Running remote commands via SSH

On occasion, I need to run a single remote command on a single system, but don’t have a tool handy (like Apple Remote Desktop’s Send Unix function) to do it. If the machine in question has SSH enabled though, there’s a simple way to do this.

1. Open Terminal

2. Run the following command:

ssh username@server.domain.com "your command here"

For example, if you wanted to use tail to display the latest entries to /var/log/system.log, you would run the following command:

ssh username@server.domain.com "tail -f /var/log/system.log"

You’ll be prompted for a password, which will be used by SSH to log into the remote system. If the password is accepted, tail should start displaying the latest entries to /var/log/system.log as they’re written. To stop, you would hit Control-C as usual. That will stop the command’s execution and close the SSH connection automatically.

When running commands that require elevated privileges, you’ll need to add the -t flag to your SSH command. -t tells SSH to force pseudo-tty allocation, which in turn provides a way to feed your account’s password to the remote server and run the command via sudo.

For example, if you wanted to restart opendirectoryd on a remote Mac running 10.8.x, you would run the following command:

ssh -t username@server.domain.com "sudo killall opendirectoryd"

You’ll be prompted for a password, which will be used by SSH to log into the remote system. You’ll then be prompted again for a password, which will be used by sudo to authenticate that your account is authorized to run the command with sudo. Once the command is run and completes successfully, the SSH connection closes automatically.