Linux Commands

Published 5 years ago.
Last updated August 22nd, 2024

About Commands

Parameters and Flags

• A single hyphen (-) can be followed by multiple single-character flags.
• A double hyphen (--) prefixes a single, multi-character option.

Consider this example:

tar -czf

In this example, -czf specifies three single-character flags: c, z, and f.

Now consider another example:

tar --exclude

In this case, --exclude specifies a single, multi-character option named exclude. The double hyphen disambiguates the command-line argument, ensuring that tar interprets it as exclude rather than a combination of e, x, c, l, u, d, and e.

Commands

File System

List files and dirs         ls, ls- l (list with more info), ls -la (list all including hidden files)

Make a file                 touch <the-new-file-name>
Remove a file               rm <the-file-name>
Make a directory            mkdir <the-new-directory-name>
Remove dir and all under it rm -rf <the-directory-name>

Copy a directory            cp -r <the-source-file-or-directory-name> <the-file-or-directory-copy-name>

Change owner                sudo chown <the-user-name> <the-file-or-directory-name>
Change user                 sudo chgrp <the-group-name> <the-file-or-directory-name>

Find a file                 find <the-path-to-search-in-or . for-current-dir> -name <the-file-name>
Find a directory            find <the-path-to-search-in . for-current-dir> -type d -name '<the-directory-name>'

List search results         ls <the-dir-path> grep <the-string-to-search-for>

Page long output            <my_command> | less (use space bar to page, ctrl+c to exit)

Apps Services and Processes

List all running processes  ps -ef (all processes), ps (running processes)

Remote File System

Login to a remote server    ssh <the-username>@<the-host-name>

Copy file from a server     scp <the-username>@<the-host-name>:/path/to/server/file /path/to/local/dir/
Copy dir from a server      scp -r <the-username>@<the-host-name>:/path/to/server/dir /path/to/local/dir/

Copy file to a server       scp /path/to/local/file <the-username>@<the-host-name>:/path/to/server/dir

Copy file between servers   scp username@remote_1:/file/to/send username@remote_2:/where/to/put

Permissions (flags)

There are four OCTAL (0..7) digits, which control the file permissions. But often, only three are used. If you use 600 it equals 0600. The missing digit is appended at the beginning of the number.

Each of three digits described permissions. Position in the number defines to which group permissions do apply!
Permissions:
1 – can execute
2 – can write
4 – can read

The octal number is the sum of those three permissions, i.e.
3 (1+2) – can execute and write
6 (2+4) – can write and read

Position of the digit in value:
1 – what owner can
2 – what users in the file group(class) can
3 – what users not in the file group(class) can

Examples:
chmod 600 file – owner can read and write
chmod 700 file – owner can read, write and execute
chmod 666 file – all can read and write
chmod 777 file – all can read, write and execute

Host

hostname                    hostname
FQDN                        hostname -f
IP Address                  hostname -I
OS                          lsb_release -da
Network Look-Up             nslookup portal-dev.ral.ucar.edu

Apache

Apache Version              /usr/sbin/apache2ctl -v
Apache status               sudo systemctl status apache2
Stop Apache                 sudo systemctl stop apache2
Start Apache                sudo systemctl start apache2
Restart Apache              sudo systemctl restart apache2

PHP

PHP Version                 php -v
php.ini                     php -i | grep php.ini
PHP Binary Location         which php

Git

clone repo to current dir   git clone git@github.com:NCAR/nrit-web-visit-app.git

Perl

Perl Version                perl -v

MySQL

MySQL Version               mysql -V
Login                       mysql -u root -p
Restart mysql               sudo /etc/init.d/mysql restart

Remote Command Line Login

In order to access a DB you must use the mysql client. If it is not installed on your local machine, you can ssh into another host that does have it, like ral-itapps-a, and then login to the DB.

mysql -u <the-sql-user> -p'<the-sql-password>' \
  -h <the-database-host-url> -P 3306 \
  -D <the-database-name>

MySQL Commands

Show the current user       SELECT USER();
Show all users              SELECT DISTINCT User FROM mysql.user;
Show user privileges        SHOW GRANTS FOR 'my_user'@localhost;
Create a user               CREATE USER 'my_user'@localhost IDENTIFIED BY 'my_users_password';
Delete a user               DROP USER 'my_user'@localhost;
User Privs                  https://phoenixnap.com/kb/how-to-create-mariadb-user-grant-privileges

Show all databases          SHOW DATABASES;
Create a new database       CREATE DATABASE 'my_db_name';
Use a database              USE my_db_name;

Show all tables             SHOW tables;
Show table fields           DESCRIBE my_table_name;
Show table full fields      SHOW FULL COLUMNS FROM my_table_name;

Host Name                   SHOW VARIABLES LIKE '%host%';
Port                        SHOW VARIABLES LIKE '%port%';
Character Sets              SHOW VARIABLES LIKE '%character_set%';
Collations                  SHOW VARIABLES LIKE '%collation%';
SQL Mode                    SHOW VARIABLES LIKE '%sql_mode%';
Socket                      SHOW VARIABLES LIKE '%socket%';
DB Data Path                SHOW VARIABLES LIKE '%datadir%';
Engine Type                 SHOW VARIABLES LIKE '%storage_engine%';
Error Log Path              SHOW VARIABLES LIKE '%log_error%';

chmod - Change Mode

The chmod command sets the permissions of files or directories.

chmod <options> <permissions> <file name>

sudo chmod -R 755 /var/www/my_website

See the Linux chmod page for more information.

chown - Change Ownership

Used to change the ownership of a file or directory to another user and or group.

sudo chown -R www-data: /path/to/directory

find - Find Files

Syntax:

find <starting path> <options> <file name>

Find a file starting at the / path, non-case sensitive, for the name linux.odt:

find / -iname linux.odt

See the Linux find page for more information.

getent - Get Entries

getent is a Unix command that helps a user get entries in a number of important text files called databases. This includes the passwd and group databases which store user information – hence getent is a common way to look up user details on Unix.

Example: Get all the entries of the /etc/passwd file:

getent passwd

Outputs something like:

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
www-data:x:33:33:www-data:/var/www:/usr/sbin/nologin
dwatts:x:34182:1500:Daniel Watts:/home/dwatts:/bin/bash

• For more info see: https://askubuntu.com/questions/725120/can-anybody-explain-output-of-getent-passwd-command

grep - Search File Content

grep is a string and pattern matching utility that displays matching lines found in files, and can also work with piped output from other commands.

Find all instances of the string my string in the files under the /path/to/search directory:

grep -r "my string" /path/to/search

See the grep command page for more information.

ls - List

List the names and information about files and directories.

Passing the -la flag lists ALL files and directories (including hidden files).

$ ls -la

total 40
drwxr-xr-x  7 root  staff   224 Apr 27 10:41 .
drwxr-xr-x  7 root  staff   224 Jan 22 15:48 ..
drwxr-xr-x  8 root  staff   256 Jan 22 15:48 .svn
-rw-r--r--  1 root  staff  1897 Apr 27 10:41 email.php
-rw-r--r--  1 root  staff  2695 Jan 22 15:48 file_downloads.php
-rw-r--r--  1 root  staff  6373 Apr 28 09:35 handle_errors.php
-rw-r--r--  1 root  staff  1191 Jan 22 15:48 mime_types.php

ls -l Output Columns

1 - File Permissions
│           2 - Number of hard links
│           │ 3 - Owner name
│           │ │              4 - Group Name
│           │ │              │      5 - File Size
│           │ │              │      │    6 - Month of last modification
│           │ │              │      │    │   7 - Day of last modification
│           │ │              │      │    │   │  8 - Time of last modification
│           │ │              │      │    │   │  │     9 - File or Directory Name
│           │ │              │      │    │   │  │     │
└─────────  └ └────────────  └────  └─── └── └─ └──── └────────  
-rw-r--r--  1 squarem1admin  staff  1897 Apr 27 10:41 email.php

mkdir - Make Directory

Use the -p flag to also create the any parent directories.

mkdir -p /path/to/directory

nano - File Editor

Create a new file and open it for editing with nano:

nano /path/to/file.ext

Use control+z to exit and save.

rm - Remove

Remove will remove a directory or a file.
Passing -r will remove everything below (inside) the directory you are removing.

rm -r directory_to_remove

tail - Read the Tail of a File

The tail command reads a file, and outputs the last part of it (the "tail").

umask - Mode Creation Mask

The umask command returns, or sets, the value of the system's file mode creation mask.

Web Server Commands

Linux Host

MySQL Database Server

Apache HTTP Server

PHP