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Part 1. Navigating the file system

The file system is a hierarchical arrangement of files, grouped into directories. The hierarchy forms a tree of the sort you learned about in CSCI 151. Each file and directory has a name and we can refer to any file in the file system by giving a path through the tree, starting at the root directory and using / to separate the directories along the path.

Here are some example paths.

  • / The root directory itself
  • /bin/bash The program bash (this is the shell we’re using right now!) which lives inside the directory bin which lives inside the root directory
  • /usr/bin/wget The program wget (this program will let us download files from the internet) which lives in the directory /usr/bin

Naming files by starting at the root / all the time is time-consuming. Each program has a notion of its current directory. We can also name files by giving a path from the current directory.

Here are some examples of relative paths.

  • paper.pdf The file paper.pdf inside the current directory
  • Music/swift.mp3 The file swift.mp3 inside the Music directory inside the current directory

The terminal you opened launched the bash shell and it should be currently running. You should see a prompt in the terminal that looks like user@mcnulty:~$ (yours will have a different username and hostname—the name of the computer). From here on out, the prompt will be specified only as $. You should not type the $ character when entering input.

Bash has several commands for navigating the file system and listing the contents. The most commonly used commands are

  • cd change current directory; $ cd /usr/bin changes the current directory to /usr/bin; with no arguments, cd changes the current directory to your home directory

  • ls lists the contents of a directory; $ ls /tmp lists the contents of the directory for temporary files, /tmp; with no arguments, ls lists the contents of the current directory

  • pwd prints the current directory (the name stands for “print working directory” where “working directory” just means the current directory).

    Info

    By convention, files and directories that start with a period are hidden from directory listings. If you want to see all files, including hidden ones, use $ ls -a.

    Every directory has two directory entries named . and ... The single period, ., is the entry for the directory itself. Two periods, .., is the entry for the parent directory.

    Perhaps the most common use of .. is to change to the parent directory using $ cd ... For example, after changing to a directory like $ cd foo/bar/, I can return to the foo directory by running $ cd ...

Your task

Create a file named task1.txt using a text editor. To do so, click the Show Applications button in the bottom left corner of the desktop, and then open Text Editor. You may also use a command-line editor like emacs or vim. Both are available on mcnulty. To open them, you can open terminal and simply type emacs {filename} or vim {filename}.

Using the shell commands cd and ls, find two different files or directories in each of the following directories

  • /
  • /usr
  • /usr/bin
  • /etc

Write the names of the 8 different files (and which directory they’re in) in task1.txt. Save this file in your home directory for now. We’re going to move it later.

Info

The shell expands a tilde, ~, into the path for your home directory. Thus, you can, for example, change back to your home directory using $ cd ~. You probably want to do so now.

You may construct paths relative to your home directory by starting the path with ~/. For example, the shell will expand ~/foo/bar into the path to the foo/bar file or directory inside your home directory.

Note that the shell treats an unquoted ~ differently than a quoted tilde, '~' or "~". Run the command $ echo ~ '~' "~" to see the difference.

Use $ ls ~ to print the contents of your home directory. You should see task1.txt in that directory listing.

Use $ cat ~/task1.txt to print the contents of the file to the terminal.

Return to your home directory by running $ cd (with no arguments) or equivalently $ cd ~.