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Execute Commands On Remote Linux Systems Via SSH On Linux

Execute Commands On Remote Linux Systems Via SSH

By sk
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The other day I was testing how to keep file permissions intact while copying files or directories to multiple locations and systems. When I wanted to check the file permissions on a remote system, I had to log-in to that system over SSH and check the attributes. The process of log-in and log-out from the remote system multiple times annoyed me a bit. I thought it would be better if I can be able to execute commands on remote Linux systems via SSH. Luckily, I find a workaround to do it after looking into the man pages of ssh command. If you ever wondered how to run a command or script on a remote system from your local system itself without having to log in to that remote system, here is how to do it.

Execute Commands On Remote Linux Systems Via SSH

The typical way to run a command or script on a remote system over SSH from the local system is:

$ ssh <[email protected]_Address-or-Doman_name> <Command-or-Script>

Allow me to show you some examples.

Run a single command on remote systems via SSH

Let us say you want to find Kernel details of your remote Linux system. To do so, simply, run:

$ ssh [email protected] uname -a

Here,

  • sk is the username of my remote system,
  • 192.168.225.22 is the IP address of the remote system,
  • And "uname -a" is the command that I want to run on the remote system from my local system.

Sample output:

Execute Commands On Remote Linux Systems Via SSH

See? I haven't actually logged-in to the remote system, but executed the uname command on the remote system over SSH and displayed the output in my local system's Terminal.

You can also specify the command in quotes like below.

$ ssh [email protected] "uname -a"

Or,

$ ssh [email protected] 'uname -a'

If you have changed default port of SSH protocol, just mention it using -p parameter like below.

$ ssh -p 2200 [email protected] uname -a

Run multiple commands on remote systems via SSH

You can also run multiple commands on the remote by specifying them within quotes like below.

$ ssh [email protected] "uname -r && lsb_release -a"

Or,

$ ssh [email protected] "uname -r ; lsb_release -a"

The above commands will display the Kernel version and distribution details of my Ubuntu server.

Sample output:

Run multiple commands on remote systems via SSH on Linux

As one one of our reader mentioned in the comment section below, you should specify multiple commands in quotes. If you don't use quotes, the first command will execute on the remote system and second command will be evaluated on local machine only. The whole command in quotes will be processed remotely as intended.


Note: Know difference between “&&” and “;” operators between commands:

The “&&” operator executes the second command only if the first command was successful.

Example:

sudo apt-get update && sudo apt-get upgrade

In the above case, the second command (sudo apt-get upgrade) will execute if the first command was successful. Otherwise, it won’t run.

The “;” operator executes the second command even if the first command was successful or fail.

Example:

sudo apt-get update ; sudo apt-get upgrade

In the above case, the second command (sudo apt-get upgrade) will execute even if the first command is failed.


Run commands with "sudo" privileges on remote systems via SSH

Some commands requires "sudo" privileges to run. For instance, the following command will install Vim on my remote system.

$ ssh -t [email protected] sudo apt install apache2

Sample output:

Run commands with "sudo" privileges on remote systems via SSH

Did you notice? I have used -t flag in the above command. We need to mention this -t flag to force pseudo-terminal allocation. This can be used to execute arbitrary screen-based programs on a remote machine, which can be very useful, e.g. when implementing menu services.

Also, I have entered password twice. The first time I entered the password of the remote user to access the remote system over SSH from my local system and the second password is required to give sudo permission to the remote user to install application (i.e. apache2 in this case) on the remote system.

Let us check if the Apache service is running using command:

$ ssh -t [email protected] sudo systemctl status apache2
[email protected]'s password: 
[sudo] password for sk: 
● apache2.service - The Apache HTTP Server
Loaded: loaded (/lib/systemd/system/apache2.service; enabled; vendor preset: enabled)
Drop-In: /lib/systemd/system/apache2.service.d
└─apache2-systemd.conf
Active: active (running) since Thu 2019-12-19 11:08:03 UTC; 52s ago
Main PID: 5251 (apache2)
Tasks: 55 (limit: 2318)
CGroup: /system.slice/apache2.service
├─5251 /usr/sbin/apache2 -k start
├─5253 /usr/sbin/apache2 -k start
└─5254 /usr/sbin/apache2 -k start

Dec 19 11:08:03 ubuntuserver systemd[1]: Starting The Apache HTTP Server...
Dec 19 11:08:03 ubuntuserver apachectl[5227]: AH00558: apache2: Could not reliably determine the server's fully qualified domain name, using 2409:4072:51f:a1b6:a00:27ff:f
Dec 19 11:08:03 ubuntuserver systemd[1]: Started The Apache HTTP Server.

Similarly, we can run any command or script on a remote system over SSH from the local system.

Run local scripts on remote systems via SSH

Let us a create a simple script on our local system to display all the available information about your remote system's distribution name, package management and base details etc.

$ vi system_information.sh

Add the following lines:

#!/bin/bash
#Name: Display System Details
#Owner: OSTechNIx
#----------------------------
echo /etc/*_ver* /etc/*-rel*; cat /etc/*_ver* /etc/*-rel*

Press ESC key and type :wq to save and quit the file.

Now run this script on your remote system over SSH using command:

$ ssh [email protected] 'bash -s' < system_information.sh

Sample output:

[email protected]'s password: 
/etc/debian_version /etc/lsb-release /etc/os-release
buster/sid
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=18.04
DISTRIB_CODENAME=bionic
DISTRIB_DESCRIPTION="Ubuntu 18.04.2 LTS"
NAME="Ubuntu"
VERSION="18.04.2 LTS (Bionic Beaver)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 18.04.2 LTS"
VERSION_ID="18.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=bionic
UBUNTU_CODENAME=bionic

If you don't specify 'bash -s' in the above command, you will get the details of the remote system but Pseudo-terminal will not be allocated.

Save output from remote system to the local system

This can be useful if you want to share the output of a command that you run on the remote system over SSH with your support team or colleague.

The following command will run "du -ah" on your remote system over SSH and save the output in diskusage.txt file in your local system.

$ ssh [email protected] du -ah > diskusage.txt

You can then analyze the disk usage details by viewing the diskusage.txt file using cat command or text viewers.

$ cat diskusage.txt 
4.0K ./.profile
4.0K ./.gnupg/private-keys-v1.d
8.0K ./.gnupg
76K ./data/image.jpg
128K ./data/file.pdf
20K ./data/text.docx
5.9M ./data/audio.mp3
6.1M ./data
0 ./.sudo_as_admin_successful
4.0K ./pacman?inline=false
4.0K ./.bash_logout
4.0K ./.wget-hsts
4.0K ./.bash_history
0 ./.cache/motd.legal-displayed
4.0K ./.cache
4.0K ./deb-pacman_1.0-0.deb
4.0K ./.bashrc
6.2M .

Configure SSH Key-based authentication to avoid password typing

If you run commands on remote systems often, you may want to configure SSH key-based authentication to skip password typing every time. More details can be found in the following link.


Suggested read:


Hope this helps.

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4 comments

Harry December 20, 2019 - 3:50 am

Great write up.
I run ssh [email protected] ‘bash -s’ < system_information.sh from windows terminal (cmd)and got message
bash: bash -s: command not found.
However , it worked when run from Gitbash.Do I need to run it from Gitbash and not run from Windows?
Thanks

Reply
sk December 20, 2019 - 12:36 pm

Hey, this guide is tested on Ubuntu and it should work on other Linux distributions as well. I don’t know about Windows. Sorry I couldn’t help.

Reply
Cray XMP December 20, 2019 - 4:35 pm

You omitted the quotes in
$ ssh [email protected] uname -r && lsb_release -a
The statement “ssh [email protected] uname -r” has a return value which is tested locally against zero by “&&” then the second statement is evaluated on the local machine only.
The whole command in quotes will be processed remotely as intended.
$ ssh [email protected] ‘uname -r && lsb_release -a’

Reply
sk December 20, 2019 - 4:49 pm

You’re right. Nice catch. Corrected it now. Thank you.

Reply

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