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Linux CLI

Tail​

  • tail -f <file>: will open the back end of the file and keep it open waiting for changes.
    • Useful for logging and/or actively waiting for changes.

Quotes​

  • Using single quotes ' won't resolve ENVIRONMENT VARIABLES inside them on the command line.
    • echo 'hello $USER' => hello $USER
  • Using double quotes " will resolve ENVIRONMENT VARIABLES inside them on the command line.
    • echo "hello $USER" => hello Dempsey (USER=Dempsey)
  • Using back ticks will execute a sub-command in the current command (equivalent of using $()
    • echo `whoami` === echo $(whoami)

$PATH​

  • This is your path variable where paths to executables live.
  • If you have for example /usr/local/bin in your $PATH, then you can run executables that live in there from anywhere on your system.
  • You can alternatively just run scripts via an alias, but if you have lots of scripts, this way would be better rather than unnecessarily bloating your alias file.

Permissions​

  • There are user (u), groups (g) and everybody else (o).
# -rwxr--r--
# ---  is the user
#    ---  is the group
#       --- is everybody else
# it goes read, write, execute
  • A few ways you can alter permissions with chmod
    • chmod +w <file> which will give write access to everyone for that file.
    • chmod ug+x <file> which will give execution access to the user and groups.
    • chmod g-x <file> will take away execution access to the group.
    • chmod 721 <file> which will give user read, write and execute. Groups read and everyone execution.
  • Octal permissions, the 1 number is for the user, 2 for the group and 3 for everyone.
    • 4 for write.
    • 2 for read.
    • 1 for execute.

Exit Codes​

  • Use $? to see the exit code of the previous command.
  • Handy to use exit codes in loops or scripts to determine how we run things.
  • 0 is successful, anything else is a failure.
  • Exit codes are what is used with operators such as && and ||
    • && is true if both commands either side of it return an exit code of 0.
    • || won't run the second command if the first command runs (short-circuiting).
    • ; will run all the next command regardless. It essentially a full stop meaning the end of a command the start of a new one.

Screen​

  • Create a screen with screen -S name.
  • Handy for having things running in the background and you can login and out of it as you please.