Linux Commands.md

1. SYSTEM

$ uname –a                          # Display linux system information
$ uname –r                          # Display kernel release information (refer uname command in detail)
$ hostname                          # Show system host name
$ hostname -i                       # Display the IP address of the host (all options hostname)
$ uptime                            # Show how long system running + load (learn uptime command)
$ last reboot                       # Show system reboot history (more examples last command)
$ cat /etc/redhat_release           # Show which version of redhat installed 
$ date                              # Show the current date and time (options of date command)
$ cal                               # Show this month calendar (what more in cal)
$ w                                 # Display who is online (learn more about w command)
$ whoami                            # Who you are logged in as (example + sreenshots)
$ finger user                       # Display information about user (many options of finger command)

2. Hardware

$ dmesg                             # Detected hardware and boot messages (dmesg many more options)
$ cat /proc/cpuinfo                 # CPU model
$ cat /proc/meminfo                 # Hardware memory
$ cat /proc/interrupts              # Lists the number of interrupts per CPU per I/O device
$ lshw                              # Displays information on hardware configuration of the system
$ lsblk                             # Displays block device related information in Linux (sudo yum install util-linux-ng)
$ free -m                           # Used and free memory (-m for MB) (free command in detail)
$ lspci -tv                         # Show PCI devices (very useful to find vendor ids)
$ lsusb -tv                         # Show USB devices (read more lsusb options)
$ lshal                             # Show a list of all devices with their properties 
$ dmidecode                         # Show hardware info from the BIOS (vendor details)
$ hdparm -i /dev/sda                # Show info about disk sda 
$ hdparm -tT /dev/sda               # Do a read speed test on disk sda
$ badblocks -s /dev/sda             # Test for unreadable blocks on disk sda

3. Statistics

$ top                               # Display and update the top cpu processes (30 example options)
$ mpstat 1                          # Display processors related statistics (learn mpstat command)
$ vmstat 2                          # Display virtual memory statistics (very useful performance tool)
$ iostat 2                          # Display I/O statistics (2sec Intervals) (more examples)
$ tail -n 500 /var/log/messages     # Last 10 kernel/syslog messages (everyday use tail options)
$ tcpdump -i eth1                   # Capture all packets flows on interface eth1 (useful to sort network issue)
$ tcpdump -i eth0 'port 80'         # Monitor all traffic on port 80 ( HTTP )
$ lsof                              # List all open files belonging to all active processes.(sysadmin favorite command)
$ lsof -u testuser                  # List files opened by specific user
$ free –m                           # Show amount of RAM (daily usage command)
$ watch df –h                       # Watch changeable data continuously(interesting linux command)

4. Users

$ id                                        # Show the active user id with login and group(with screenshot)
$ last                                      # Show last logins on the system (few more examples)
$ who                                       # Show who is logged on the system (real user who logged in)
$ groupadd   admin                          # Add group "admin" (force add existing group)
$ useradd -c "Sam Tomshi" -g admin -m sam   # Create user "sam" and add to group "admin" (here read all parameter)
$ userdel sam                               # Delete user sam (force,file removal)
$ adduser sam                               # Add user "sam" 
$ usermod                                   # Modify user information (mostly useful for linux system admins)

5. File Commands

$ ls –al                                # Display all information about files/ directories(20 examples)
$ pwd                                   # Show current directory path(simple but need every day)
$ mkdir directory-name                  # Create a directory(create mutiple directory)
$ rm file-name                          # Delete file(be careful of using rm command)
$ rm -r directory-name                  # Delete directory recursively 
$ rm -f file-name                       # Forcefully  remove file
$ rm -rf directory-name                 # Forcefully remove directory recursively
$ cp file1 file2                        # Copy file1 to file2 (15 cd command examples)
$ cp -r dir1 dir2                       # Copy dir1 to dir2, create dir2 if it doesn’t  exist
$ mv file1 file2                        # Move files from one place to another(with 10 examples)
$ ln –s  /path/to/file-name  link-name  # Create symbolic link to file-name (examples)
$ touch file                            # Create or update file (timestamp change)
$ cat > file                            # Place standard input into file (15 cat command examples)
$ more file                             # Output the contents of file (help display long tail files)
$ head file                             # Output the first 10 lines of file (with different parameters)
$ tail file                             # Output the last 10 lines of file (detailed article with tail options)
$ tail -f file                          # Output the contents of file as it grows starting with the last 10 lines
$ gpg -c file                           # Encrypt file (how to use gpg)
$ gpg file.gpg                          # Decrypt file

6. Process Related

$ ps                                # Display your currently active processes (many parameters to learn)
$ ps aux | grep 'telnet'            # Find all process id related to telnet process
$ pmap                              # Memory map of process (kernel,user memory etc)
$ top                               # Display all running processes (30 examples)
$ kill pid                          # Kill process with mentioned pid id (types of signals)
$ killall proc                      # Kill all processes named proc
$ pkill processname                 # Send signal to a process with its name
$ bg                                # Resumes suspended jobs without bringing them to foreground (bg and fg command)
$ fg                                # Brings the most recent job to foreground
$ fg n                              # Brings job n to the foreground

7. File Permission Related

$ chmod octal file-name                     # Change the permissions of file to octal , which can be found separately for user, group and world
                                            # Octal value (more examples)
                                            # 4 - read
                                            # 2 – write
                                            # 1 – execute
# Example 
$ chmod 777 /data/test.c                    # Set rwx permission for owner , rwx  permission for group, rwx permission for world
$ chmod 755 /data/test.c                    # Set rwx permission for owner,rx for group and world
$ chown owner-user file                     # Change owner of the file (chown more examples)
$ chown owner-user:owner-group  file-name   # Change owner and group owner of the file
$ chown owner-user:owner-group directory    # Change owner and group owner of the directory
# Example 
$ chown bobbin:linoxide test.txt
$ ls -l test.txt                            # -rw-r--r-- 1 bobbin linoxide 0 Mar 04 08:56 test.txt

8. Network

$ ifconfig –a                           # Display all network ports and ip address (set mtu and other all options)
$ ifconfig eth0                         # Display specific  ethernet port ip address and details
$ ip addr show                          # Display all network interfaces and ip address (available in iproute2 package, more powerful than ifconfig)
$ ip address add 192.168.0.1 dev eth0   # Set ip address
$ ethtool eth0                          # Linux tool to show ethernet status (set full duplex , pause parameter)
$ mii-tool  eth0                        # Linux tool to show  ethernet status (more or like ethtool)
$ ping host                             # Send echo request to test connection (learn sing enhanced ping tool)
$ whois domain                          # Get who is information for domain
$ dig domain                            # Get DNS information for domain (screenshots with other available parameters)
$ dig  -x host                          # Reverse lookup host 
$ host google.com                       # Lookup DNS ip address for the name (8 examples of host command)
$ hostname –i                           # Lookup local ip address (set hostname too)
$ wget file                             # Download file (very useful other option)
$ netstat  -tupl                        # Listing all active listening ports(tcp,udp,pid) (13 examples)

9. Compression / Archives

$ tar cf home.tar  home             # Create tar named home.tar containing home/ (11 tar examples)
$ tar xf file.tar                   # Extract the files from file.tar
$ tar czf  file.tar.gz  files       # Create a tar with gzip compression
$ gzip file                         # Compress file and renames it to file.gz (untar gzip file)

10. Install Package

$ rpm -i pkgname.rpm                # Install rpm based package (Installing, Uninstalling, Updating, Querying, Verifying)
$ rpm -e pkgname                    # Remove package
# Install from source
$ ./configure
$ make
$ make install (what it is)

11. Search

$ grep pattern files                # Search for pattern in files (you will this command often)
$ grep  -r pattern dir              # Search recursively for pattern in dir
$ locate file                       # Find all instances of file
$ find /home/tom -name 'index*'     # Find files names that start with "index"(10 find examples)
$ find /home -size +10000k          # Find files larger than 10000k in /home

12. Login (ssh and telnet)

$ ssh user@host                     # Connect to host as user (secure data communication command)
$ ssh  -p port user@host            # Connect to host using specific port
$ telnet host                       # Connect to the system using  telnet port

13. File Transfer

scp
$ scp file.txt   server2:/tmp                           # Secure copy file.txt to remote host  /tmp folder
$ scp nixsavy@server2:/www/*.html   /www/tmp            # Copy *.html files from remote host to current system /www/tmp folder
$ scp -r nixsavy@server2:/www   /www/tmp                # Copy all files and folders recursively from remote server to the current system /www/tmp folder
rsync
$ rsync -a /home/apps /backup/                          # Synchronize source to destination
$ rsync -avz /home/apps linoxide@192.168.10.1:/backup   # Synchronize files/directories between the local and remote system with compression enabled

14. Disk Usage

$ df –h                             # Show free space on mounted filesystems(commonly used command)
$ df -i	                            # Show free inodes on mounted filesystems
$ fdisk -l	                        # Show disks partitions sizes and types(fdisk command output)
$ du -ah                            # Display disk usage in human readable form (command variations)
$ du -sh                            # Display total disk usage on the current directory
$ findmnt                           # Displays target mount point for all filesystem (refer type,list,evaluate output)
$ mount device-path mount-point     # Mount a device 

15. Directory Traverse

$ cd /test                          # Change to /test directory
$ cd ..                             # To go up one level of the directory tree(simple & most needed)
$ cd	                            # Go to $HOME directory