Linux Administration notes

User management:

• Switch user: $su - <username>

• If the username is not given then it will be treated as root.

  $su - is equivalent to $su - root

• After executing the switch command you need to enter the password.

• If you are switching from root user to any user password is not required. Note from the user is important here i.e. current user, not the user to which you are switching.

• Let's say you are logged into a machine with ec2-user, then switch to the root using su - then you need to enter the password of the root user.

• After some time if want to switch back to ec2-user then use su - ec2-user. Here password is not required to enter because your current user was root before switching.

• If you want to make a user superuser then add him to the sudoer`s group(wheel in redhat distors and sudo in debian distros)

  sudo usermod -aG sudo username

• Another way is to update /etc/sudoers file, by executing visudo command

  # User privilege specification

  username ALL=(ALL:ALL) ALL

• If you want to get rid of password prompts for a user everytime you switch back then use

  username ALL=(ALL:ALL) NOPASSWD:ALL

    # User privilege specification
    root    ALL=(ALL:ALL) ALL
  
    # Members of the admin group may gain root privileges
    %admin ALL=(ALL) ALL
    k8    ALL=(ALL:ALL) ALL
    # Allow members of group sudo to execute any command
    %sudo   ALL=(ALL:ALL) ALL
  
    bmaddi  ALL=(ALL:ALL) NOPASSWD:ALL
    k8    ALL=(ALL:ALL) NOPASSWD:ALL
  

  AWK vs CUT:

• Both awk and cut can be used to extract fields and substrings from text, but they have some key differences:

  Delimiter

  cut uses a single character as the field delimiter by default (\t). You can specify a custom delimiter using -d:

      cut -d ',' -f1  # For comma-separated file
  

  awk uses whitespace as the default delimiter, which includes spaces, tabs, and newlines. You can specify a custom delimiter using -F:

      awk -F'|' '{print $1}' # For pipe-delimited file
  

  Empty Fields

  cut will print empty fields if they exist, while awk will suppress leading whitespace and empty fields by default.

      echo "abc   def" | cut -f2    # Prints empty field
      echo "abc   def" | awk '{print $2}' # Prints def
  

  Flexibility

  awk is more flexible as it allows you to use regular expressions as delimiters. You can also loop through columns using a for loop.

      awk -F'[ ]+' '{for (i=1;i<=NF;i++) print $i}'
  

  You can use functions like substr() to extract substrings:

      awk '{print substr($1,1,6),$2}'
  

  Performance

  cut is typically faster than awk since it is a simpler tool designed for a single purpose.