• Mastering Linux Administration

Create User

su root
useradd foo
pass foo

# Add user to the sudoers group
usermod -aG wheel foo
su - foo
whoami

Package Management

# apt is a frontend to dpkg
dpkg --list {name}
dpkg --remove {name}
apt remove {name}

Linux File System

• check default shell

cat /etc/passwd | grep foo
echo $0

• wildcards
  • * : any string of none or more characters
  • ? : single character
  • [] : any of the characters inside the brackets
• metacharacters
  • >: Output redirection.
  • >>: Output redirection – append.
  • <: Input redirection.
  • <<: Input redirection.
  • *: File substitution wildcard (explained previously).
  • ?: File substitution wildcard (explained previously).
  • [ ]: File substitution wildcard (explained previously).
  • |: Pipe for using multiple commands.
  • ;: Command execution sequence.
  • ( ): Group of commands in the execution sequence.
  • ||: Conditional execution (OR).
  • &&: Conditional execution (AND).
  • &: Run a command in the background.
  • #: Use a command directly in the shell.
  • $: Variable value expansion.
  • \: The escape character.
  • cmd: Command substitution.
  • $(cmd): Command substitution.

ls -l /etc/ | less

touch myfile-1
touch myfile-2
# brace expansion
rm ~/{myfile-1, myfile-2}

• Bash Shell variables
  • HOME
  • LOGNAME : login name
  • PWD
  • OLDPWD
  • PATH
    • This variable is essential in Linux. It helps the shell know where all the programs are located
    • When you enter a command into your Bash shell, it first has to search for that command through the Linux filesystem
    • To make any changes permanent, you must modify the PATH variable inside a file called ~/.bash_profile or ~/.bashrc
  • SHELL
  • USER
  • TERM

printenv | grep PATH
echo $PATH

• Links
  • Symbolic Links
  • Hard Links
  • Difference between soft vs hard

# Soft Link
# ln -s [original_filename] [link_filename]
touch a.txt
ln -s a.txt b.txt
echo "aaa" >> a.txt
cat a.txt
cat b.txt
# return the name of the file the symbolic link points to
readlink b.txt

# Hard Link
# different virtual file that points to the original file
# they are physically the same
touch a.txt
ln a.txt b.txt
echo "aaa" >> a.txt
cat a.txt
cat b.txt
readlink b.txt

# more detailed info-name,size,type
stat a.txt
# report type of a file
file a.txt

• less print one page and allows navigation
• head print first 10 lines
• tail print last 10 lines
  • -f output appends as the file grows