SSH automation in Bash Scripting - Step-by-Step Execution

Concept Flow - SSH automation

Start script

↓

Check SSH key

Yes↓

Run ssh command

↓

Receive output

↓

Process output or error

↓

End script

The script checks for SSH keys, runs a remote command via SSH, receives output, and ends.

Execution Sample

Bash Scripting

ssh user@host 'echo Hello from remote'
echo Done

Connects to remote host via SSH and runs a simple echo command, then prints Done locally.

Execution Table

Step	Action	Command/Condition	Result	Output
1	Start script	N/A	Script begins
2	Check SSH key	ssh-keygen -F host	Key found or not	Entry printed if key exists
3	Run ssh command	ssh user@host 'echo Hello from remote'	Connected and command executed	Hello from remote
4	Print local message	echo Done	Printed locally	Done
5	End script	N/A	Script ends

💡 Script ends after running remote command and printing local message.

Variable Tracker

Variable	Start	After SSH command	Final
ssh_output		Hello from remote	Hello from remote
local_message			Done

Key Moments - 2 Insights

Why does the script not ask for a password every time?

Because SSH keys are set up and found in step 2 (see execution_table row 2), allowing passwordless login.

What happens if the remote command fails?

The ssh command returns an error and no output is captured in ssh_output (see execution_table row 3). The script still prints Done locally.

Visual Quiz - 3 Questions

Look at the execution table, what is the output of the ssh command at step 3?

A Done

B Hello from remote

C No output

D Error message

💡 Hint

Check the Output column in execution_table row 3.

At which step does the script print the local message 'Done'?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the Action and Output columns in execution_table row 4.

If SSH keys are not set up, what would change in the execution flow?

A The script would ask for a password at step 3

B The script would skip the ssh command

C The script would print 'Done' before ssh command

D The script would end immediately

💡 Hint

Think about what happens when ssh-keygen -F host does not find a key (execution_table row 2).

Concept Snapshot

SSH automation runs commands on remote machines without manual login.
Use SSH keys for passwordless access.
Basic command: ssh user@host 'command'.
Capture output to variables for processing.
Automate repetitive remote tasks easily.

Full Transcript

This visual execution shows how SSH automation works in a bash script. The script starts and checks if SSH keys exist for passwordless login. Then it runs a remote command using ssh, capturing the output 'Hello from remote'. After that, it prints a local message 'Done'. The script ends after these steps. Key points include the importance of SSH keys to avoid password prompts and how output is handled. The execution table tracks each step, the commands run, and their results. Variables like ssh_output store remote command output. This helps beginners see how SSH automation scripts flow and behave.

Practice

1. What is the main benefit of using SSH keys for automation instead of passwords?

easy

A. It allows passwordless login for automated scripts.

B. It makes the connection slower but more secure.

C. It requires manual password entry every time.

D. It disables encryption for faster transfers.

Solution

1. Step 1: Understand SSH key usage

   SSH keys allow scripts to connect without typing passwords manually.

2. Step 2: Compare with password login

   Password login needs manual input, which breaks automation.

3. Final Answer:

   It allows passwordless login for automated scripts. -> Option A

4. Quick Check:

   SSH keys enable passwordless login = B [OK]

Hint: SSH keys = no password prompts for scripts [OK]

Common Mistakes:

• Thinking SSH keys slow down connection
• Assuming passwords work without manual input
• Believing SSH keys disable encryption

2. Which of the following is the correct syntax to copy a local file file.txt to a remote server using SSH automation?

easy

A. ssh file.txt user@remote:/path/

B. scp file.txt user@remote:/path/

C. ssh-copy file.txt user@remote:/path/

D. copy file.txt user@remote:/path/

Solution

1. Step 1: Identify the command for secure copy

   scp is used to copy files over SSH securely.

2. Step 2: Check other options

   ssh runs commands, ssh-copy is not a valid command, and copy is not for SSH.

3. Final Answer:

   scp file.txt user@remote:/path/ -> Option B

4. Quick Check:

   Use scp to copy files over SSH = A [OK]

Hint: Use scp to copy files remotely [OK]

Common Mistakes:

• Using ssh instead of scp for file copy
• Typing ssh-copy which is invalid
• Using copy command which is local only

3. What will be the output of this command if the remote server is reachable and the user has SSH key access?
ssh user@remote 'echo Hello from remote'

medium

A. Connection timed out.

B. Permission denied (publickey).

C. Hello from remote

D. ssh: Could not resolve hostname remote: Name or service not known

Solution

1. Step 1: Understand the command

   The command runs echo Hello from remote on the remote server via SSH.

2. Step 2: Check conditions

   If the server is reachable and SSH keys are set, the command outputs the echoed text.

3. Final Answer:

   Hello from remote -> Option C

4. Quick Check:

   SSH runs remote echo command = Hello from remote [OK]

Hint: SSH runs remote commands and shows output [OK]

Common Mistakes:

• Assuming permission denied without checking keys
• Confusing hostname errors with command output
• Thinking SSH command outputs nothing

4. You wrote this script to automate a remote command:

ssh user@remote 'uptime'

But it fails with "Permission denied (publickey)". What is the most likely fix?

medium

A. Generate SSH keys and add the public key to remote's authorized_keys.

B. Change the command to ssh user@remote uptime without quotes.

C. Use scp instead of ssh to run commands.

D. Restart the remote server to reset SSH.

Solution

1. Step 1: Understand the error

   "Permission denied (publickey)" means SSH keys are missing or not accepted.

2. Step 2: Fix SSH key setup

   Generate SSH keys locally and add the public key to ~/.ssh/authorized_keys on remote.

3. Final Answer:

   Generate SSH keys and add the public key to remote's authorized_keys. -> Option A

4. Quick Check:

   Permission denied means missing keys = A [OK]

Hint: Add your public key to remote authorized_keys [OK]

Common Mistakes:

• Removing quotes does not fix permission errors
• Using scp instead of ssh for commands
• Restarting server won't fix key issues

5. You want to automate running multiple commands on a remote server via SSH in one script. Which is the best way to do this?

hard

A. Use a single SSH command with multiple commands separated by semicolons inside quotes.

B. Run separate SSH commands for each remote command in the script.

C. Use scp to send commands instead of SSH.

D. Copy a script file to remote and run it with one SSH command.

Solution

1. Step 1: Consider multiple commands execution

   Running many commands inline can be complex and error-prone.

2. Step 2: Use script file for clarity and reuse

   Copying a script file to remote and running it via SSH is cleaner and easier to maintain.

3. Final Answer:

   Copy a script file to remote and run it with one SSH command. -> Option D

4. Quick Check:

   Script file on remote is best for multiple commands = C [OK]

Hint: Send script file and run remotely for many commands [OK]

Common Mistakes:

• Running many commands inline can cause quoting errors
• Running separate SSH commands is slower and complex
• Using scp to send commands is incorrect