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20140819-3 选题
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A Pocket Guide for Linux ssh Command with Examples
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================================================================================
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If you have been in the IT world for quite some time you probably have heard about SSH, how great a tool it is and all its cool security features. In this tutorial you will learn how to use SSH in a few minutes and login to your remote computers seamlessly and securely.
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If you have no clue what SSH is you can visit [Wikipedia][1] before proceeding.
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### Basic Usage ###
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The simplest usage of SSH is where you specify a user and the hostname. The hostname could be an IP address or a domain name an in the following format.
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$ ssh user@hostname
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For example to login to a Raspberry Pi on my LAN, I would simply type the command in the terminal as follows:
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$ ssh pi@10.42.0.47
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Where pi is the user and 10.42.0.47 is the IP of the Raspberry Pi on my LAN. Change this accordingly to reflect your LAN configuration or your remote computer’s IP address.
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If you have logged in successfully then the rest of the guide shall be a breeze for you.
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### Using A Different Port ###
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By default ssh uses port 22, but for various reasons you may want to connect to another port.
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$ ssh -p 10022 user@hostname
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This will connect to ssh via port 10022 instead of port 22.
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### Execute Commands Remotely ###
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At times its convenient to execute a command on the remote host and get the output and continue working on the local machine. Well SSH has catered for this need,
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$ ssh pi@10.42.0.47 ls -l
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This command for example will list the contents of the home directory and return the prompt to you. Cool? Try it out with other commands as well.
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### Mounting remote filesystems ###
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Another great tool based on ssh is sshfs. With sshfs you can mount remote filesystems and have the remote files on the local machine.
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$ sshfs -o idmap=user user@hostname:/home/user ~/Remote
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For example this command can be used as:
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$ sshfs -o idmap=user pi@10.42.0.47:/home/pi ~/Pi
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This will mount pi’s home directory to a folder on the local machine called Pi.
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For more details on sshfs [look at our sshfs tutorial][2].
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### X11 Forwarding ###
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Suppose now you want to run a GUI program on your remote computer? SSH had you in mind! Login to the remote machine with the basic SSH command but -X option. This will allow X11 forwarding. After you login you might not see any difference, but once you invoke a GUI based program you notice the difference.
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$ ssh -X pi@10.42.0.47
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$ pistore
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Now you may want to do other stuff on the command line while running the GUI program. Simply suffix the command with &.
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$ pistore&
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### Escape Sequences ###
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There are various escape sequences provided by SSH. To view them, SSH to any remote machine then type tilde(~) followed by a question mark. You will see a couple of other supported escape sequences. In this example you can the output of **~#** and **~C**.
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### Edit SSH Configuration ###
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If you need to change SSH configuration, open the file **/etc/ssh/sshd_config** with your favourite text editor and edit whatever you need to. For example we might need to change the banner. In your text editor find the following line:
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#Banner none
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Uncomment the line by deleting the # then add a path to the file with the message you want displayed. The line should now read as:
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Banner /etc/issue
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In this /etc/ssh/sshd_config file you will also find the options of changing the port number, idle logout timeout e.t.c . These are fairly straight forward, but refer to the ssh manual for anything that might not be familiar before attempting to make changes.
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### Generate SSH Key Pair ###
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To generate a new key pair run the command as follows:
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$ ssh-keygen -t dsa
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You will be asked for a passphrase then the key pair will be generated. This command will also give you the key’s randomart image.
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### Finding A Hostkey ###
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Now before you add that key pair it does no harm to see if it exists already.
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$ ssh-keygen -F 10.42.0.47
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### Removing A Hostkey ###
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Sometimes its necessary to remove a key pair you had generated, for example when the host has changed or perhaps when you need to remove keys that are no longer used.
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$ ssh-keygen -R 10.42.0.47
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This is much more convenient than opening **~/.ssh/known_hosts**
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and removing the keys manually.
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### Conclusion ###
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With the above commands you will be able to use SSH with ease. There is more to explore and your imagination is your limitation.
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--------------------------------------------------------------------------------
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via: http://linoxide.com/linux-command/pocket-guide-linux-ssh-command/
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作者：[Bobbin Zachariah][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://linoxide.com/author/bobbin/
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[1]:http://en.wikipedia.org/wiki/Secure_Shell
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[2]:http://linoxide.com/how-tos/sshfs-mount-remote-directories/
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Linux Systemd - Start/Stop/Restart Services in RHEL / CentOS 7
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================================================================================
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One of the major changes in RHEL / CentOS 7.0 is the swtich to **systemd**, a system and service manager, that replaces SysV and Upstart used in previous releases of Red Hat Enterprise Linux. systemd is compatible with SysV and Linux Standard Base init scripts.
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**Systemd** is a system and service manager for Linux operating systems. It is designed to be backwards compatible with SysV init scripts, and provides a number of features such as parallel startup of system services at boot time, on-demand activation of daemons, support for system state snapshots, or dependency-based service control logic.
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Previous versions of Red Hat Enterprise Linux, which were distributed with SysV init or Upstart, used init scripts written in bash located in the /etc/rc.d/init.d/ directory. In RHEL 7 / CentOS 7, these init scripts have been replaced with service units. Service units end with the .service file extension and serve a similar purpose as init scripts. To view, start, stop, restart, enable, or disable system services you will use the systemctl instead of the old service command.
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> Note: for backwards compatibility the old service command is still available in CentOS 7 and it will redirect any command to the new systemctl utility.
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### Start/Stop/Restart Services with systemctl ###
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To start a service with systemctl you will need to use the command like this:
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# systemctl start httpd.service
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This will start the httpd service, in our case Apache HTTP Server.
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To stop it use this command as root:
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# systemctl stop httpd.service
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To restart you can use either the restart options, it will restart the service if it’s running or start it if it’s not running. You can also use the try-restart option that will restart the service only if it’s already running. Also you have the reload option that will reload the configuration files.
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# systemctl restart httpd.service
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# systemctl try-restart httpd.service
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# systemctl reload httpd.service
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The commands in our example look like this:
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### Checking the status of a service ###
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To check the status of a service you can use the status option like this:
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# systemctl status httpd.service
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And the output should look like this:
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Informing you of various aspects of the running service.
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### Enable / Disable services to run at boot time ###
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You can also use the enable / disable options to make a service run at boot time, using the command like this:
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# systemctl enable httpd.service
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# systemctl disable httpd.service
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The output looks like this:
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Although the adoption of systemd has been very controversial in the last few years, slowly most of the major Linux distributions have either adopted or are planning to have it in the next point release, so it’s a useful tool to get used to.
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--------------------------------------------------------------------------------
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via: http://linoxide.com/linux-command/start-stop-services-systemd/
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作者：[Adrian Dinu][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创翻译，[Linux中国](http://linux.cn/) 荣誉推出
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[a]:http://linoxide.com/author/adriand/
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