Some Linux shell commands

Some linux commands listed here are only available on Ubuntu/Debian. Other linux distributions may have differences.

| || && symbols

  • &&: do the latter command only if the former command succeeds. E.g. echo 1 && echo "success"
  • ||: do the latter command only if the former command fails. E.g. cat xxx || echo "fail"
  • The above two symbols can appear in one sentence, like {do something} && {echo "success"} || {echo "fail"}
  • | denotes a ‘pipe’ command. It lets you use 2 or more commands s.t. the output of one command serves as input to the next.
    • | can only handle stdout from the last command. It cannot handle stderr directly. If you want to make stderr available for the pipe, redirect the data stream by using 2>&1.
    • The latter command (followed by |) must be able to handle inputs from stdin. For example, ls, cp, mv does not accept datas from stdin and they are not pipe commands.
    • E.g. ifconfig | grep inet

$-related symbols

Variables

  • $xxx or ${xxx} fetches the value of a variable or the environment variable

Calculate the result

  • $( ) and `` (back quote) executes the command inside the bracket or back quote, and returns the output of that command

  • $(( )) $[] calculates the formula

Others

  • $# indicates ‘argc’, the count of arguments
  • $n is the value of the n-th argument, where n=1,2,…,9
  • $* shows all arguments’ value (can exceed 9) in a single string
  • $@ is almost the same as $*, except that $@ will add quotation marks to pass multiple arguments, unlike $* which combines all arguments into one single argument
  • $$ is the PID (Process ID) of the shell itself
  • $? is the return value of the last command, where 0 indicates success, and all other values means an error
  • $! is the PID of the last background command (the last background process)

dpkg

  • Install deb type softwares on Ubuntu or Debian: sudo dpkg -i xxx.deb

scp Commands

scp command can be used to copy files between local and remote, which is based on ssh protocal.

Copy local files to a remote host

Copy file(s)

  • Copy local files to remote host’s specified directory (the file name keeps same)
1
scp path/to/local/file RemoteUsrName@IP:/path/to/remote/folder/
  • Specify the file name, and rename the file to specified filename when copying
1
scp path/to/local/file RemoteUsrName@IP:/path/to/remote/file

Copy directory

  • Copy a directory, but hidden files (such as .git directory) will not be copied
1
scp -r /path/to/local/folder RemoteUsrName@IP:/path/to/remote/folder/
  • Copy a directory, including all hidden directories
1
scp -rp /path/to/local/folder RemoteUsrName@IP:/path/to/remote/folder/

Download remote files to local directory

To download files, we only need to swap the last two arguments (file or directory path) in the commands. For example:

1
scp -rp RemoteUsrName@IP:/path/to/remote/folder/  /path/to/local/folder`

scp uses the same protocal as ssh, and will also use port 22 by default.

If you want to use another port, you need to add -P xxx after scp command, for instance, scp -P 10000 -rp /path/to/local/folder RemoteUsrName@IP:/path/to/remote/folder/. Note that you have to use uppercase P

Want to avoid typing password everytime you use scp? You need to configure public key authentication for ssh. For reference, ssh-keygen 是什么？

View and change hostname

  • See your hostname on Ubuntu: use hostname command or uname -n command

  • Change your hostname on Ubuntu:

    • sudo vim /etc/hostname to change the hostname forever (Restart to take effect)

    • hostname xxx to change the hostname temporarily (takes effect immediately, but restart will restore the original hostname)

Modify the prompt after a successful login

  • Ubuntu： cd /etc/update-motd.d and modify the shell scripts inside it.

    (You may add exit 0 command near the beginning of the scripts, to ‘cancel’ executing it by letting it exit earlier)

Cannot SSH to your machine?

If you cannot ssh to your machine remotely, check these settings:

  • Check whether openssh-server is installed
1
sudo apt install openssh-server
  • ufw
1
sudo ufw disable

  • Modify your sshd configuration

    1
    sudo vim /etc/ssh/sshd_config

    To allow SSH to root user, set the following config in the file

    1
    2
    PermitRootLogin prohibit-password  # disable this
    PermitRootLogin yes   # enable this

  • Restart SSH service

    1
    sudo service ssh restart

Terminal timeout with SSH?

It is sometimes annoying that, when SSHing to a remote machine, the session gets timeout after minutes of inactivity, and you have to reinitiate the connection. Well, this can be solved under a simple fix.

On the local machine, open ~/.ssh/config and add the following line:

1
2
Host *
    ServerAliveInterval 60

The meaning is, for any remote host, send a null packet every 60 seconds to keep the connection alive.

The alternative is to configure on remote host. On the server, edit the /etc/ssh/sshd_config file, and find the following parameters: ClientAliveInterval and ClientAliveCountMax.

  • ClientAliveInterval specifies the time (in seconds) that the server will wait before sending a null packet to client to keep connection alive.

  • ClientAliveCountMax defines the number of client alive messages which are sent without getting any msg from the client. When this limit is reached, the remote host will drop this connection to terminate this SSH session.

For instance, you can set

1
2
ClientAliveInterval 1200
ClientAliveCountMax 3

The timeout value is given by the product of the two parameters.
(timeoutvalue = ClientAliveInterval * ClientAliveCountMax)

After that, reload OpenSSH-server service.

1
sudo systemctl reload sshd

How to add a user?

First we need to know the difference between adduser and useradd.

Both commands are used for creating a new user, but follow different ways to execute it.

The main difference is that,

  • adduser will setup the user’s home folder, password and other necessary functions, asking you to fill up the information.
  • useradd only creates a new user, without creating the home directory, adding password and so on.

For beginners, it is highly recommended to use adduser command.

Only root user is allowed to add or modify users. If you are a normal user, you need to use sudo adduser to add a user.

How to add a user to sudoers

There’re two ways to grant sudo privileges to a user. The recommended one is to add the user to sudo group. An alternative is to edit the sudoers file.

Add the user to sudo group

The easiest way to grant sudo privileges to a user is by adding it to the ‘sudo’ group. Members in this group can execute any command as ‘root’ via sudo.

A 'group' refers to a collection of users who are given a similar permissions. Note that one user may belong to multiple groups.

To add a user to ‘sudo’ group, use the following command (replace USERNAME with the username you want to grant sudo access):

1
usermod -aG sudo USERNAME

To ensure you have successfully granted the user sudo privileges, use sudo whoami and see the output. If the output is ‘root’, you succeeds.

Add the user to sudoers file

This way is not recommended compared to the above one.

The users’ and groups’ sudo privileges configurations are stored in /etc/sudoers file. You can configure the sudo access by modifying theis file, or by creating a new configuration file in /etc/sudoers.d/ directory, since the files in this dir are included in the sudoers file.

You should always use visudo command to edit the sudoers file. This command will check for syntax errors when you save it. If an error is detected, the file is not saved to prevent unexpected disaters.

Open the file by visudo /etc/sudoers and add the following line at the end of the file (change ‘USERNAME’ with the username you want to grant access to)

1
USERNAME ALL=(ALL:ALL) NOPASSWD: ALL

This grants the user permission to execute all commands via sudo. If you want to give partial permissions only, consider this example:

1
USERNAME ALL=(ALL:ALL) NOPASSWD: /bin/rm, /bin/mkdir

This allows only mkdir and rm commands to be run as root, which is safer under some special circumstances.

Create a service and autostart

Some tasks requires the terminal to be active all the times. When a terminal window is closed, the process inside it might stop. You can use screen to make something always alive in the terminal, however it cannot autostart at system startup. To configure the autostart, we need to create a ‘service’.

In /lib/systemd/system/, create a file named xxx.service (you may need sudo), and write the following things:

1
2
3
4
5
6
7
8
9
10
11
[Unit]
Description=Write your description here

[Service]
Type=simple
User=your_username
Group=your_users_group_name
ExecStart=Enter_your_command_here

[Install]
WantedBy=multi-user.target

Without the line User=xxx and Group=xxx, the service will run as root by default.

Then you can start this service, and it will be running in the background

1
sudo systemctl start xxx

To make it start automatically at system startup, enter

1
sudo systemctl enable xxx

At any time, to check the service status and its output, use

1
sudo systemctl status xxx

Let nginx reverse proxy ‘code-server’

Suppose your code-server (VSCode on the web) is running on localhost:12345. Then, to proxy it, add the following to your nginx configuation

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
server {
    listen 80; 
    server_name domain.name.com;   # replace with your domain name
    location / {
        proxy_pass  http://localhost:12345;   # enter your localhost address and port here
        proxy_redirect     off;
        proxy_set_header   Host             $host;        
        proxy_set_header   X-Real-IP        $remote_addr;  
        proxy_set_header   X-Scheme         $scheme;       
        proxy_set_header   X-Forwarded-For  $proxy_add_x_forwarded_for;
        proxy_set_header   Accept-Encoding  gzip;
        proxy_set_header   Upgrade          $http_upgrade;
        proxy_set_header   Connection       upgrade; 
    }
}

Use sudo without password

To use sudo without password, we need to configure /etc/sudoers file.

It’s recommended to use visudo command to edit it, instead of using normal editors like vim.

1
visudo

and in the end of this file, add a line

1
<your_username>  ALL=(ALL:ALL) NOPASSWD: ALL

After that, save the file.

The default editor for root user might be nano. If you prefer vim, you can switch to the root user (sudo -i), and add a line export EDITOR=/usr/bin/vim in /root/.bashrc to let vim be the default editor.

Clean up journal (log files) to save disk space

You may find that /var/log/journal/ occpies a large amount of your disk usage. These are log files created by ‘journal’ service (systemd), and can be disabled or limited to save space.

To limit its storage, use

1
2
3
sudo journalctl --disk-usage      # see the disk usage
sudo journalctl --vacuum-time=1d  # remove logs more than 1 day ago
sudo journalctl --vacuum-size=10M # limit every log's size to 10MB

After that, your /var/log/journal/ space will be significantly reduced

Linux
#Linux
Some Linux shell commands
https://www.billhu.us/2022/19_linux_commands/
Author
Bill Hu
Posted on
February 10, 2022
Licensed under