apt-get, apt-cache, dpkgyum, rpm, dnffstabls, cp, mv, rm, touchcat, more, less, head, tailchmod, chown, chgrpps, top, kill, jobs, bg, fgtop, htop, vmstat, iostat/dev directory, major and minor numbersfdisk, parted, mkfs, fsck/, /home, /var, /etc, /usrbash, sh, zshPATH, HOME, USER#!/bin/bash), variables, loops, conditionalstar, gzip, rsyncuseradd, usermod, userdelgroupadd, groupmod, groupdelchmod, chown, chgrpifconfig, ip, nmcliping, traceroute, netstat, sstcpdump, nmapiptables, firewalld/var/log)top, vmstat, iostatsystemctljournalctl, /var/log/messagesCPU (Central Processing Unit):
RAM (Random Access Memory):
Storage Devices:
HDD (Hard Disk Drive):
SSD (Solid State Drive):
Motherboard:
x86 Architecture:
ARM Architecture:
BIOS (Basic Input/Output System):
UEFI (Unified Extensible Firmware Interface):
GRUB (GRand Unified Bootloader):
/boot/grub/grub.cfg is used to define boot options and settings.LILO (LInux LOader):
/etc/lilo.conf was used to configure boot options.Ubuntu:
CentOS:
Debian:
Filesystem Types:
Partitioning Schemes:
MBR (Master Boot Record):
GPT (GUID Partition Table):
Common Partitions:
APT (Advanced Package Tool):
sudo apt-get update: Updates the package list.sudo apt-get upgrade: Upgrades all installed packages.sudo apt-get install <package_name>: Installs a package.YUM (Yellowdog Updater, Modified):
sudo yum update: Updates all packages.sudo yum install <package_name>: Installs a package.RPM (Red Hat Package Manager):
rpm -ivh <package_name>.rpm: Installs a package.rpm -e <package_name>: Uninstalls a package.DNF (Dandified YUM):
sudo dnf install <package_name>: Installs a package.sudo dnf update: Updates installed packages.apt-get:
apt-get is used for managing packages from the terminal.apt-get install <package_name>: Installs a new package.apt-get remove <package_name>: Removes a package.apt-get update: Refreshes the package index.apt-get upgrade: Upgrades all packages to the latest versions.apt-cache:
apt-cache search <keyword>: Searches for packages matching the keyword.apt-cache show <package_name>: Displays detailed information about a package.dpkg:
dpkg is the base command for handling .deb packages on Debian-based systems.dpkg -i <package_name>.deb: Installs a package.dpkg -r <package_name>: Removes a package.dpkg -l: Lists all installed packages.yum:
yum is the default package manager for RPM-based distributions like CentOS and RHEL.yum install <package_name>: Installs a package.yum remove <package_name>: Removes a package.yum update: Updates installed packages.yum list installed: Lists all installed packages.rpm:
rpm is a package management utility for installing, querying, verifying, updating, and removing RPM packages.rpm -ivh <package_name>.rpm: Installs a package with verbose output and progress.rpm -e <package_name>: Removes a package.rpm -qa: Lists all installed RPM packages.dnf:
dnf is the next-generation version of yum and is the default package manager in Fedora and newer versions of CentOS and RHEL.dnf install <package_name>: Installs a package.dnf remove <package_name>: Removes a package.dnf update: Updates installed packages.dnf list installed: Lists all installed packages.zypper is the package manager used by openSUSE and SUSE Linux Enterprise.zypper install <package_name>: Installs a package.zypper remove <package_name>: Removes a package.zypper update: Updates all installed packages.zypper search <package_name>: Searches for a package in the repositories.zypper info <package_name>: Displays detailed information about a package.zypper list-updates: Lists available updates for installed packages.zypper refresh: Refreshes the package repositories.zypper addrepo <repository_url> <repo_alias>: Adds a new repository.zypper removerepo <repo_alias>: Removes a repository.zypper clean: Cleans up the local repository cache.zypper patch: Applies all necessary patches for the system.zypper dist-upgrade: Performs a distribution upgrade, updating the entire system to a new release.Understanding of File System Hierarchy:
ls, cp).fstab:
fstab (File System Table) file is used by the system to automatically mount partitions and devices at boot time./mnt/data).ext4, xfs).defaults, noatime).ls (List)
ls -l: Lists files in long format, showing permissions, owner, size, and modification time.ls -a: Includes hidden files (those starting with a dot).ls -h: Human-readable file sizes.ls -R: Lists directories and their contents recursively.cp (Copy)
cp [options] source destinationcp -r: Recursively copies directories and their contents.cp -i: Prompts before overwriting files.cp -u: Copies only when the source file is newer than the destination file.cp -p: Preserves the file attributes (permissions, timestamps, etc.).mv (Move/Rename)
mv [options] source destinationmv -i: Prompts before overwriting files.mv -u: Moves only when the source file is newer than the destination file.mv -v: Verbose output showing the actions being performed.rm (Remove)
rm [options] filerm -r: Recursively removes directories and their contents.rm -i: Prompts before each removal.rm -f: Forces removal without prompts (use with caution).rm -v: Verbose output showing the files being removed.touch
touch [options] filetouch newfile.txt creates an empty file named newfile.txt.touch -c: Does not create any files; only changes the timestamps if the file exists.cat (Concatenate)
cat [options] filecat file.txt displays the content of file.txt.cat -n: Numbers all output lines.more
more filespace to scroll down one screen, Enter to scroll one line.less
more, but with the ability to scroll both forward and backward.less filespace to scroll down, b to scroll up, and q to quit.less -S: Truncates long lines instead of wrapping them.head
head [options] filehead -n N: Displays the first N lines of the file.head -c N: Displays the first N bytes of the file.tail
tail [options] filetail -n N: Displays the last N lines of the file.tail -f: Continuously displays new lines as they are added to the file (useful for log files).tail -c N: Displays the last N bytes of the file.file
file [options] filefile myfile.txt displays the type of myfile.txt.du (Disk Usage)
du [options] [directory]du -h: Human-readable format (e.g., KB, MB).du -s: Summary (total size of the specified directory).df (Disk Free)
df [options] [directory]df -h: Human-readable format (e.g., GB, TB).mkdir (Make Directory)
mkdir [options] directorymkdir -p: Creates parent directories as needed.rmdir (Remove Directory)
rmdir directoryrmdir -p: Removes parent directories if they are empty.These commands cover a broad range of basic file and directory operations, useful for day-to-day Linux command-line tasks.
cat (Concatenate)
cat [options] filecat file.txt displays the content of file.txt.more
more filespace to scroll down one screen, Enter to scroll one line.less
more, but with the ability to scroll both forward and backward.less filespace to scroll down, b to scroll up, and q to quit.head
head [options] filehead -n N: Displays the first N lines of the file.tail
tail [options] filetail -n N: Displays the last N lines of the file.tail -f: Continuously displays new lines as they are added to the file (useful for log files).chmod (Change Mode)
chmod [options] mode filechmod u+x file adds execute permission to the user (owner).chmod 755 file sets permissions to rwxr-xr-x.chown (Change Owner)
chown [options] owner filechown user file: Changes the owner to user.chown user:group file: Changes the owner to user and group to group.chgrp (Change Group)
chgrp [options] group filechgrp group file changes the group ownership to group.ps (Process Status)
ps [options]ps aux: Shows detailed information about all running processes.top
topq to quit, h for help, and k to kill a process.kill
kill [options] PIDkill -9 PID forcefully terminates the process with the given PID.jobs
jobs [options]jobs -l: Lists the PID along with job information.bg (Background)
bg [job_spec]bg %1 resumes job number 1 in the background.fg (Foreground)
fg [job_spec]fg %1 brings job number 1 to the foreground.top
toptop to monitor system performance and resource usage.htop
top, offering a more user-friendly and interactive interface.htopvmstat (Virtual Memory Statistics)
vmstat [options] [delay [count]]vmstat 1 5: Displays stats every 1 second for 5 iterations.iostat (Input/Output Statistics)
iostat [options] [interval] [count]iostat -x: Provides extended statistics, including disk utilization and response times./dev Directory:
/dev directory contains special files representing devices attached to the system. These files allow software to interact with hardware through file-like interfaces.Major and Minor Numbers:
fdisk:
fdisk -l: Lists all partitions on all devices.fdisk /dev/sda: Opens the partition table of the specified device for editing.parted:
fdisk, used for managing disk partitions, especially on large or GPT-partitioned disks.parted /dev/sda: Opens the specified device for partitioning.mklabel gpt: Creates a new GPT partition table.mkpart primary ext4 1MiB 100%: Creates a primary partition of type ext4 using the entire disk.mkfs:
mkfs -t [filesystem_type] /dev/[partition]mkfs.ext4 /dev/sda1 creates an ext4 filesystem on partition /dev/sda1.fsck:
fsck [options] [filesystem]fsck /dev/sda1 checks the filesystem on /dev/sda1 for errors and attempts to fix them.Here’s the updated section with comments and additional details:
/ (Root Directory):
/bin, /etc, /lib, and /sbin./home:
/home for personal files and settings (e.g., /home/username)./var:
/var/log: Contains log files./var/spool: Holds spools for mail, printing, etc./var/tmp: Temporary files that need to persist between reboots./etc:
/etc/passwd: Contains user account information./etc/fstab: Lists filesystems to be mounted at boot./etc/network/interfaces: Configures network interfaces./usr:
/usr/bin: User commands and applications./usr/lib: Shared libraries./usr/share: Shared data, including documentation and application-specific data./opt:
/mnt:
/media:
/lib:
/lib64:
/proc:
/proc/cpuinfo for CPU details./sys:
/dev:
/dev/sda for disks)./boot:
initrd).ext4 (Fourth Extended Filesystem):
XFS:
Btrfs (B-tree Filesystem):
FAT32 (File Allocation Table 32):
NTFS (New Technology File System):
ntfs-3g driver.XFS (Extended Filesystem X):
ReiserFS:
JFS (Journaled File System):
ZFS (Zettabyte File System):
Bash (Bourne Again Shell):
sh shell, offering more features like improved scripting, command-line editing, and command history.Sh (Bourne Shell):
sh highly portable.Zsh (Z Shell):
PATH:
PATH to find the corresponding executable.export PATH=$PATH:/usr/local/bin adds /usr/local/bin to the existing PATH.HOME:
cd $HOME changes the current directory to the user’s home directory.USER:
echo $USER prints the username of the current user.Shebang (#!/bin/bash):
#!/bin/bash tells the system to use the Bash shell to run the script.#!/bin/bash
echo "Hello, World!"
Variables:
name="Alice"
echo "Hello, $name!"
Loops:
for i in {1..5}
do
echo "Iteration $i"
done
count=1
while [ $count -le 5 ]
do
echo "Count: $count"
count=$((count + 1))
done
Conditionals:
if [ "$name" == "Alice" ]; then
echo "Hello, Alice!"
else
echo "You are not Alice."
fi
Input/Output Redirection:
> writes output to a file (overwrites existing content), >> appends output to a file.echo "Hello, World!" > output.txt
echo "Appended line" >> output.txt
< takes input from a file.sort < unsorted.txt
Pipes:
|) are used to pass the output of one command as input to another command.ls | grep "file"
tar:
tar -cvf archive.tar /path/to/filestar -xvf archive.targzip:
tar to create compressed tarballs (.tar.gz).gzip file.txt creates file.txt.gzgunzip file.txt.gzrsync:
rsync -avz /source/path /destination/pathCreating Archives:
tar is commonly used for this.tar -czvf archive.tar.gz /path/to/directory creates a compressed archive.Extracting Archives:
tar or unzip depending on the format.tar -xzvf archive.tar.gz extracts the contents of the compressed archive.useradd:
useradd [options] username-m: Creates a home directory for the user if it doesn’t exist.-s /bin/bash: Specifies the default shell for the user.-G groupname: Adds the user to a specified group.-u UID: Sets a specific user ID (UID).-d /home/username: Specifies a custom home directory.useradd -m -s /bin/bash -G sudo -u 1001 -d /home/alice alice creates a user named “alice” with a home directory /home/alice, Bash as the default shell, adds them to the “sudo” group, and assigns the UID 1001.passwd:
passwd [options] username-d: Deletes the user’s password (making it possible for the user to log in without a password).-l: Locks the user account by disabling the password.-u: Unlocks the user account.passwd alice prompts you to enter and confirm a new password for the user “alice”.usermod:
usermod [options] username-c "Comment": Adds or modifies the comment field (often used for the full name).-G groupname: Changes or adds supplementary groups.-l newname: Changes the user’s login name.-aG groupname: Adds the user to a supplementary group without removing them from other groups.usermod -aG developers alice adds user “alice” to the “developers” group while retaining their existing group memberships.userdel:
userdel [options] username-r: Removes the user’s home directory and mail spool.-f: Forces the deletion of the user account even if the user is logged in.userdel -r alice deletes the user “alice” and their home directory.groupadd:
groupadd [options] groupname-g GID: Specifies a custom group ID (GID).groupadd -g 1002 developers creates a group named “developers” with the GID 1002.groupmod:
groupmod [options] groupname-n newname: Renames the group.-g GID: Changes the group ID (GID).groupmod -n devs developers renames the “developers” group to “devs.”groupdel:
groupdel groupnamegroupdel devs deletes the group “devs.”Create a New User:
useradduseradd [options] username-m: Creates a home directory for the user if it doesn’t exist.-s /bin/bash: Specifies the default shell for the user.-G groupname: Adds the user to a specified group.-u UID: Sets a specific user ID (UID).-d /home/username: Specifies a custom home directory.useradd -m -s /bin/bash -G sudo -u 1001 -d /home/alice alice creates a user named “alice” with a home directory /home/alice, Bash as the default shell, adds them to the “sudo” group, and assigns the UID 1001.Set or Change User Password:
passwdpasswd [options] username-d: Deletes the user’s password (making it possible for the user to log in without a password).-l: Locks the user account by disabling the password.-u: Unlocks the user account.passwd alice prompts you to enter and confirm a new password for the user “alice”.Modify Existing User:
usermodusermod [options] username-c "Comment": Adds or modifies the comment field (often used for the full name).-G groupname: Changes or adds supplementary groups.-l newname: Changes the user’s login name.-aG groupname: Adds the user to a supplementary group without removing them from other groups.usermod -aG developers alice adds user “alice” to the “developers” group while retaining their existing group memberships.Delete a User:
userdeluserdel [options] username-r: Removes the user’s home directory and mail spool.-f: Forces the deletion of the user account even if the user is logged in.userdel -r alice deletes the user “alice” and their home directory.Lock a User Account:
passwd -lpasswd -l usernamepasswd -l alice locks the user account for “alice”, preventing login.Unlock a User Account:
passwd -upasswd -u usernamepasswd -u alice unlocks the user account for “alice”, allowing login.Set Password Expiration Policies:
chagechage [options] username-M DAYS: Sets the maximum number of days between password changes.-W DAYS: Sets the number of days before expiration during which the user is warned.-I DAYS: Sets the number of days after password expiration before the account is disabled.chage -M 90 -W 14 alice sets the password for “alice” to expire every 90 days and warns them 14 days before expiration.View User Account Information:
finger (if installed)finger usernamefinger alice provides details about the user “alice”, such as login name, home directory, and shell.chmod:
chmod [options] mode fileu (user), g (group), o (others), r (read), w (write), x (execute).r=4, w=2, x=1.chmod u+rwx,g+rx,o+r file.txt: Gives the owner full permissions, the group read and execute, and others read permission.chmod 755 file.txt: Sets the permissions to rwxr-xr-x.chown:
chown [options] owner[:group] filechown alice file.txt: Changes the owner of “file.txt” to “alice”.chown alice:developers file.txt: Changes the owner to “alice” and the group to “developers”.chgrp:
chgrp [options] group filechgrp developers file.txt changes the group of “file.txt” to “developers.”setfacl:
setfacl [options] acl_spec filesetfacl -m u:alice:rwx file.txt gives user “alice” read, write, and execute permissions on “file.txt”.getfacl:
getfacl filegetfacl file.txt displays the ACLs set on “file.txt”.ifconfig:
ifconfig [interface] [options]ifconfigifconfig eth0 192.168.1.100 netmask 255.255.255.0ifconfig eth0 up or ifconfig eth0 downifconfig is now deprecated on many Linux distributions in favor of the ip command.ip:
ifconfig for managing network interfaces, IP addresses, routes, and tunnels.ip [options] object commandip addr showip addr add 192.168.1.100/24 dev eth0ip link set eth0 up or ip link set eth0 downip route shownmcli:
nmcli [options] [command]nmcli connection shownmcli connection up id "connection_name"nmcli connection modify "connection_name" ipv4.addresses 192.168.1.100/24nmcli networking off && nmcli networking onPurpose: Network interfaces are physical or virtual devices that enable a system to connect to a network.
Configuration:
/etc/network/interfaces (Debian-based systems) or /etc/sysconfig/network-scripts/ifcfg-* (Red Hat-based systems).dhclient.Commands:
ip link: Used to manage network interfaces.
ip link set eth0 up to bring an interface up.ethtool: Used to display or change Ethernet device settings.
ethtool eth0 displays the link status and speed.ping:
ping [options] destinationping google.com sends ICMP echo requests to google.com to check connectivity.traceroute:
traceroute [options] destinationtraceroute google.com displays the route packets take to reach google.com.netstat:
netstat [options]-t: Shows TCP connections.-u: Shows UDP connections.-l: Lists listening ports.netstat -tuln displays listening TCP and UDP ports in numerical form.ss:
netstat and is considered faster.ss [options]-t: Shows TCP sockets.-u: Shows UDP sockets.-l: Lists listening sockets.ss -tuln displays listening TCP and UDP sockets.tcpdump:
tcpdump [options]tcpdump -i eth0tcpdump port 80tcpdump -w file.pcaptcpdump -i eth0 port 80 captures all HTTP traffic on interface eth0.nmap:
nmap [options] targetnmap 192.168.1.1-254nmap -sT targetnmap -p 22,80 targetnmap -sT -p 22,80 google.com scans for SSH and HTTP ports on google.com.-rwxr-xr--).- for a regular file, d for a directory).chmod: Command to change file or directory permissions.
chmod [permissions] [file]chmod 755 myfile gives read, write, and execute permissions to the owner, and read and execute permissions to the group and others.SUID (Set User ID):
s replaces the execute (x) bit in the owner’s permission set.chmod u+s /path/to/file sets the SUID bit.SGID (Set Group ID):
s replaces the execute (x) bit in the group’s permission set.chmod g+s /path/to/directory sets the SGID bit.Sticky Bit:
t replaces the execute (x) bit in the others’ permission set.chmod +t /path/to/directory sets the sticky bit.iptables:
iptables [options] [chain] [rule-specification]iptables -A INPUT -s 192.168.1.100 -j DROPiptables -A INPUT -p tcp --dport 22 -j ACCEPTiptables-save > /etc/iptables/rules.v4firewalld:
firewall-cmd [options]firewall-cmd --get-active-zonesfirewall-cmd --zone=public --add-service=http --permanentfirewall-cmd --reloadPassword Policies:
/etc/login.defs to set password expiration rules.chage [options] [username]chage -M 90 username sets the maximum number of days before a password must be changed to 90.Account Locking:
passwd -l [username]: Lock a user account.passwd -u [username]: Unlock a user account.faillog: Display and manage user account failed login attempts.faillog -u [username] shows the failed login attempts for a user.Keeping Software Up-to-Date:
Common Package Managers:
sudo apt-get updatesudo apt-get upgradesudo apt-get dist-upgrade (handles dependencies more intelligently and might remove some obsolete packages).sudo yum check-update or sudo dnf check-updatesudo yum update or sudo dnf upgradesudo zypper refreshsudo zypper updateImportance of System Logs:
/var/log directory, with various logs tracking different aspects of system operation.Common Log Files:
Viewing and Managing Logs:
tail -f /var/log/syslog for live updates)./etc/logrotate.conf and /etc/logrotate.d/.Monitoring System Performance:
Key Tools:
top, and press q to quit.vmstat [interval] [count]vmstat 2 5 provides five updates every two seconds.iostat [options] [interval] [count]iostat -d 2 5 reports disk I/O statistics every two seconds, five times.Optimizing System Performance:
Key Performance Metrics:
GRUB Issues:
sudo mount /dev/sda1 /mnt
sudo grub-install --boot-directory=/mnt/boot /dev/sda
sudo update-grub
set root=(hd0,1)
linux /vmlinuz root=/dev/sda1
initrd /initrd.img
boot
Kernel Panics:
fstab or boot parameters).systemctl to check the status:sudo systemctl status <service_name>
sudo systemctl restart <service_name>
journalctl:sudo journalctl -u <service_name>
Using journalctl:
journalctl is used to view system logs managed by systemd, which includes logs for boot processes, service management, and more.sudo journalctl
sudo journalctl -b
sudo journalctl -u <service_name>
sudo journalctl -f
sudo journalctl | grep "<search_term>"
/var/log/messages:
cat /var/log/messages
grep -i 'error\|warning' /var/log/messages
tail -n 100 /var/log/messages
Booting into Recovery Mode:
Shift (for BIOS systems) or Esc (for UEFI systems).Rescuing Files:
sudo mount /dev/sda1 /mnt
sudo cp -r /mnt/home/user/important_files /external_drive/