Linux Basics

Link to Slides

https://bits-sos.github.io/slides/linux-basics.html

Before We Start

Get the same setup:

git clone https://github.com/BITS-SOS/linux-basics
cd linux-basics

Some Absolute Basics

Files

In Linux, everything is a file.
File system hierarchy: /, /bin, /lib, /usr, etc.
Files have ownership and access permissions.
Linux supports various file types like regular files, directories, symlinks, etc.

Users and Groups

Users are accounts that access system resources with unique usernames and IDs.
Groups are collections of users that share permissions and access levels.
Users can be assigned permissions to files and directories.
New users and groups can be created with useradd and groupadd respectively.
(IMPORTANT) Only use the root user or sudo when you absolutely know what you are doing.

Permissions

There are three types of permissions: read, write, and execute, which can be assigned to three different classes of users: owner, group, and others.
The chmod command is used to modify permissions and the chown and chgrp commands are used to change the ownership and group of a file or directory.
The umask command can be used to set default permissions for newly created files and directories.

History of Terminals

1960s and 1970s - Mainframes

Mainframes were big computers a lot of people accessed at the same time
People used "terminals" to access these mainframes and perform tasks on them
Earliest form was teletype machine (tty)
Video terminals came around later, but were expensive
- They came with color output, fancy cursor movements and other features

Personal Computers

Terminal Emulators simulated video terminals on pesonal computers
First ever terminal emulator was on the PDP-11

Shells

User interface that lets users interact with the OS through a CLI
First shell was Multic's MIDAS
"Thompson shell" or sh came with UNIX
Bourne shell, C shell, Korn shell came around later
Linux comes with Bourne Again Shell (bash)
ZSH and Fish are other popular options

Navigation

pwd

pwd

pwd

pwd
-> /home/<user>/linux-basics

cd

cd dir
pwd
cd ..
pwd

cd

cd dir
pwd
-> /home/<user>/linux-basics/dir
cd ..
pwd
-> /home/<user>/linux-basics

ls

ls dir

ls

ls dir
-> file.txt

tree

tree dir

tree

tree dir
dir
└── file.txt

1 directory, 1 file

mv

mv hello.c hello2.c
ls

mv

mv hello.c hello2.c
ls
-> dir hello2.c ...

mkdir

mkdir dir/dir2
ls dir

mkdir

mkdir dir/dir2
ls dir
-> file.txt dir2

touch

touch dir/file2.txt
ls dir

touch

touch dir/file2.txt
ls dir
-> dir2 file.txt file2.txt

rm

rm dir/file2.txt
rm -r dir/dir2
ls dir

rm

rm dir/file2.txt
rm -r dir/dir2
ls dir
-> file.txt

cp

cp dir/file.txt dir/file2.txt
ls dir

cp

cp dir/file.txt dir/file2.txt
ls dir
-> file.txt file2.txt

Common Commands

man

man man

echo

echo "Hello, World!"

echo

echo "Hello, World!"
-> Hello, World!

cat

cat linux-basics.sh

less

less linux-basics.sh

head

head -n 1 linux-basics.sh

head

head -n 1 linux-basics.sh
-> #!/usr/bin/bash

tail

tail -n 1 linux-basics.sh

tail

tail -n 1 linux-basics.sh
-> echo $(expr $STEP + 1) > /tmp/STEP

grep

grep "getopts" *

|

cat linux-basics.sh | grep "bash"

>

echo "ok" > ok.txt

>>

echo "still ok" >> ok.txt

cut

echo "h e l l o" | cut -d" " -f2

tee

echo "hello" | tee hello.txt

chmod

chmod -x linux_basics.sh

chown

chown -R $user file/dir

chattr

sudo chattr +i hello.txt

wget

wget https://bits-sos.github.io/images/bits_sos.png

curl

curl https://wttr.in

tar

tar -xvf archive.tar

unzip

unzip zipped.zip

find

find . -name "*.sh"

fzf

fzf

sleep

sleep 3s;

pkill

pkill -SIGUSR1 <process>

killall

killall process

alias

alias vi="nvim"

function

mkcd() { mkdir $1 && cd $1; }

Branching and Looping

variables

w="world"
echo "hello $w"
$? -> exit status
read var
getopts

test

test expr
[ expr ]
[ -f file ]
[ -z empty_string ]
[ "stra" != "strb" ]
[ $x -gt 4 ]
[[]] -> not really standard, but has neat features like regex matching

exit

exit 0 -> success
exit 1 -> failure

arithmetic

let "a = 2 + 6/3"
expr "2 + 6/3"
$(( 2 + 6/3 ))
${#w} -> length of str in w

if

if predicate; then
    action
elif predicate2; then
    action2
else
    action3
fi

match case

case $i in
    a)
        s1
        ;;
    b | c)
        s2
        ;;
    d)
        s3
        ;;
    *)
        s4
        ;;
esac

while loop

while pred; do
    something
done

for loop

for i in 1 2 3 4 5; do
    echo "count: $i"
done

Shell Scripting Examples

Remove Prefix

#!/bin/bash
read -p "Enter the name of the directory: " dirname
read -p "Enter the prefix to be removed: " prefix

# Check if the given directory exists
if [ -d "$dirname" ]
then
  # Loop through all the files in the directory
  for file in "$dirname"/*"$prefix"*
  do
    # Remove prefix from filename
    newname=$(echo $file | sed "s/$prefix//")
    mv "$file" "$newname"
    echo "File $file renamed to $newname"
  done
else
  echo "Directory $dirname does not exist."
fi

Resize Images in a Directory

#!/bin/bash

read -p "Enter the name of the directory: " dirname
read -p "Enter the desired width of images: " width

for file in $dirname/*.{jpg,png,jpeg}; do
  if [ -f "$file" ]; then
    echo "Resizing $file..."
    convert "$file" -resize "$width" "$file"
  fi
done

echo "All images in $dirname have been resized to $width pixels wide."

Task

Write a script to recursively copy all the files in the demo directory to a directory called BACKUP