Linux Commands Notes

06.08.2024 — linux, job search, bash, notes — 13 min read

Linux is a free and open source operating system based on Unix. The difference between Linux and Unix is that Unix is a proprietary operating system, while Linux is open source.

kernel: the core of the operating system, manages the hardware resources of the computer
shell: a command line interface that allows users to interact with the operating system (bash, zsh, ksh)
distribution: a version of the Linux operating system that includes the Linux kernel and additional software

Basics

outputs the string that is passed to it as an argument

$ echo "Hello World"
Hello World

outputs the hostname of the system

$ hostname
w4nd

outputs the username of the current user

$ whoami
p0tt3r

outputs the present working directory

$ pwd
/Users/p0tt3r/dev

relative path: relative to the present working directory, start from current directory
absolute path: relative to the root directory, start from root directory
creates a new directory

$ mkdir test

change the current directory to test

$ cd test

creates a new file

$ touch file.txt

edit an existing file or create a new file

$ nano file.txt

display the contents of a file

$ cat file.txt
Hi

copy a file, copy contents of file.txt to file2.txt

$ cp file.txt file2.txt

rename a file, move the contents of file2.txt to file3.txt

$ mv file2.txt file3.txt

search for a string or pattern in a file

$ grep pattern files

$ grep "Hi" file.txt

recursively search for a string in a directory

$ grep -r pattern dir

$ grep -r "Hi" test

search for multiple strings in a file

$ egrep "Lorem|dolor|id" file3.txt

read a file without using the cat command

$ less file3.txt
$ more file3.txt

advantage of using less or more is that it allows you to scroll through the file

list all files in the current directory

$ ls
file.txt file3.txt

list all files in the current directory with permissions

$ ll
$ ls -l
total 16
-rw-r--r--  1 p0tt3r  staff    3 Aug  5 14:31 file.txt
-rw-r--r--  1 p0tt3r  staff  446 Aug  5 15:08 file3.txt

getfacl command is used to get file access control lists

displays the IP address of the system

$ ip addr show
$ ifconfig

displays the first 5 lines of a file and the last 5 lines of a file

$ head -5 file3.txt
$ tail -5 file3.txt

list all files including hidden files

$ la
$ ls -a

displays the history of commands that have been executed

$ history

root is the admin/superuser, has all the permissions

/root - home directory of the root user
/ - root directory

inode (index node): a data structure that stores information about a file or directory except its name and actual data, every file has a unique identifier number

$ ls -li
total 16
115609748 -rw-r--r--  1 p0tt3r  staff    3 Aug  5 14:31 file.txt
115610387 -rw-r--r--  1 p0tt3r  staff  446 Aug  5 15:08 file3.txt

find a file in the filesystem

$ find dir -name file

$ find . -name file.txt
./file.txt

$ locate file.txt

count the number of words and lines in a file

$ wc file3.txt
       6      69     446 file3.txt

pipe (|): takes the output of one command and uses it as input for another command

$ ls -l | grep file3
-rw-r--r--  1 p0tt3r  staff  446 Aug  5 15:08 file3.txt

check the difference between two files

$ diff file.txt file3.txt

remove a file

$ rm file.txt

remove a directory

$ rmdir test

permanently remove a file (unable to recover)

$ shred -u file.txt

check system architecture and hardware information

$ lscpu

combine two files

$ cat file.txt file3.txt > file4.txt

check the type of the file

$ file file3.txt
file3.txt: ASCII text

sort the contents of a file

$ cat file3.txt| sort
$ sort file3.txt

access linux server from another linux server: putty, ssh
create a tar archive

$ tar -cvf archive.tar file3.txt

extract a tar archive

$ tar -xvf archive.tar

compress a file

$ gzip file3.txt

decompress a file

$ gunzip file3.txt.gz

compress a directory

$ tar -zcvf archive.tar.gz test

decompress a directory

$ tar -zxvf archive.tar.gz

system and kernel information

$ uname -a
Darwin w4nd 23.5.0 Darwin Kernel Version 23.5.0: Wed May  1 20:19:05 PDT 2024; root:xnu-10063.121.3~5/RELEASE_ARM64_T8112 arm64

check the system uptime

$ uptime
0:54  up 2 days,  5:06, 2 users, load averages: 1.24 1.78 2.03

set environment variables

$ export VAR=value

check the environment variables

$ env
$ printenv

package manager: command line tool that allows you to install, update, and remove software packages (apt-get, yum, dnf)

# apt-get is a package manager for debian based systems

# install and remove a package
$ apt-get install package
$ apt-get remove package

# update package list
$ apt-get update

# upgrade all packages
$ apt-get upgrade

File Permissions

different types of permissions
- read (r) : 4
- write (w) : 2
- execute (x) : 1
different types of users
- user owner (u)
- user group (g)
- others (o)
- all (a)
check the permissions of a file

$ ls -l file3.txt
-rw-r--r--  1 p0tt3r  staff  446 Aug  5 15:08 file3.txt

$ ls -l
drwxr-xr-x  2 p0tt3r  staff   64 Aug  6 00:57 dir
-rw-r--r--  1 p0tt3r  staff    0 Aug  5 17:52 file.txt

# File type: -
# Permission settings: rw-r--r--
# User owner: p0tt3r
# Group owner: staff
# File size: 0
# Last modification time: Aug 5 17:52
# Filename: file.txt

when permissions and users are represented by letters, that is called symbolic mode
when permissions and users are represented by numbers, that is called octal mode
rwxr--r– is a set of three different permissions
- rwx: user owner has read, write and execute permissions
- r--: user group has read permissions
- r--: others have read permissions
rwxr--r– is represented by 744 in octal mode
- rwx = 4 + 2 + 1 = 7
- r-- = 4 + 0 + 0 = 4
- r-- = 4 + 0 + 0 = 4
change the permissions of a file

$ chmod 777 file3.txt

$ ls -l file3.txt
-rwxrwxrwx  1 p0tt3r  staff  446 Aug  5 15:08 file3.txt

change the owner of a file

$ chown user file3.txt

change the group of a file

$ chgrp group file3.txt

create a symbolic link (l denotes a symbolic link)

$ ln -s file3.txt file4.txt

$ ls -l
lrwxr-xr-x  1 p0tt3r  staff    9 Aug  6 01:19 file4.txt -> file3.txt

symbolic link or symlink is a pointer to a file or directory, can be used to create shortcuts
- A hard link is a direct reference to the inode of a file. Multiple hard links to a file share the same inode and, therefore, the same data. Deleting the original file does not affect the hard link.
```
$ ln file3.txt file5.txt
```
- A soft link is a pointer to a file or directory. It is a separate file that contains a path to the original file. Deleting the original file makes the soft link invalid, resulting in a broken link.
```
$ ln -s file3.txt file6.txt
```
setuid/setguid permissions are used to run a program with the permissions of the owner of the file (s/S denotes setuid/setguid)
- setuid: set user id
- setguid: set group id

$ chmod u+s file3.txt
$ chmod g+s file3.txt

sticky bit is used to restrict the deletion of a file (t/T denotes sticky bit)

$ chmod +t file3.txt

Redirection

write something to a file, redirect the output of a command to a file

$ echo "Hi" > file.txt
$ cat file3.txt > file4.txt
$ ls -l > files.txt

append something to a file

$ echo "Hello" >> file.txt
$ cat file3.txt >> file4.txt
$ ls -l >> files.txt

redirect an error to a file

$ cat abc
cat: abc: No such file or directory

$ cat abc 2> error.txt

redirect both output and error to a file

$ cat abc 2>&1 output.txt

Automation

cronjob: automate any task or script
cron: a daemon that runs in the background and executes scheduled tasks
crontab: a file that contains the cronjobs
create a cronjob

$ crontab -e

check the cronjobs that are running

$ crontab -l

* * * * * represents minute (0-59) hour (0-23) day (1-31) month (1-12) day_of_week (0-6)
run a script every minute

* * * * * echo "1 minute"

run a script every hour

0 * * * * echo "1 hour"

run a script every day

0 0 * * * echo "1 day"

run a script every day at 12:30

30 12 * * * echo "12:30"

debug a cronjob
- check system time
- crontab entry
- check /var/log/messages

Services

daemon: a background process that runs continuously, eg. apache, mysql, httpd
check the status of a service

$ systemctl status sshd

start/stop/restart a service

$ systemctl start sshd
$ systemctl stop sshd
$ systemctl restart sshd

enable/disable a service

$ systemctl enable sshd
$ systemctl disable sshd

httpd is the daemon that runs on the server which helps in establishing a connection over HTTP/HTTPS

System Monitoring

check free disk space

$ df -ah

# -a for all filesystems, -h for human readable format

check disk usage, size of the directory

$ du -sh /home

# -s for summary, -h for human readable format

show free memory

$ free -h

check CPU usage

$ top
$ htop
$ btop

Process

process: a running instance of a program
PID (Process ID): unique identifier for each process
check the process that is running

$ ps aux

$ ps aux | grep process_name

kill a process

$ kill <PID>

Signal	Value	Action	Command
SIGTERM	15	Terminate	`kill -15 <PID>`
SIGKILL	9	Kill	`kill -9 <PID>`
SIGINT	2	Interrupt	`kill -2 <PID>`

SIGTERM: signal terminate, the process can catch the signal and perform cleanup operations before terminating, gracefully kills the process, can be caught and ignored and does not kill child processes

$ sleep 100 &
[1] 27093

$ kill -15 27093
[1]  + 27093 terminated  sleep 100

SIGKILL: signal kill, the process cannot catch the signal, the process is killed immediately, inevitable and kills child processes

$ sleep 100 &
[1] 27038

$ kill -9 27038
[1]  + 27038 killed     sleep 100

SIGINT: signal interrupt (keyboard interrupt), the process can catch the signal and perform cleanup operations before terminating

$ sleep 100 &
[1] 26982

$ kill -2 26982
[1]  + 26982 interrupt  sleep 100

kill all processes that match a name

$ killall process_name

Networking

check the IP address of the system

$ ip addr show
$ ifconfig

check if a IP/server is accessible

$ ping www.google.com
PING www.google.com (142.250.72.100): 56 data bytes
64 bytes from 142.250.72.100: icmp_seq=0 ttl=60 time=41.297 ms
^C
--- www.google.com ping statistics ---
1 packets transmitted, 1 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 41.297/41.297/41.297/0.000 ms

# 0.0% packet loss means the server is accessible

$ telnet www.google.com 80
Trying 142.250.80.4...
Connected to www.google.com.

check network interfaces and IP addresses

$ netstat
$ ifconfig

display listening ports and the programs that are using them

$ netstat -tulnp

check the process that is running on a specific port

$ lsof -i :port

$ netstat -tulnp | grep port

configure network packet filtering rules, define the rules for incoming and outgoing packets

$ iptables

check the routing table

$ route -n

check the path that packets take to reach a destination

$ tracepath www.google.com

dump socket statistics

$ ss -tulnp

# -t for TCP, -u for UDP, -l for listening, -n for numeric, -p for process

query internet name servers interactively

$ nslookup www.google.com

check the DNS records of a domain (DNS lookup utility)

$ dig www.google.com

SSH

SSH (Secure Shell): communication protocol that provides a secure way to access a remote computer, communiucation between the host and the client is encrypted
default port for SSH is 22
sshd is the daemon that runs on the server which helps in establishing a secure connection over SSH
connect to a remote server

$ ssh user@hostname
$ ssh user@192.168.x.x

connect to a remote server with a different port

$ ssh -p port user@hostname

copy files from local to remote server

$ scp file.txt user@hostname:/path

copy files from remote server to local

$ scp user@hostname:/path/file.txt .

generate rsa key pair

$ ssh-keygen -t rsa

copy the public key to the remote server

$ ssh-copy-id user@hostname

ssh security can be improved by disabling root login, changing the default port, using key-based authentication, and using a firewall

File System

filesystem: a method of organizing and storing files on a computer
root directory: the top-level directory in the filesystem, represented by /
home directory: the directory where a user's files are stored, represented by ~
/etc directory contains configuration files
- /etc/passwd file contains information about the users on the system
- /etc/shadow file contains the encrypted passwords of the users
- /etc/group file contains information about the groups on the system
- /etc/fsatb file contains information about the filesystems on the system
- /etc/hosts file contains information about the hostname and IP address mappings
- /etc/resolv.conf file contains information about the DNS servers
- /etc/hostname file contains the hostname of the system
- /etc/ssh directory contains configuration files for the SSH server
- /etc/cron.* directories contain cronjobs
/var directory contains variable data files, such as logs, databases, and mail
- /var/log directory contains system log files
- /var/mail directory contains mail files
/tmp directory contains temporary files
/dev directory contains device files
/home directory contains the home directories of the users
/bin directory contains binary files and executable programs
/sbin directory contains system binaries and executable programs
/lib directory contains library files
/usr directory contains user binaries, libraries, and documentation
/opt directory contains optional software packages that are not part of the default installation
/boot directory contains boot files
/proc directory contains information about processes and system resources
swap space is used when the system runs out of physical memory, it is a portion of the hard disk that is used as virtual memory