March 2023 - ILUG-D - lists.fsci.in

1D1C - test
by Dhanasekar 31 Mar '23

31 Mar '23

test - check file types and compare values To print “Yes, that’s true.” when 100 is greater than 99. $ test 100 -gt 99 && echo "Yes, that's true." || echo "No, that's false." To print "No, that's false." when 99 is greater than 100. $ test 99 -gt 100 && echo "Yes, that's true." || echo "No, that's false." To check if a file exists and is a regular file $ test -f /etc/resolv.conf && echo "File /etc/resolv.conf found." || echo "File /etc/resolv.conf not found." $ test -f /etc/ssh/sshd_config && echo "File /etc/ssh found." || echo "File /etc/ssh not found." To print 0 when the expression is true and two strings are identical. $ [ "awesome" = "awesome" ]; echo $? To print 1 when the expression is false and two strings are not identical. $ [ "awesome" = "1awesome" ]; echo $? To print "1" because the expression is false $ [ 7 -eq 10 ]; echo $? To print "0" because the expression is true $ [ 10 -eq 10 ]; echo $? regards, T.Dhanasekar

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1D1C - telnet
by Dhanasekar 30 Mar '23

30 Mar '23

telnet - is used for interactive communication with another host using the TELNET protocol $ telnet -l [username] [remote machine IP] [port (optional)] To connect to the device with specific IP address $ telnet 192.168.122.105 To quit the connected machine $ logout To connect remote server with user and port number $ telnet -l remote_user 192.168.122.105 3000 To connect to the telnet shell $ telnet telnet> To print help options $ telnet telnet > h To exit the telnet shell telnet > quit regards, T.Dhanasekar

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1D1C - telinit
by Dhanasekar 29 Mar '23

29 Mar '23

telinit - Change SysV runlevel syntax $ sudo telinit [options] {runlevel} To change the runlevel for a system without rebooting or changing the /etc/inittab $ sudo telinit 0 halt $ sudo telinit 1 single-user mode $ sudo telinit 2 Not used (user-definable) $ sudo telinit 3 Full multi-user mode $ sudo telinit 4 Not used (user-definable) $ sudo telinit 5 Full multi-user mode (with an X-based login screen) $ sudo telinit 6 Reboot $ sudo telinit q Reload daemon configuration equivalent systemctl daemon-reload regards, T.Dhanasekar

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1D1C - tcpdump
by Dhanasekar 26 Mar '23

26 Mar '23

tcpdump - prints out a description of the contents of packets on a network interface To capture packets from Specific Interface $ sudo tcpdump -i eth1 To capture only N number of packets $ sudo tcpdump -c 4 -i eth1 To display the package in ASCII format $ sudo tcpdump -A -i eth1 To Display Available Interfaces $ sudo tcpdump -D To Display Captured Packets in HEX and ASCII $ sudo tcpdump -XX -i eth1 To capture and save packets in a file $ sudo tcpdump -w 0002.pcap -i eth1 To read captured packets file $ sudo tcpdump -r 0002.pcap To capture IP address packets $ sudo tcpdump -n -i eth1 To capture only TCP packets. $ sudo tcpdump -i eth1 tcp To capture packet from specific port $ sudo tcpdump -i eth1 port 22 To capture packets from source IP $ sudo tcpdump -i eth1 src 192.168.122.50 To capture packets from destination IP $ sudo tcpdump -i eth1 dst 192.168.122.100 regards, T.Dhanasekar

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1D1C - tar
by Dhanasekar 25 Mar '23

25 Mar '23

tar - an archiving utility syntax $ tar <options> <files> To create an archive file $ tar -cvf myfile.txt /home/ilugc/ To create tar.gz Archive File $ tar cvzf mydoc.tar.gz /home/ilugc/Documents To create tar.bz2 archive file $ tar cvfj mydownloads.tar.bz2 /home/ilugc/Downloads To untar tar Archive file $ tar -xvf mydoc.tar To untar tar archive file in different location $ tar -xvf mydoc.tar -C /home/ilugc/Music/ To uncompress tar.gz Archive File $ tar -xvf mydoc.tar.gz To Uncompress tar.bz2 Archive File $ tar -xvf mydownloads.tar.bz2 To list Content of tar Archive File $ tar -tvf mydoc.tar To List Content tar.gz Archive File $ tar -tvf mydoc.tar.gz To List Content tar.bz2 Archive File $ tar -tvf mydownloads.tar.bz2 To Untar Single file from tar File $ tar -xvf files.sh.tar file.sh To Untar Single file from tar.gz File $ tar -zxvf backup.tar.gz backup.html To Untar Single file from tar.bz2 File $ tar -jxvf Python.tar.bz2 demo.py To Untar Multiple files from tar, tar.gz, and tar.bz2 File $ tar -xvf file.tar "file1" "file2" "file3" $ tar -zxvf myfile.tar.gz "file1" "file2" "file3" $ tar -jxvf myfiles.tar.bz2 "file1" "file2" "file3" To Extract Group of Files using Wildcard $ tar -xvf Python.tar --wildcards '*.py' $ tar -zxvf Perl.tar.gz --wildcards '*.pl' $ tar -jxvf Bash.tar.bz2 --wildcards '*.sh' To add files or directories to the existing tar archive files $ tar -rvf file.tar file.txt $ tar -rvf file.tar demo.py To Add Files or Directories to tar.gz and tar.bz2 Files $ tar -rvf file.tar.gz sample.txt $ tar -rvf Php.tar.bz2 sample.php To verify any tar or compressed archive file $ tar tvfW sample.tar To Check the Size of the tar, tar.gz, and tar.bz2 Archive File $ tar -czf - sample.tar | wc -c $ tar -czf - mydocs.tar.gz | wc -c $ tar -czf - mydownloads.tar.bz2 | wc -c To Exclude Files and Directories When Creating Tar File $ tar --exclude='example.txt' -zcvf file.tar.gz /home/ilugc $ tar --exclude='/home/ilugc/songs' -zcvf backup.tar.gz /home/ilugc To exclude files with specific file extensions when creating a tar archive file, $ tar --exclude='*.txt' -zcvf backup.tar.gz /home/ilugc $ tar --exclude='*.py' -zcvf backup.tar.gz /home/ilugc To remove File and Directory from Tar Archive $ tar --delete -f backup.tar.gz sample.txt $ tar --delete -f backup.tar.gz '/home/tecmint/songs' To extract Specific File Extension in Tar Archive $ tar -xvf backup.tar.gz --wildcards '*.mp4' regards, T.Dhanasekar

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1D1C - taskset
by Dhanasekar 24 Mar '23

24 Mar '23

taskset - is used to set or retrieve the CPU affinity of a running process To fetch the CPU Affinity of a Process $ pidof sshd $ taskset -p <PID_of_sshd> To get the CPU range of a process $ taskset -cp <PID_of_sshd> To change the CPU affinity of a process by assigning the new value to the PID of the existing process $ taskset -p 0x7 <PID_of_sshd> To Change CPU range $ taskset -cp 0,2 <PID_of_sshd> To set the CPU core $ taskset -c 3,5 sshd $ taskset -c 0-4 sshd regards, T.Dhanasekar

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1D1C - tail
by Dhanasekar 23 Mar '23

23 Mar '23

tail - output the last part of files To display the last 10 lines of a file $ tail /var/log/messages To Display the last N lines in a file $ tail -n 4 /var/log/messages To Print filename header $ tail -v example.txt $ tail -v /var/log/messages To Display the last n lines from multiple files $ tail -n 4 file1.txt file2.txt $ tail -n 4 /var/log/messages /var/log/apache2/access.log To Save the output of tail command to a text file $ tail -n 10 /var/log/apache2/access.log > output.txt To append to output.txt $ tail -n 10 /var/log/messages >> output.txt To monitor real-time log files $ tail -f /var/log/messages To use pipes $ tail -file.txt | sort $ tail /var/log/messages | sort $ tail /var/log/messages | tail -n 6 | sort To Print N number of bytes data from a file $ tail -c 500 /var/log/messages $ tail -c 10M /var/log/messages To print help options $ tail --help regards, T.Dhanasekar

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1D1C - tac
by Dhanasekar 22 Mar '23

22 Mar '23

tac - is used to concatenate and print files in reverse syntax $ tac [OPTION]... [FILE]... $ cat example.txt This is line 1 This is line 2 This is line 3 This is line 4 This is line 5 $ cat example1.txt This is msg 1 This is msg 2 This is msg 3 This is msg 4 This is msg 5 To print files in reverse $ tac example.txt To attach the separator before instead of after $ tac -b example.txt example1.txt To make the STRING as the separator $ tac --separator="STRING" example1.txt example.txt To interpret the separator as a regular expression $ tac -r example.txt example1.txt regards, T.Dhanasekar

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1D1C - systemd-machine-id-setup
by Dhanasekar 21 Mar '23

21 Mar '23

systemd-machine-id-setup - Initialize the machine ID in /etc/machine-id To clear the original “machine-id” $ sudo rm /etc/machine-id To create the new “machine-id” value $ sudo systemd-machine-id-setup To verify the new value of machine-id. $ cat /etc/machine-id regards, T.Dhanasekar

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1D1C - systemd-analyse
by Dhanasekar 20 Mar '23

20 Mar '23

systemd-analyse - Analyze and debug system manager To determine system boot-up performance statistics $ systemd-analyze To Print time required to boot the machine $ systemd-analyze time To Print list of running units ordered by time to init $ systemd-analyze blame To Print a tree of the time critical chain of units $ systemd-analyze critical-chain Output state serialization of service manager $ systemd-analyze dump Output SVG graphic showing service initialization $ systemd-analyze plot To List load directories for units $ systemd-analyze unit-paths To List exit status definitions $ systemd-analyze exit-status To Analyze security of unit $ systemd-analyze security regards, T.Dhanasekar

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1D1C - systemd
by Dhanasekar 19 Mar '23

19 Mar '23

systemd - is a system and service manager for Linux operating systems To find boot process duration $ systemd-analyze $ systemd-analyze time To analyze time taken by each process at boot $ systemd-analyze blame To analyze critical chain at boot $ systemd-analyze critical-chain To Determine the initial start-up transaction $ systemd --test To make not to pipe output into a pager $ systemd --test --no-pager To Dump understood unit configuration items $ systemd --dump-configuration-items To dump exposed bus properties $ systemd --dump-bus-properties regards, T.Dhanasekar

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1D1C - systemctl
by Dhanasekar 18 Mar '23

18 Mar '23

systemctl - Control the systemd system and service manager syntax $ systemctl [OPTION] [SERVICE] To start service $ sudo systemctl start mariadb.service $ sudo systemctl start sshd To stop service $ sudo systemctl stop mariadb.service $ sudo systemctl stop sshd To restart or reload service $ sudo systemctl reload mariadb.service $ sudo systemctl restart mariadb.service To reload a service default, but if reload is not available for that service. It will restart it $ sudo systemctl reload-or-restart mariadb.service To check status of service $ sudo systemctl status mariadb.service $ sudo systemctl status sshd To enable service $ sudo systemctl enable mariadb.service $ sudo systemctl enable sshd To disable service $ sudo systemctl disable mariadb.service $ sudo systemctl disable sshd To check Service is enabled or disabled $ sudo systemctl is-active mariadb.service $ sudo systemctl is-active sshd $ sudo systemctl is-enabled mariadb.service $ sudo systemctl is-enabled sshd To see the status of all services $ sudo systemctl list-units --type=service To List services by status $ sudo systemctl list-units --type=service --state=active $ systemctl list-units --type=service --state=running $ systemctl list-units --type=service --state=stopped $ systemctl list-units --type=service --state=enabled $ systemctl list-units --type=service --state=disabled $ systemctl list-units --type=service --state=failed To prevent service from starting $ sudo systemctl mask {servicename} To Kill a service with signal 15 $ sudo systemctl kill {servicename} To Kill a service with signal 9 $ sudo systemctl kill -s 9 {servicename} To Reboot $ sudo systemctl reboot To shutdown $ sudo systemctl poweroff To change default to GUI $ sudo systemctl set-default graphical.target To switch to multi-user.target $ sudo systemctl isolate multi-user.target To reload daemon $ systemctl daemon-reload To find version $ systemctl --version regards, T.Dhanasekar

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1D1C - sysctl
by Dhanasekar 17 Mar '23

17 Mar '23

sysctl - configure kernel parameters at runtime To view all current kernel parameters invoke the sysctl $ sudo sysctl -a To view the swappiness value $ sudo sysctl vm.swappiness $ sysctl kernel.hostname To Modify the Kernel Parameters $ sudo sysctl -w parameter=value To enable IPv4 packet forwarding $ sudo sysctl -w net.ipv4.ip_forward=1 To set a parameter permanently $ sudo sysctl -w net.ipv4.ip_forward=1 >> /etc/sysctl.conf To read values from file $ sudo sysctl -p /etc/sysctl.d/file_name.conf To print variable names without values $ sudo sysctl -N vm.swappiness To print values only of the given variable $ sudo sysctl -n vm.swappiness To read values from all system directories $ sudo sysctl --system To print version and help options $ sudo sysctl --version $ sudo sysctl --help regards, T.Dhanasekar

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1D1C - sync
by Dhanasekar 16 Mar '23

16 Mar '23

sync - is used to synchronize the data of the temporary memory to the permanent storage memory syntax $ sync [option] [file] To sync all the cached files to the permanent memory which belongs to the current user $ sudo sync To sync only file data, no unneeded metadata $ sudo sync -d /home/ilugc/file1.txt To sync all the files in dir $ sudo sync -f /home/ilugc/Downloads To sync the cache data of the mounted partition $ sudo sync /dev/vda1 $ sudo sync /var/log/syslog regards, T.Dhanasekar

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1D1C - swapon
by Dhanasekar 15 Mar '23

15 Mar '23

swapon - enable devices and files for paging and swapping syntax $ swapon <option> To enable all swap devices and files listed in the /etc/fstab $ sudo swapon -a $ sudo swapon -av To enable swap device with device name $ sudo swapon -a /dev/vda5 $ sudo swapon -av /dev/vda5 To display the swap usage summary of all enabled swap devices and files $ sudo swapon -s To enable the swap discards if the device supports the discard operation $ sudo swapon -d To reinitialize the swap space $ sudo swapon -f To display a summary table with selected columns $ swapon --show=NAME,SIZE To print the raw output format $ sudo swapon --raw regards, T.Dhanasekar

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1D1C - swapoff
by Dhanasekar 14 Mar '23

14 Mar '23

swapoff - disables swapping on the specified devices and files To deactivate the swap $ sudo swapoff /dev/vda5 $ sudo swapoff -v /dev/vda5 To disable all swaps from /proc/swaps $ sudo swapoff -a $ sudo swapoff -av To check if the swap area has been disabled $ free -h or $ sudo swapon --show regards, T.Dhanasekar

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1D1C - swaplabel
by Dhanasekar 13 Mar '23

13 Mar '23

swaplabel - print or change the label or UUID of a swap area To print the swaplabel $ sudo swaplabel /dev/vda5 To change the swaplabel, first make swapoff $ sudo swapoff -a then, change the swaplabel name $ sudo swaplabel -L mynewswap /dev/vda5 then, $ sudo swapon -a check with $ sudo swapon --show now new swaplabel is fixed $ sudo swaplabel /dev/vda5 regards, T.Dhanasekar

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1D1C - sudo
by Dhanasekar 12 Mar '23

12 Mar '23

sudo - allows a permitted user to execute a command as the superuser or another user To run command as a root user $ sudo <command> $ sudo pwd To run command as a different user $ sudo -u user3 <command> $ sudo -u user3 pwd To List user privileges with sudo $ sudo -l To Display privileges for another user with sudo $ sudo -l -U user3 To add a user to the sudo group $ sudo usermod -aG sudo user3 To add users to the sudoers file $ sudo visudo To run command in the background $ sudo -b <command> To update sudoers files $ sudo -e <file> To update the user’s cached credentials $ sudo -v To invalidate user’s cached credentials $ sudo -k regards, T.Dhanasekar

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1D1C - su
by Dhanasekar 11 Mar '23

11 Mar '23

su - run a command with substitute user and group ID su command without any option $ su su command to make the shell a login shell $ su - user3 To switch to root user $ su - To print help options $ su -h To switch to a Different User $ su -l user3 To Use su with sudo command $ sudo su - user3 To run specific command as a different user $ su -c pwd user3 To use a different shell $ su -s /usr/bin/csh To Execute a command as different user with su command $ su -c <command> user3 To use a different user in the same environment $ su -p user3 $ su -m user3 regards, T.Dhanasekar

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1D1C - strip
by Dhanasekar 10 Mar '23

10 Mar '23

strip - discard symbols and other data from object files syntax $ strip [options] objfile... $ cat hello.c #include <stdio.h> int main() { // printf() displays the string inside quotation printf("Hello, World!"); return 0; } $ gcc hello.c -o hello.out To strip the symbol table $ readelf -s hello.out $ strip -s hello.out check the symbol table $ readelf -s hello.out To remove debug symbols only $ strip --strip-debug hello.out check the symbol table $ readelf -a hello.out To Remove a particular section $ readelf -S hello.out strip the .gnu.version section from the executable $ strip -R .gnu.version hello.out check the list of sections $ readelf -S hello.out To Remove unneeded symbols $ strip --strip-unneeded hello.out $ readelf -a hello.out To Shield a particular symbol from stripping $ strip -s -K hello.c hello.out $ readelf -s hello.out To strip off a particular symbol $ strip -N hello.c hello.out $ readelf -s hello.out To create a new stripped off file $ strip -s -o stripped_hello hello.out $ ls -lart stripped_hello To preserve the access and modification date/time check the access and modification time of the original file $ stat hello.out $ strip -s -p hello.out regards, T.Dhanasekar

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1D1C - strace
by Dhanasekar 09 Mar '23

09 Mar '23

strace - is a CLI tool for debugging and troubleshooting programs To trace of all system calls made command free -h $ strace free -h $ strace df -Th To Trace Linux Process PID $ sudo strace -p <PID> To get summary of linux process $ sudo strace -c -p <PID> To Print Instruction Pointer During System Call $ sudo strace -i df -Th $ sudo strace -i free -h To show Time of Day For Each Trace Output Line $ sudo strace -t df -Th $ sudo strace -t free -h To Print Command Time Spent in System Calls $ sudo strace -T df -Th $ sudo strace -T free -h To trace Only Specific System Calls $ sudo strace -e trace=write df -Th $ sudo strace -e trace=write free -h $ sudo strace -e trace=all df -Th $ sudo strace -e trace=all free -h To trace System Calls Based on a Certain Condition To trace all system calls involving process management $ sudo strace -q -e trace=process df -Th $ sudo strace -q -e trace=process free -h To trace all system calls that take a filename as an argument, $ sudo strace -q -e trace=file df -Th $ sudo strace -q -e trace=file free -h To trace all system calls involving memory mapping $ sudo strace -q -e trace=memory df -Th $ sudo strace -q -e trace=memory free -h To redirect Trace Output to File $ sudo strace -o df_debug.txt df -Th $ sudo strace -o free_debug.txt free -h To show debugging information for strace $ strace -d df -Th $ strace -d free -h regards, T.Dhanasekar

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1D1C - stat
by Dhanasekar 08 Mar '23

08 Mar '23

stat - display file or file system status To view the file details $ stat example.txt To view information about multiple files $ stat sample.pdf example.txt To display file system status $ stat -f /home To display information in terse form $ stat -t sample.pdf $ stat -t example.txt $ stat -t sample.pdf example.txt To enable following of symbolic links $ stat /usr/share/zoneinfo/Asia/Calcutta To get information on the file that the links points $ stat -L /usr/share/zoneinfo/Asia/Calcutta To format sequencing To display inode of a file only $ stat --printf='%i\n' example.txt To display access rights and uid (User ID) $ stat --printf='%a:%u\n' example.txt To print out a new line without requiring an additional operand. $ stat --format='%a:%F' example.txt To display help options $ stat --help regards, T.Dhanasekar

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1D1C - startx
by Dhanasekar 07 Mar '23

07 Mar '23

startx command launches an X11 session. To start an X session at 16 bits color depth. $ startx -- -depth 16 To start an X session at 100 dpi (dots per inch) resolution. $ startx -- -dpi 100 To start an X session with support for multiple displays. $ startx -- -layout Multihead regards, T.Dhanasekar

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1D1C - ssh-keyscan
by Dhanasekar 06 Mar '23

06 Mar '23

ssh-keyscan - is a utility for gathering the public SSH host keys of a number of hosts To read all public keys from the host $ ssh-keyscan 192.168.122.50 To read all public keys from the host at port number 22 $ ssh-keyscan -p 22 192.168.122.50 To read all public keys of the rsa type $ ssh-keyscan -t rsa 192.168.122.50 To update the known_hosts file located in the path, ~/.ssh/known_hosts $ ssh-keyscan -H 192.168.122.50 >> ~/.ssh/known_hosts regards, T.Dhanasekar

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1D1C - ssh-keygen
by Dhanasekar 05 Mar '23

05 Mar '23

ssh-keygen - is used to generate a public/private authentication key pair To generate ssh key without any arguments $ ssh-keygen To define key type $ ssh-keygen -t rsa $ ssh-keygen -t dsa To define bit size default is 2048 $ ssh-keygen -b 4096 To assign passphrase $ ssh-keygen -P "myp@ssword" To change passphrase of the private key $ ssh-keygen -p To create keys with custom filename $ ssh-keygen -f my-rsa-key To add custom comment to the key $ ssh-keygen -C "This key is for my.server.in" To change comment of the key $ ssh-keygen -c To hash the content of known_hosts file $ ssh-keygen -H To delete all the keys related to 192.168.122.101 host from known_hosts file $ ssh-keygen -R 192.168.122.101 regards, T.Dhanasekar

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