Bash ssh

title: Bash ssh (Secure SHell)

Bash command: ssh

SSH (Secure SHell) is a secure method of remotely connecting to a Linux machine.

The basic syntax of the command is ssh [email protected].

Used to connect to a remote computer It’s full form is ‘Secure Shell’. It’s purpose is to connect to another remote computer using tcp. The address of the remote computer can be provided using an IP address or, if provided, an identifier.

SSH is a replacement for Telnet as well as any remote shell program that predates it. The main benefit it offers over other remote shells is that it is cryptographically secured by default, a now necessary feature as systems are more often than not connected via untrusted networks.

If the remote computer requires user login, the form ssh [email protected]_address can be used, which will then prompt for the user password on successful connection.

Example command:
ssh [email protected]

This will connect to the computer in the local ip address of and log in with the username pi.

Another way to use this command is to pipe a command directly to it.

For example:
To execute the command
ls /tmp/doc

on the computer with ip address, type the following command at a shell prompt:

ssh ls /tmp/doc

After authenticating to the remote server, the contents of the remote directory will be displayed, and you will return to your local shell prompt.

  • Hostname can be either an IP address or a FQDN.
  • [email protected]’ can be excluded if the user currently logged into the shell has the same username as the desired user on the remote system.
  • if the server is configured to listen to any port other than 22, then you have to use -p option to specify the port. For example: ssh -p 2024 123.456.789.012.

SSH can rely on password-based authentication or key-based authentication. Password-based authentication is becoming less common as cloud solutions such as Amazon Web Services, Microsoft Azure, and Google Cloud Platform promote the usage of keys. If a system is configured for password-based authentication, once the connection is opened the system will prompt the user for the password. If the system is configured to use keys, the key can be added to the ssh-agent (more below) or specified with the -i switch like so: ssh -i my_private_key.pem [email protected].

Logging in without a password

It is also possible to set up your Linux servers to connect without entering a password.

This can be important when servers need to connect to each other without user input.

Examples of this include:

  • transferring files back and forth (using SCP)
  • using automatic scripts for backups

Step 1 – Generate an SSH authentication-key file

[[email protected] ~]$ ssh-keygen

When you are prompted for a passphrase, leave it blank

By default the key files should be stored in /home/user/.ssh/

Step 2 – Copy the public part of the key to other servers

[[email protected] ~]$ ssh-copy-id -i ~/.ssh/ [email protected]

If the server is not installed with openssh-clients you can copy the authentication key with the command:

[[email protected] ~]$ cat ~/.ssh/ | ssh [email protected] "cat >> ~/.ssh/authorized_keys"

Step 3 – Verification

Attempt to connect to the other servers va SSH

If access is denied, it is likely a permission error on the .ssh directory.

Check/change the permissions on both the local and remote user’s files:

chmod 600 ~/.ssh/authorized_keys chmod 700 ~/.ssh/

Try to connect again after the permission change.

More Information:

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