Linux Training Notes

Linux Root Directories

/ - The root directory
/boot - Boot loader files, contains grub.cfg
/dev - System device files
/bin - Everyday user commands
/sbin - System/filesystem commands
/etc - System configuration files
/home - User home directories
/lib - Essential shared libraries and kernel modules
/media - Mount points for removable media
/mnt - Temporarily mounted filesystems
/opt - Optional application software packages
/proc - Virtual filesystem providing process and system information
/root - Home directory for the root user
/run - Runtime data for running processes
/srv - Service data
/sys - Virtual filesystem providing an interface to kernel data structures
/tmp - Temporary files
/usr - User binaries, libraries, and documentation
/var - Variable data such as logs and spool files

Important Filesystem Commands

cd - Change directory
ls - List directory contents
pwd - Print working directory

File Management

touch - Create an empty file
cp - Copy files and directories
mv - Move or rename files and directories
vi - Edit files
mkdir - Create a directory
rmdir - Remove an directory
chgrp - Change group ownership
chmod - Change file permissions

Find Files and Diectories

find
locate

File Types

- - Regular file
d - Directory
l - Symbolic link
c - Special file or device file
s - Socket file
p - Named pipe (FIFO)
b - Block device file

Soft and Hard Links

inode: A number that uniquely identifies a file or directory in the filesystem
Soft link (symbolic link): A file that points to another file or directory by its path
Hard link: A directory entry that points to an inode

Filters / Text Procesors Commands

cut - Extract columns from a file
cut -d "," -f1 merged.txt > items.txt
-d: Delimiter
-f1: Extract from the first field
awk - Designed for data extraction, mostly used to extract fields from output
cat seinfeld-characters | awk '{$2="Osborne" ; print $0}'
awk 'length($0) > 15' seinfeld-characters
ls -l | awk '{if($9 == "seinfeld") print $0;}'
ls -l | awk '{print NF}'

grep - Search for keyword in files/output
grep Seinfeld seinfeld-characters
grep -c Seinfeld seinfeld-characters (count entries)
grep -n Seinfeld seinfeld-characters (show line numbers)
grep -i Seinfeld seinfeld-characters (ignore case)
grep -v Seinfeld seinfeld-characters (show those without a match)
grep -v Seinfeld seinfeld-characters | awk '{print $1}' | cut -c1-3

egrep - Search for multiple keywords
egrep -i "Seinfeld|Kramer" seinfeld-characters

sort - Sort lines of text files
uniq - Report or filter out duplicate lines
wc - Word count

Text Processing Commands

sort - Sort lines of text files
sort seinfeld-characters
sort -r file (reversed)
sort -k2 file (sorts by second column value)
ls -l | sort -k9

uniq - Hides duplicate sequential entries
Always use with sort first!
sort seinfeld-characters | uniq
sort file | uniq -c (Shows count of each unique)
sort file | uniq -d (Only show duplicates)
ls -l | sort | uniq

wc - Word count
wc file - Shows line count, words, and bytes
wc -l file - Shows line count
wc -w file - Shows word count
wc -c file - Shows byte count

Compare Files

diff - Compare files line by line
diff file1 file2
cmp - Compare files byte by byte
cmp file1 file2

sed Command

Replace a string in a file
sed 's/old_string/new_string/g' file
sed -i : Actually changes the file
new_string can be blank to remove content

Find and delete a line
sed '/Seinfeld/d' seinfeld-characters

Remove empty lines
sed '/^$/d' file

Remove first or n lines in a file
sed '1d' file (removes first line)
sed '1,3d' file (removes first 3 lines)

Replace tabs with spaces
sed 's/\t/ /g' file

Show defined lines from a file
sed -n 12,18p file (shows lines 12-18)
sed 12,18d file (removes lines 12-18)
sed G file (inserts blank line after each line)
sed '8!s/Seinfeld/S/g' file (replaces all but line 8)
:%s/Seinfeld/Peter/ (replace all in vim)

User Account Mgmt

useradd {}
groupadd {}
userdel {}
groupdel {}
usermod -G superheroes spiderman
chgrp -R superheroes spiderman
useradd -g superheroes -s /bin/bash -c "Ironman Character" -m -d /home/ironman ironman

systemtctl

systemctl start|stop|status servicename.service
systemctl enable|disable servicename.service
systemctl restart|reload servicename.service
systemctl list-units --all

To add service: Create unit file in /etc/systemd/system/servicename.service
systemctl poweroff|halt|reboot

Kill Process Signals

kill -{} PID
SIGTERM - Graceful termination
SIGKILL - Forceful termination
SIGSTOP - Pause process
SIGCONT - Resume process
SIGSEGV - Segmentation fault, invalid memory location

Kill Real-Time Signals

Send extra information
SIGRTMIN - Marks beginning of signal range
SIGRTMAX - Marks end of signal range

System Health Lab Task 1:

Check system usage and resources
Manage and monitor processes
Check disk usage: df -h
Check memory usage: free -h
Check running processes: top

Review system log files
List Log directory: ls /var/log
Check the contents of a log file
head -n 10 /var/log/auth.log
tail -n 10 /var/log/auth.log
less /var/log/auth.log

Manage and monitor processes
Start a process in the background: sleep 100 &
View background jobs: jobs
Move job to background: bg %1
Move job to foreground: fg %1
Check user processes: ps -u labsuser
Run process with modified priority: nice -n 5 sleep 200 &

Summarize system health information
View active sessions and activity: w
Check logged-in users: who
Check login history: last | head -n 10

System Maintenance Commands

shutdown
init
reboot
halt - Forces a shutdown as if holding the power button

Changing the Hostname

hostnamectl set-hostname newhostname
Must reboot for changes to take effect

Finding System Information

General Info: uname -a
lsb_release -a
cat /etc/os-release
dmidecode

Terminal Control Keys

CTRL-u: Erase everything you've typed on current line
CTRL-c: Stop/kill a command
CTRL-z: Suspend a command
CTRL-d: Exit from interactive program