Navigating Files and Directories

1. Files and Directories

• File System: an Operating System component responsible for managing files and directories.

• Perspective:

  • On a GUI, you click to move from one place to another, so you are outside the file system space looking in.

  • On a CLI, you need to explicitly provide direction (path) for the command to know with which file/directory it is supposed to interact. The perspective is more inside the file system space.

2. Where are we, what do we have here, and how do we go somewhere else?

• Three basic commands: pwd, ls, cd

• pwd returns the absolute path to the current working directory (i.e.: where you are when you are in the terminal).

pwd

• ls returns the list of current files and directories in the target directory.

ls /

• There are many options available for different commands. To view the documentation, run the followings:

ls --help

• Run ls by itself will list the contents of the current directory.

ls

• cd allows users to change the current directory (outcome of pwd) to the target directory.

  • Run man cd or cd --help to read the documentation for cd.

  • The generate syntax for cd is cd DESTINATION with DESTINATION can either be absolute or relative paths or special paths.

• Change to root directory and view contents of root:

cd /
ls 

• Special paths:

  • ~: home direcrory

  • .: current directory

  • ..: a directory that is one level above the current directory

• Change to your home directory using the special path

  • Check the content of your home directory to confirm that you have the shell-lesson-data directory.

  • Change into shell-lesson-data directory and view the contents of this directory

cd ~
ls Desktop/
cd Desktop/shell-lesson-data
ls 

3. Challenge: exploring more ls flags

• You can also use two options at the same time. What does the command ls do when used with the -l option? What about if you use both the -l and the -h option?

• Some of its output is about properties that we do not cover in this lesson (such as file permissions and ownership), but the rest should be useful nevertheless.

Solution

• The -l option makes ls use a long listing format, showing not only the file/directory names but also additional information, such as the file size and the time of its last modification.

• If you use both the -h option and the -l option, this makes the file size human readable, i.e. displaying something like 5.3K instead of 5369.

4. Challenge: Listing in reverse chronological order

• By default, ls lists the contents of a directory in alphabetical order by name. The command ls -t lists items by time of last change instead of alphabetically. The command ls -r lists the contents of a directory in reverse order.

• Which file is displayed last when you combine the -t and -r options? Hint: You may need to use the -l option to see the last changed dates.

Solution

The most recently changed file is listed last when using -rt. This can be very useful for finding your most recent edits or checking to see if a new output file was written.

5. Challenge: ls Reading comprehension

• Using the filesystem diagram below.

• If pwd displays /Users/backup and -r tells ls to display things in reverse order, what command(s) will result in the following output:

pnas_sub/ pnas_final/ original/

1. ls pwd

2. ls -r -F

3. ls -r -F /Users/backup

Solution

1. No: pwd is not the name of a directory.

2. Yes: ls without directory argument lists files and directories in the current directory.

3. Yes: uses the absolute path explicitly.

6. General syntax of a shell command

• ls is the command, with an option -F and an argument /.

• Option:

  • either start with a single dash (-) or two dashes (--),

  • change the behavior of a command.

  • can be referred to as either switches or flags.

• Arguments tell the command what to operate on (e.g. files and directories).

• Sometimes options and arguments are referred to as parameters.

  • The shell is in fact just a process/function and these options and arguments are being passed as parameters to the shell’s function that is responsible for executing the command.

• A command can be called with more than one option and more than one argument, but a command doesn’t always require an argument or an option.

• Each part is separated by spaces: if you omit the space between ls and -F the shell will look for a command called ls-F, which doesn’t exist.

• Capitalization can be important.

  • ls -s will display the size of files and directories alongside the names

  • ls -S will sort the files and directories by size