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Bash Cheat sheet

Find where you are: pwd What is in here: ls

  • A program is an executable file.
  • Any command you call in bash is a program and it has to be somewhere in the system. Where bash looks is called PATH. To find where that program is, use which.
  • If you have a program in your folder, you must prepend ./ to the program so that shell knows that the program is here.

https://www.rozmichelle.com/pipes-forks-dups/

  • Programs take in something and spit out something (although they might do something else in the background).

  • Two channels that programs take in.

    • Arguments
      • These are all the words we put in behind the program. Usually specify some configurations. Ordering matters.
      • Arguments without dashes are usually required.
      • An argument with a double dash is usually an option and may/may not have extra info. Equal signs can be use or not. Ex. --color=red or --color red. Another one is --fast. This one doesn't have extra info.
      • An argument with a dash is always a single character and usually an abbreviation of the double dash. Ex. -c red. If you see a single dash with multiple characters, ex. -cd, that usually means -c -d.
    • STDIN
      • A channel that data flows in.

  • Two channels where data flow out.

    • STDOUT
    • STDERR

      • These are effectively the same but separated for organizational reasons. By default, these just flow into the terminal.

      • STDOUT is usually the data.

      • STDERR is usually the log or notes the program made.

      • To save STDOUT to a file, use >. Ex. ls > file.out saves the output of ls to a file named file.out.

      • To save STDERR to a file, use 2>. Ex. ls 2> file.out creates file.out, which has nothing inside because ls doesn't output STDERR.

  • With this, you can chain programs. For example, ls | head -n 1 ls outputs what we have in our current directory through STDOUT, which is passed into STDIN via the pipe '|' operator of head. Using the argument -n 1, head spits out through STDOUT only the first line it received.

Go here: cd

  • Absolute paths always start with /.
  • Your home directory (think of it as desktop) is at /home/$USERNAME
  • Relative paths (not started with /) are related to where you are, use pwd.
  • . (the dot) means current directory.
  • .. (two dots) mean the parent directory (directory above).
  • To go to a sibling folder, you can do cd ../folder/
  • Directory paths can end with / but not files.

I don't want to type this again and again. Use variables. = defines a variable. You don't need to quote the thing you're setting if you're not using spaces or some weird characters. For example, VAR=Hello is fine. VAR=Hello world is not. Use VAR="Hello world".

How do we discern literal texts and variables? The $ dollar sign. Example: suppose we set F=folder cd folder is the same as cd $F.

Strings can be concatenated or stick together using braces {}.

  • ${F}${F}${F} => folderfolderfolder
  • ${F}here => folderhere
  • $Fhere => Nothing. Bash tried to look for a variable called Fhere.