Finding System Information

• cat /etc/redhat-release → Red Hat version
• uname -a → Linux hostname, kernel, architecture
• dmidecode → hardware, BIOS, system info
• arch → x86_64 or i386/32-bit

Common Keyboard Shortcuts

• Ctrl + U → erase command line
• Ctrl + C → stop/kill current command
• Ctrl + Z → suspend command
• Ctrl + D → exit interactive session

Record Terminal Session

• script mylog.log → record shell session

  • Use exit to stop recording

Root Password Recovery

1. Reboot → enter GRUB menu
2. Press e → edit boot params
3. Add rd.break at end of kernel line
4. Ctrl + X to boot
5. Mount root with chroot /sysroot
6. passwd root → change password
7. touch /.autorelabel → SELinux relabeling on reboot

View Environment Variables

• printenv, echo $PATH, echo $SHELL

Set Environment Variables

• Temporary: export TEST=1, echo $TEST

• Permanent:

  vi ~/.bashrc
  TEST=123
  export TEST

Global → vi /etc/profile or /etc/bashrc

• Basic Replace Syntax:

  • sed -i 's/KENNY/LENNY/g' filename

  • Substitute all occurrences of "KENNY" with "LENNY"

• Delete Line Containing String:

  • sed -i '/SEINFELD/d' filename

• Delete Empty Lines:

  • sed -i '/^$/d' filename

• Delete First Line:

  • sed '1d' filename

• sed '1,2d' filename → delete the first two lines

• sed 's/\t/ /g' filename → replace tabs with spaces

• sed -n '12,18p' filename → print only lines 12 to 18

• sed '12,18d' filename → delete lines 12 to 18

• sed G filename → insert an empty line after every line

• sed '8!s/seinfeld/S1/' filename → replace "seinfeld" with "S1" on every line except line 8

Crontab Basics

• crontab -e → edit crontab
• crontab -l → list crontab entries
• crontab -r → remove crontab entries
• systemctl status crond → check crond service status

AT: One-time Scheduled Tasks

• Used to schedule jobs only once.

Scheduling Examples:

• at HH:MM PM → schedule a job interactively

• at 11:23AM 12/24/25

  • Enter job (e.g., echo "test") and press Ctrl + D to finish
• at 2:45PM 01/30/25
• at 4PM + 4 days
• at now + 5 hours
• at 8:00AM Sun
• at 10:00AM next month

Managing AT Jobs

• atq → list the scheduled at jobs
• atrm # → remove a specific at job (use number from atq)
• systemctl status atd → check atd daemon status

File Display Commands

cat → Show entire content
cat -A → Show non-printable characters

more → Paginate output
less → Same as more but allows navigation with arrow keys

head → Show top lines (default: 10)
tail → Show bottom lines (default: 10)

head -2 myfile
tail -2 myfile

Filters / Text Processing Input

• cut → Cut input
• awk → List by columns
• grep & egrep → Search by keyword
• sort → Sort in alphabetical order
• uniq → Remove duplicate lines
• wc → Word count (including lines)

cut Command Examples

• cut -c1 filename → First letter of each line
• cut -c1,2,4 filename → Characters 1, 2, and 4
• cut -c1-3 filename → Range: characters 1 to 3
• cut -c1-3,6-8 filename → Ranges: characters 1-3 and 6-8
• cut -b1-3 filename → Byte range 1 to 3
• cut -d: -f6 /etc/passwd → Field 6 using : delimiter
• cut -d: -f6-7 → Fields 6 and 7
• ls -l | cut -c2-4 → Extract character range 2 to 4 from ls output

awk Command Examples

• awk '{print $1}' file → Print 1st column of file
• ls -l | awk '{print $1, $3}' → Print 1st and 3rd columns from ls -l
• ls -l | awk '{print $NF}' → Print last column
• awk '/jerry/ {print}' file → Search for "jerry" and print matching lines
• awk -F: '{print $1}' /etc/passwd → Use : as delimiter, print 1st field
• echo "hello tom" | awk '{$2="adam"; print}' → Replace column 2 with "adam"
• awk 'length($0) > 15' file → Print lines longer than 15 characters

grep Command Examples

• grep keyword file → Search for keyword
• grep -c keyword file → Count matching lines
• grep -i keyword file → Ignore case
• grep -n keyword file → Show line numbers
• grep -v keyword file → Invert match (everything except keyword)
• grep keyword file | awk '{print $1}' → Pipe grep to awk to extract 1st column
• ls -l | grep keyword → Filter ls -l output by keyword
• egrep -i "keyword1|keyword2" file → Search for multiple keywords (case-insensitive)

sort Command Examples

• sort file → Sort alphabetically
• sort -r file → Sort in reverse order
• sort -u file → Sort and remove duplicates
• sort file | uniq → Same as above with explicit uniq
• sort file | uniq -d → Show only duplicate lines
• sort file | uniq -c → Count duplicates
• sort -k4 -n file → Sort by 4th field (numeric)
• ls -l | sort -k4 -n → Sort by size from ls -l

wc Command Examples

• wc -c filename → Byte count
• wc -w filename → Word count
• wc -l filename → Line count
• ls -l | wc -l → Count lines from ls -l
• wc -l filename → Count number of lines in file

cmp / diff

• diff → Compare files line by line
• cmp → Compare files byte by byte

tar / gzip

• tar cvf export.tar somedir → Compress directory
• tar xvf export.tar → Extract tar archive
• gzip export.tar → Compress with gzip
• gzip -d export.tar.gz → Decompress gzip archive

truncate Command

• truncate -s10 filename → Chop file to 10 bytes
• truncate -s60 filename → Extend file to 60 bytes

Combining & Splitting Files

• cat file1 file2 file3 > fileN → Combine multiple files into one

Disk and Memory Info

• df → disk partition info
• df -h → human-readable format
• du → size of file/directory
• free → memory info
• cat /proc/cpuinfo → CPU info
• cat /proc/meminfo → memory info

System Logs & Diagnostics

• dmesg → check kernel ring buffer logs
• iostat → check disk I/O stats
• ip route | column -t → check routing table
• ss → view open ports/sockets

Log Directory Structure

• Logs in /var/log/

  • boot → startup records
  • chronyd → NTP info
  • cron → cron jobs
  • maillog → mail daemon
  • secure → login/logout tracking
  • messages → general trace logs
  • httpd → web server errors

Log Monitoring

• tail -f filename → follow log file in real time

System Shutdown & Boot Control

• shutdown → graceful shutdown
• init 0-6 → runlevels (0=off, 3=multi-user, 6=reboot)
• reboot → reboot system
• halt → force shutdown immediately

Hostname Management

• cat /etc/hostname → current hostname
• hostnamectl set-hostname myhostname → change hostname

• Essential Commands:

  • useradd, groupadd, userdel, groupdel, usermod
  • Modify 3 files: /etc/passwd, /etc/group, /etc/shadow

Create User with Custom Options

useradd -G mygroup -s /bin/bash \
  -c "User Description" -m \
  -d "/home/myuser" myuser

Other Useful User Commands

• useradd myuser → create user
• id myuser → check if user exists
• groupadd → create group
• cat /etc/group → check group
• userdel -r myuser → delete user and home directory
• groupdel mygroup → delete group
• usermod -G mygroup myuser → change user's group
• chgrp -R mygroup myuser → change group ownership recursively
• passwd myuser → set or update user's password

Password Aging with chage

• chage → manage password aging

  • -m → minimum days
  • -M → maximum days
  • -d → last password change
  • -I → inactive days
  • -E → expiration date
  • -W → warning days

• Example:

  • chage -M 5 -M 90 -W 10 -I 10 username

Edit Password Policies

• File: /etc/login.defs

  • PASS_MAX_DAYS 9999
  • PASS_MIN_DAYS 0
  • PASS_MIN_LEN 5
  • PASS_WARN_AGE 7

Switch User Privileges

• visudo → edit sudo permissions config (/etc/sudoers)

  • Add user to wheel group for sudo access
• usermod -aG wheel myuser → grant sudo rights

User Info and Monitoring Commands

• who → see logged in users
• last → list last login records
• w → who is logged in with more details
• finger → show user details
• id → show UID, GID and groups
• id myuser → check user info

Send Messages to Users

• wall → broadcast message to all users

  • Type message, then Ctrl + D to send

• write myuser → send message to specific user

  • Type message, then Ctrl + D to send

• touch one two three → Create 3 files

• mkdir folder → Make directory

• mv -R / cp -R → Move or copy folder including subfolders

• find . -name "george" → Search from current directory

• locate → Faster, but relies on a database

• updatedb → Update the locate database

Wildcards

Links

Soft Link (Symbolic Link)

• ln -s
• Link will be removed if the original file is removed or renamed.

Hard Link

• ln
• Deleting, renaming, or moving the original file will not affect the hard link.

Diagram

       INODE
        ↑
MY HARD LINK ← MYFILE.TXT → MY-SOFT-LINK

Command to check links:

ls -lTri

File Permissions

Permission Types

Each Permission (R, W, X) Can Be Controlled at Three Levels

• U → User
• G → Group
• O → Others (Everyone)

Permission Strings

Example:

-rwxrwxrwx
 |   |  |
 |   |  └── Others
 |   └───── Group
 └───────── User

To remove write permission from all:

chmod a-w filename

To add read and write permission for user:

chmod u+rw filename

Directory Permissions

Example:

drwxrwxrwx
^   ^  ^
|   |  └── Others
|   └───── Group
└───────── User

Numeric Permission Mapping

Example to set permissions:

chmod 764 file

Breakdown:

• User (7) → rwx
• Group (6) → rw-
• Others (4) → r--

Basic System Info Commands

• uptime → time, uptime, users, load average
• hostname → current hostname
• ip hostname → show IP & hostname
• uname -a → current OS and kernel info
• which command → path to a command

• cal → calendar for current month/year

  • cal 2025 → full year
• bc → basic calculator

System Monitoring

• systemctl → interact with systemd system manager
• ps → view current processes
• top → dynamic view of process usage

Systemctl Usage

• systemctl start|stop|status servicename.service → manage service state
• systemctl enable servicename.service → start service at boot
• systemctl restart|reload servicename → restart or reload service
• systemctl list-units --all → list all units

• To add a service:

  • Create a unit file in /etc/systemd/system/servicename.service

Process Management

• Start a process in the background:

  • Ctrl + Z then jobs → suspend and background
  • bg → continue in background
  • fg → bring back to foreground

• Run process detached from terminal:

  • nohup process & → keep running after logout
  • nohup process > /dev/null 2>&1 & → suppress all logs/output

Killing and Prioritizing

• pkill name → kill process by name

• nice → start process with a priority (range -20 to 19)

  • lower = higher priority
  • nice -20 top

Jobs Monitoring

• jobs → list background/foreground job states

Vi Editor Commands

• i → insert mode
• I → insert at line start
• ESC → escape to command mode
• R → replace mode
• x → delete character
• dd → delete line
• yy → copy line
• p → paste
• v → visual mode
• o → open new line below and insert
• O → open new line above and insert
• a → append after cursor
• A → append at end of line
• :q! → quit without saving
• :wq or ZZ → save and quit

ps Command

• ps → Current shell process status
• ps -E → All running processes
• ps aux → All in BSD style with details
• ps -EF → Running process with details
• ps -u root → Processes of specific user
• ps -EF | grep ? → Find process by pattern

top Command

• top -u username → Show only that user's processes
• top +d c → Absolute path
• top +d k → Kill by PID
• top +d M or +d P → Sort by memory or process usage

kill Commands

• kill -l → List all signals
• kill PID → Default terminate
• kill -1 → Restart
• kill -2 → Equivalent to Ctrl+C
• kill -9 → Force kill
• kill -15 → Graceful kill
• killall → Kill all processes by name
• pkill → Kill by process name

File Ownership

• chown → Change ownership
• chgrp → Change group

Access Control List (ACL)

• setfacl → Set file ACL
• getfacl → Get file ACL

Add Permission to User

setfacl -m u:user:rwx /path/to/file
setfacl -m g:group:rw /path/to/file

Recursive Inheritance from Folder

setfacl -R -m entry /path/to/dir

Remove Specific ACL Entry

setfacl -x u:user /path/to/file

Remove All ACL Entries

setfacl -b /path/to/file

Check ACL

ls -ltr

Example output:

-rw-rw-r--+

Help Commands

• whatis command
• command --help
• man command

Add Text to a File

echo "my text" > myfile    # Overwrite
cat myfile                 # Read the text
echo "hello" >> myfile     # Append

Input and Output Redirects
3 Redirect Types

• stdin → 0 → < or << → Feeding file content to a command
• stdout → 1 → > or >>
• stderr → 2 → 2> or 2>> → Write error output to a file

tee Command

• tee → Output + Save

echo "test" | tee myfile
tee -a file  # Append

• ls -l → List
• pwd → Print working directory

dr-xr-xr-x
|
directories

-rw-r--r--
|
file

Linux File Types

Inode = Pointer or number of a file on a hard disk.

/boot -> Grub.cfg
/root -> home of root
/dev -> system device (mouse, keyboard)
/etc -> configuration files
/bin -> /usr/bin -> everyday user commands
/sbin -> /usr/sbin -> system/filesystem commands
/opt -> third party app (not part of os)
/proc -> running process (only in memory)
/lib -> /usr/lib -> C prog lib
/tmp -> temporary folder
/home -> user dir
/var -> system logs
/run -> system daemon -> store temporary runtime / PID file
/mnt -> mount external file system
/media -> CDrom