Linux Commands

bash_command.md

clear
sudo apt=get update && sudo apt-get upgrade

whoami
hostname
ls
ls -l

To list hidden folder and files
ls -la
Folders
ls -l -G
ls -l --no-group
man ls

Print Working Directory
pwd

**Print Environment Variable printenv

change directory
cd /bin

Home directory
cd ~

Entries that starts with p
ls -l /bin/p*

At root
ls -l /

ls -l /var

Files and User Permissions

Below is the result of ls -l command:
-rwxr-xr-x 1 vinkrish staff 3818 Feb 1 2000 program.sh

Observations:

• - beginning character tells it's a file, d is for directory
• Next 3 characters specify read, write and execute permissions for owner
• Next 3 characters specify group permissions
• Next 3 characters specify other user permissions
• 1 specifies file references
• vinkrish: Owner
• staff: Group
• 3818: file size in bytes
• 02/01/2000: Last modified date
• program.sh: name of the file

Switch user to root user for the session, terminal will show now running as root (avoids entering sudo each time!)
sudo su

Environment Variable

export PORT=3000
process.env.PORT

To print out content of file
cat /var/file.txt

To print line numbers
cat -n /var/file.txt

tail /var/file.txt

To open just content without previous commands
less /var/file.txt

g -> go to beginning of file  
Shift + g -> go to end of file  
/word + enter -> searches the word in file  
:q -> quit

Open folder or file in default app

open .
open folder/file.txt
open folder/file.asd -a TextEdit

To create a file
touch file.txt

Write to a file (overwrites)
echo 'hello world!' > file.txt

To append file
echo ' appended' >> file.txt

To create folder + intermediary folders with -p flag

mkdir folder_name
mkdir -p outer_folder/middle_folder/inner_folder
ls outer_folder/middle_folder
outputs inner_folder

Delete file and folder

rm file.txt
rm -r folder_name
rm -rf folder/
rm -r *

Move Files

mv source destination
mv file.txt folder_name/file.txt

Rename file
mv file.txt renamed_file.txt

Rename folder
mv folder_name/ renamed_folder_name

Move all content from one folder to another
mv source_folder/* destination_folder/

Copy file
cp file.txt destination_folder/file.txt

Copy all files
cp -R source_folder/* destination_folder/

Symbolic Link

ln -s existing_file symbolic_link_file
ln -l symbolic_link_file
rm symbolic_link_file
unlink symbolic_link_file

ln -sf existing_file overwrite_symbolic_link_file

ln -s /Users/vinkrish/Workspace ~/workspace
man ln

Extract tgz file

tar -xvf file.tgz
tar -xvzf file.tar.gz
tar -xvf file.tar.gz

Find png images in a folder
find image_folder/ -name "*.png"

Case insensitive search
find image_folder/ -iname "*.jpg"

All folders (outer + inner)
find . -type d

Find folders having images in their name
find . -type d -name "images"

Deleting with find
find dist/ -name "*.built.js" -delete

Add execute permission to user
chmod u+x script.sh

Add Write persmission to all users
chmod a+w /var/file.txt

Remove Write persmission to all users
chmod a-w /var/file.txt

Change owner
sudo chown vinay /var/file.txt

Change permission so that you don't have to use sudo
chmod u+w /var/file.txt

find /var/log/syslog* -print0 | xargs
find /var/log/syslog* -type f

To use pipes properly
find /var/log/syslog* -type f -print0 | xargs -0 chmod a+r

To search in concatinated file content
grep looks for matching lines in command input

find /var/log/syslog* -type f -print0 | xargs -0 cat | grep "SearchTerm"
find /var/log/syslog* -type f -print0 | xargs -0 cat | grep "SearchTerm.*filterFurther"

To count number of occurances:
find /var/log/syslog* -type f -print0 | xargs -0 cat | grep -c "SearchTerm.*filterFurther"

ifconfig
ifconfig | grep -E "inet6.*autoconf"
ifconfig | grep -E "inet6.*autoconf" | head -n 1
ifconfig | grep -E "inet6.*autoconf" | head -n 1 |  cut -d " " -f 2

Text editor
nano file-name.sh

To generate public and private key with comment to identify

ssh-keygen -b 2048 -t rsa -C vinay@localhost.com
(or)
ssh-keygen -t ed25519 -C "<comment>"

cd ~/.ssh

look at the permission ls -l

ls .ssh
cat .ssh/id_rsa.pub

Save the public key on server
ssh-copy-id -i .ssh/id_rsa.pub vinay@machine-name

SCP (Secure Copy)

Copy file from a remote host to local host
scp username@from_host:file.txt /local/directory/

Copy file from local host to a remote host
scp file.txt username@to_host:/remote/directory/

Copy directory from a remote host to local host
scp -r username@from_host:/remote/directory/ /local/directory/

Copy directory from local host to a remote host
scp -r /local/directory/ username@to_host:/remote/directory/

Copy file from remote host to remote host
scp username@from_host:/remote/directory/file.txt username@to_host:/remote/directory/

Bash Script

#!/bin/bash

while [ : ]
do
  originalAddress=$(ifconfig | grep -E "inet6.*autoconf" | head -n 1 |  cut -d " " -f 2)
  echo "$(date) $originalAddress" >> ~/ip.txt
  sleep 10
done

• Specify BASH shell
• assign value to variable (note: no spaces around =)
• make script executable
• run script
• watch file for appended content with flag -f

chomod u+x script.sh
./script.sh
tail -f ip.txt

CRON Job

#!/bin/bash

originalAddress=$(ifconfig | grep -E "inet6.*autoconf" | head -n 1 |  cut -d " " -f 2)
echo "$(date) $originalAddress" >> ~/ip.txt

crontab -e
* * * * * ~/script.sh

crontab -r