Introducing the Shell

  • A shell is a program whose primary purpose is to read commands and run other programs.
  • This lesson uses Bash, the default shell in many implementations of Unix.
  • Programs can be run in Bash by entering commands at the command-line prompt.
  • The shell’s main advantages are its high action-to-keystroke ratio, its support for automating repetitive tasks, and its capacity to access networked machines.
  • The shell’s main disadvantages are its primarily textual nature and how cryptic its commands and operation can be.

Navigating Files and Directories

  • The file system is responsible for managing information on the disk.
  • Information is stored in files, which are stored in directories (folders).
  • Directories can also store other directories, which then form a directory tree.
  • pwd prints the user’s current working directory.
  • ls [path] prints a listing of a specific file or directory; ls on its own lists the current working directory.
  • cd [path] changes the current working directory.
  • Most commands take options that begin with a single -.
  • Directory names in a path are separated with / on Unix, but \ on Windows.
  • / on its own is the root directory of the whole file system.
  • An absolute path specifies a location from the root of the file system.
  • A relative path specifies a location starting from the current location.
  • . on its own means ‘the current directory’; .. means ‘the directory above the current one’.

Working With Files and Directories

  • cp [old] [new] copies a file.
  • mkdir [path] creates a new directory.
  • mv [old] [new] moves (renames) a file or directory.
  • rm [path] removes (deletes) a file.
  • * matches zero or more characters in a filename, so *.txt matches all files ending in .txt.
  • ? matches any single character in a filename, so ?.txt matches a.txt but not any.txt.
  • Use of the Control key may be described in many ways, including Ctrl-X, Control-X, and ^X.
  • The shell does not have a trash bin: once something is deleted, it’s really gone.
  • Most files’ names are something.extension. The extension isn’t required, and doesn’t guarantee anything, but is normally used to indicate the type of data in the file.
  • Depending on the type of work you do, you may need a more powerful text editor than Nano.

Pipes and Filters

  • wc counts lines, words, and characters in its inputs.
  • cat displays the contents of its inputs.
  • sort sorts its inputs.
  • head displays the first 10 lines of its input.
  • tail displays the last 10 lines of its input.
  • command > [file] redirects a command’s output to a file (overwriting any existing content).
  • command >> [file] appends a command’s output to a file.
  • [first] | [second] is a pipeline: the output of the first command is used as the input to the second.
  • The best way to use the shell is to use pipes to combine simple single-purpose programs (filters).

Loops

  • A for loop repeats commands once for every thing in a list.
  • Every for loop needs a variable to refer to the thing it is currently operating on.
  • Use $name to expand a variable (i.e., get its value). ${name} can also be used.
  • Do not use spaces, quotes, or wildcard characters such as ‘*’ or ‘?’ in filenames, as it complicates variable expansion.
  • Give files consistent names that are easy to match with wildcard patterns to make it easy to select them for looping.
  • Use the up-arrow key to scroll up through previous commands to edit and repeat them.
  • Use Ctrl+R to search through the previously entered commands.
  • Use history to display recent commands, and ![number] to repeat a command by number.

Shell Scripts

  • Save commands in files (usually called shell scripts) for re-use.
  • bash [filename] runs the commands saved in a file.
  • $@ refers to all of a shell script’s command-line arguments.
  • $1, $2, etc., refer to the first command-line argument, the second command-line argument, etc.
  • Place variables in quotes if the values might have spaces in them.
  • Letting users decide what files to process is more flexible and more consistent with built-in Unix commands.

Finding Things

  • find finds files with specific properties that match patterns.
  • grep selects lines in files that match patterns.
  • --help is an option supported by many bash commands, and programs that can be run from within Bash, to display more information on how to use these commands or programs.
  • man [command] displays the manual page for a given command.
  • $([command]) inserts a command’s output in place.