Linux commands

Linux Commands
1. Tar Command Examples: Archiving and Extracting Files
Create a new uncompressed tar archive from a directory.	$ tar cvf archive_name.tar dirname/
Extract files from an existing uncompressed tar archive.	$ tar xvf archive_name.tar
List the contents of an existing uncompressed tar archive.	$ tar tvf archive_name.tar
2. Grep Command Examples: Powerful Text Searching
Perform a case-insensitive search for a string within a file.	$ grep -i "the" demo_file
Display the matched line and the three lines immediately following it.	$ grep -A 3 -i "example" demo_text
Recursively search for a string across all files in the current directory and its subdirectories.	$ grep -r "ramesh" *
3. Find Command Examples: Locating Files and Directories
Locate files by name, performing a case-insensitive search.	# find -iname "MyCProgram.c"
Execute an arbitrary command on files discovered by the 'find' command (e.g., calculate MD5 checksum).	$ find -iname "MyCProgram.c" -exec md5sum {} \;
Identify all empty files within the user's home directory.	# find ~ -empty
4. SSH Command Examples: Secure Remote Access
Establish a secure shell connection to a remote host with a specified username.	ssh -l jsmith remotehost.example.com
Enable verbose debugging output for the SSH client during connection attempts.	ssh -v -l jsmith remotehost.example.com
Show the installed SSH client version.	$ ssh –V OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003
5. Sed Command Examples: Stream Editor for Text Transformation
Convert a DOS-formatted text file to Unix format by removing carriage returns.	$sed 's/.$//' filename
Reverse the order of lines in a file.	$ sed -n '1!G;h;$p' thegeekstuff.txt
Prepend line numbers to all non-empty lines in a file.	$ sed '/./=' thegeekstuff.txt | sed 'N; s/\n/ /'
6. Awk Command Examples: Pattern Scanning and Processing Language
Filter out and display unique lines from a file.	$ awk '!($0 in array) { array[$0]; print }' temp
Display lines from '/etc/passwd' where the User ID (UID) matches the Group ID (GID).	$awk -F ':' '$3==$4' passwd.txt
Extract and display specific fields from a delimited file.	$ awk '{print $2,$5;}' employee.txt
7. Vim Command Examples: Efficient Text Editing
Open a file and navigate directly to a specified line number.	$ vim +143 filename.txt
Open a file and jump to the first occurrence of a search term.	$ vim +/search-term filename.txt
Open a file in read-only mode to prevent accidental modifications.	$ vim -R /etc/passwd
8. Diff Command Examples: Comparing Files
Compare two files, ignoring differences in whitespace.	# diff -w name_list.txt name_list_new.txt 2c2,3 < John Doe --- > John M Doe > Jason Bourne
9. Sort Command Examples: Ordering Text Files
Sort the lines of a file in ascending alphabetical order.	$ sort names.txt
Sort the lines of a file in descending alphabetical order.	$ sort -r names.txt
Sort the '/etc/passwd' file numerically by the third colon-delimited field (UID).	$ sort -t: -k 3n /etc/passwd | more
10. Export Command Examples: Managing Environment Variables
Display all environment variables related to 'ORACLE'.	$ export | grep ORACLE declare -x ORACLE_BASE="/u01/app/oracle" declare -x ORACLE_HOME="/u01/app/oracle/product/10.2.0" declare -x ORACLE_SID="med" declare -x ORACLE_TERM="xterm
Set and export a new environment variable.	$ export ORACLE_HOME=/u01/app/oracle/product/10.2.0
11. Xargs Command Examples: Building and Executing Command Lines
Copy all JPG images from the current directory to an external hard drive.	# ls *.jpg | xargs -n1 -i cp {} /external-hard-drive/directory
Find all JPG images on the system and create a compressed tar archive of them.	# find / -name *.jpg -type f -print | xargs tar -cvzf images.tar.gz
Download files from all URLs listed in 'url-list.txt', allowing for continued downloads.	# cat url-list.txt | xargs wget –c
12. LS Command Examples: Listing Directory Contents
List files with sizes presented in human-readable formats (KB, MB, GB).	$ ls -lh -rw-r----- 1 ramesh team-dev 8.9M Jun 12 15:27 arch-linux.txt.gz
List files sorted by last modification time in reverse order (oldest first).	$ ls -ltr
Classify and list files, appending indicators to distinguish directories, executables, and symbolic links.	$ ls -F
13. PWD Command: Print Working Directory	The 'pwd' command is used to print the name of the current working directory.
14. CD Command Examples: Changing Directories
Toggle between the current and previous working directories.	$ cd -
15. Gzip Command Examples: File Compression and Decompression
Compress a file using gzip, creating a '.gz' archive.	$ gzip test.txt
Decompress a '.gz' file.	$ gzip -d test.txt.gz
16. Bzip2 Command Examples: Advanced File Compression
Compress a file using bzip2, creating a '.bz2' archive.	$ bzip2 test.txt
Decompress a '.bz2' file.	bzip2 -d test.txt.bz2
Display compression ratio and details of a '.gz' file (using gzip's list option).	$ gzip -l *.gz compressed uncompressed ratio uncompressed_name 23709 97975 75.8% asp-patch-rpms.txt
17. Unzip Command Examples: Extracting Zip Archives
Extract all files from a '.zip' archive.	$ unzip test.zip
List the contents of a '.zip' archive without extracting them.	$ unzip -l jasper.zip Archive: jasper.zip Length Date Time Name -------- ---- ---- --- 40995 11-30-98 23:50 META-INF/MANIFEST.MF 32169 08-25-98 21:07 classes_ 15964 08-25-98 21:07 classes_names 10542 08-25-98 21:07 classes_ncomp
18. Shutdown Command Examples: System Control
Immediately shut down the system and power off.	# shutdown -h now
Schedule a system shutdown to occur in 10 minutes.	# shutdown -h +10
Immediately reboot the system.	# shutdown -r now
Reboot the system and force a filesystem check during startup.	# shutdown -Fr now
19. FTP Command Examples: File Transfer Protocol
Connect to an FTP server and download multiple files using wildcards.	$ ftp IP/hostname ftp> mget *.html
List specific files on the remote FTP server before initiating a download.	ftp> mls *.html - /ftptest/features.html /ftptest/index.html /ftptest/othertools.html /ftptest/samplereport.html /ftptest/usage.html
20. Crontab Command Examples: Scheduling Tasks
Display the crontab entries for a specified user.	# crontab -u john -l
Schedule a script to run every 10 minutes.	*/10 * * * * /home/ramesh/check-disk-space
21. Service Command Examples: Managing System Services
Check the operational status of a specified system service.	# service ssh status
Display the status of all currently managed system services.	service --status-all
Restart a specified system service.	# service ssh restart
22. PS Command Examples: Process Status
Display comprehensive information about all running processes.	$ ps -ef | more
Display running processes in a hierarchical tree format.	$ ps -efH | more
23. Free Command Examples: Displaying Memory Usage
Show total, used, and free memory, swap, buffers, and cache in bytes.	$ free total used free shared buffers cached Mem: 3566408 1580220 1986188 0 203988 902960 -/+ buffers/cache: 473272 3093136 Swap: 4000176 0 4000176
Display memory usage in gigabytes. (Use -b for bytes, -k for kilobytes, -m for megabytes).	$ free -g total used free shared buffers cached Mem: 3 1 1 0 0 0 -/+ buffers/cache: 0 2 Swap: 3 0 3
Show a total line for memory and swap usage.	ramesh@ramesh-laptop:~$ free -t total used free shared buffers cached Mem: 3566408 1592148 1974260 0 204260 912556 -/+ buffers/cache: 475332 3091076 Swap: 4000176 0 4000176 Total: 7566584 1592148 5974436
24. Top Command Examples: Real-time Process Monitoring
Monitor running processes in real-time. (Interactive: Press 'O' to sort by various columns like PID, USER, etc.)	$ top Current Sort Field: P for window 1:Def Select sort field via field letter, type any other key to return a: PID = Process Id v: nDRT = Dirty Pages count d: UID = User Id y: WCHAN = Sleeping in Function e: USER = User Name z: Flags = Task Flag
Display real-time process information filtered to a specific user.	$ top -u oracle
25. DF Command Examples: Reporting Disk Space Usage
Report filesystem disk space usage in kilobytes (default for '-k').	$ df -k Filesystem 1K-blocks Used Available Use% Mounted on /dev/sda1 29530400 3233104 24797232 12% / /dev/sda2 120367992 50171596 64082060 44% /home
Display filesystem disk space usage in human-readable format (e.g., GB, MB).	ramesh@ramesh-laptop:~$ df -h Filesystem Size Used Avail Use% Mounted on /dev/sda1 29G 3.1G 24G 12% / /dev/sda2 115G 48G 62G 44% /home
26. Kill Command Examples: Terminating Processes
Forcefully terminate a process by its Process ID (PID). (Find PID with 'ps -ef | grep process_name').	$ ps -ef | grep vim ramesh 7243 7222 9 22:43 pts/2 00:00:00 vim $ kill -9 7243
27. RM Command Examples: Removing Files and Directories
Remove a file with an interactive confirmation prompt.	$ rm -i filename.txt
Remove multiple files matching a pattern with interactive confirmation.	$ rm -i file*
Recursively remove a directory and all its contents.	$ rm -r example
28. CP Command Examples: Copying Files and Directories
Copy a file, preserving its mode, ownership, and timestamp.	$ cp -p file1 file2
Copy a file with an interactive prompt before overwriting an existing destination.	$ cp -i file1 file2
29. MV Command Examples: Moving and Renaming Files
Rename or move a file, prompting for confirmation before overwriting an existing destination. (Use '-f' to force overwrite without prompt).	$ mv -i file1 file2
Display verbose output during file move/rename operations.	$ mv -v file1 file2
30. Cat Command Examples: Concatenating and Displaying Files
Concatenate and display the content of multiple files to standard output.	$ cat file1 file2
Display file contents with line numbers prepended to each line.	$ cat -n /etc/logrotate.conf 1 /var/log/btmp { 2 missingok 3 monthly 4 create 0660 root utmp 5 rotate 1 6 }
31. Mount Command Examples: Attaching Filesystems
Mount a filesystem to a specified directory.	# mkdir /u01 # mount /dev/sdb1 /u01
Add a filesystem entry to '/etc/fstab' for automatic mounting on system startup.	# /dev/sdb1 /u01 ext2 defaults 0 2
32. Chmod Command Examples: Changing File Permissions
Grant full read, write, and execute permissions to the user and group for a specified file.	$ chmod ug+rwx file.txt
Remove all read, write, and execute permissions for the group on a specified file.	$ chmod g-rwx file.txt
Recursively apply file permissions to a directory and all its contents.	$ chmod -R ug+rwx file.txt
33. Chown Command Examples: Changing File Ownership
Change both the owner and group of a file simultaneously.	$ chown oracle:dba dbora.sh
Recursively change the owner and group of a directory and its contents.	$ chown -R oracle:dba /home/oracle
34. Passwd Command Examples: User Password Management
Change the password for the current user (prompts for old and new passwords).	$ passwd
(Root Only) Reset the password for a specified user without requiring their current password.	# passwd USERNAME
(Root Only) Disable password authentication for a specified user, allowing passwordless login.	# passwd -d USERNAME
35. Mkdir Command Examples: Creating Directories
Create a new directory within the current user's home directory.	$ mkdir ~/temp
Create nested directories, creating parent directories as needed, and suppress errors if directories already exist.	$ mkdir -p dir1/dir2/dir3/dir4/
36. Ifconfig Command Examples: Network Interface Configuration
Display configuration and status of all network interfaces.	$ ifconfig -a
Activate or deactivate a specific network interface.	$ ifconfig eth0 up $ ifconfig eth0 down
37. Uname Command Examples: System Information
Display comprehensive system information, including kernel name, hostname, and operating system details.	$ uname -a Linux john-laptop 2.6.32-24-generic #41-Ubuntu SMP Thu Aug 19 01:12:52 UTC 2010 i686 GNU/Linux
38. Whereis Command Examples: Locating Command Binaries
Locate the binary, source, and manual page files for a specified command.	$ whereis ls ls: /bin/ls /usr/share/man/man1/ls.1.gz/usr/share/man/man1p/ls.1p.gz
Search for an executable in a non-standard specified directory.	$ whereis -u -B /tmp -f lsmk lsmk: /tmp/lsmk
39. Whatis Command Examples: Brief Command Descriptions
Display a concise, one-line description for a command.	$ whatis ls ls (1) - list directory contents $ whatis ifconfig ifconfig (8) - configure a network interface
View the manual page for a command from a specific section.	$ whatis crontab crontab (1) - maintain crontab files for individual users (V3) crontab (5) - tables for driving cron $ man 5 crontab
Following 8 sections are available in the man page. 1. General commands 2. System calls 3. C library functions	4. Special files (usually devices, those found in /dev) and drivers 5. File formats and conventions 6. Games and screensavers 7. Miscellaneous 8. System administration commands and daemons
40. Locate Command Examples: Fast File Search
Quickly search for files and directories by name using a pre-built database.	$ locate crontab /etc/anacrontab /etc/crontab /usr/bin/crontab /usr/share/doc/cron/examples/crontab2english.pl.gz /usr/share/man/man1/crontab.1.gz /usr/share/man/man5/anacrontab.5.gz /usr/share/man/man5/crontab.5.gz /usr/share/vim/vim72/syntax/crontab.vim
41. Man Command Examples: Accessing Manual Pages
View the complete manual page for a specified command.	$ man crontab
Access a specific section of a command's manual page (e.g., section 5 for file formats).	$ man SECTION-NUMBER commandname
42. Tail Command Examples: Viewing the End of Files
Display the last 10 lines of a file (default behavior).	$ tail filename.txt
Display a specified number (N) of lines from the end of a file.	$ tail -n N filename.txt
Monitor a file in real-time as it grows (useful for log files). Terminate with Ctrl+C.	$ tail -f log-file
43. Less Command Examples: Interactive File Viewing
Interactively view large files page by page without loading the entire file into memory.	$ less huge-log-file.log
Navigate through a file: CTRL+F (forward), CTRL+B (backward).	CTRL+F – forward one window CTRL+B – backward one window
44. Su Command Examples: Switching User Identity
Switch to another user account (superusers can switch without a password).	$ su - USERNAME
Execute a single command as another user, then return to the original user session.	[john@dev-server]$ su - raj -c 'ls' [john@dev-server]$
Login as a specified user and invoke a custom shell instead of their default.	$ su -s 'SHELLNAME' USERNAME
45. MySQL Command Examples: Database Interaction
Connect to a remote MySQL database server as a specified user (prompts for password).	$ mysql -u root -p -h 192.168.1.2
Connect to the local MySQL database server as a specified user (prompts for password).	$ mysql -u root -p
46. YUM Command Examples: Package Management (RHEL/CentOS)
Install the Apache HTTP server package.	$ yum install httpd
Upgrade the Apache HTTP server package.	$ yum update httpd
Remove the Apache HTTP server package.	$ yum remove httpd
47. RPM Command Examples: Red Hat Package Manager
Install an RPM package, showing verbose output and hash marks.	# rpm -ivh httpd-2.2.3-22.0.1.el5.i386.rpm
Upgrade an RPM package, showing verbose output and hash marks.	# rpm -uvh httpd-2.2.3-22.0.1.el5.i386.rpm
Remove an installed RPM package.	# rpm -ev httpd
48. Ping Command Examples: Network Connectivity Test
Send a limited number of ICMP echo requests (packets) to a remote host to test connectivity.	$ ping -c 5 gmail.com
49. Date Command Examples: System Date and Time Management
Set the system's date and time to a specific value.	# date -s "01/31/2010 23:59:53"
Synchronize the hardware clock with the system's current date and time.	# hwclock –systohc # hwclock --systohc –utc
50. Wget Command Examples: Non-Interactive Network Downloader
Download a file from a specified URL.	$ wget http://prdownloads.sourceforge.net/sourceforge/nagios/nagios-3.2.1.tar.gz
Download a file and save it with a different local filename.	$ wget -O taglist.zip http://www.vim.org/scripts/download_script.php?src_id=7701