen key.ppk -O private-openssh -o key.pem
ssh -i ~/key.pem {user}@{ip}

  1. Install PuttyTools

    apt-get install putty-tools
  2. Generate a pem file form the ppk

    puttygen server.ppk -O private-openssh -o server.pem  

The file server.pem file will be saved on same location

First, install PuTTY for Mac using

brew install putty

Then, use the following command to convert the .ppk format private key to a standard PEM format private key:

puttygen privatekey.ppk -O private-openssh -o privatekey.pem

Make sure permissions on the private key file are set properly. It should only be readable by the user that owns it.

chmod go-rw privatekey.pem

You can now use the key for logins from scripts and command line with:

ssh -i privatekey.pem user@hostname

If you're on a Mac and you've previously installed Homebrew, from Terminal:

$ brew install putty
$ puttygen server.ppk -O private-openssh -o server.pem

The first command was suggested in this comment and the second from Emizen Tech's answer.