“Shell” is a command-line interpreter usually found in Unix and Unix-like systems (including Linux and Mac OS). It is what you get when you access a Ubuntu system via ssh.

When the shell is ready for your commands, it displays a shell prompt. On our VM, it has the form USER@HOSTNAME:current_path$, where current_path tells you where you currently are in the directory (folder) hierarchy. For example, the prompt may be vagrant@trusty64:~$; here, ~ stands for your home directory. You type in a command, hit return, and the command will execute. When the command finishes, the shell gives your another prompt. If a command hangs and you want to stop it, [CTRL]-c will usually do the trick.

If you are new to shell, please check out this tutorial for beginners. At least go through the sections “Navigation,” “Looking Around,” and “Manipulating Files.” We highly recommend going through the entire tutorial; it is not that long.

Shell provides access to a wide range of useful commands with text interfaces. We sample several of them below.

Viewing a File

To see what’s in a text file named myfile, simply use the command:
less myfile
It enters a “paging” mode, which lets you view/scroll/search the file inside your terminal. When less is running, you can use the following key strokes/sequences:
[SPACE] or f goes forward one page, and b goes back one page.
[UP] scrolls up one line, and [DOWN] scrolls down one line.
g goes to the beginning of the file, and G goes to the end of the file. More generally, type in a number and then g, and you will go to that line of the file.
/ lets you search for a string, and n goes to the next match in the output.
? lets you search in the reverse direction.
h lets you view the help page.
q quits less and brings you back to the shell.

Editing a File

Both nano and vim come pre-installed on your VM and lets you edit text files within a shell. nano is very easy to learn, and we recommend that if you are new to shell. To edit a (new or existing) text file named myfile, simply use the command:
nano myfile
The rest is pretty self-explanatory. The bottom of the screen always show the list of commands. ^x means [CTRL]-x.

vim is very powerful but can take quite a while to get used to. To learn vim, we recommend a nice interactive tutorial.

Searching Through Big Files or Lots of Files

If you have big files or many files to search through, use grep. Here are some common usage examples:
grep 'foo' file finds lines in file containing the string foo (anywhere within the line).
grep -n 'foo' file also give you the line numbers.
grep 'foo' *.sql searches for foo in all files in the current directory whose name ends with .sql.
grep -r 'foo' dir/ recursively searches for foo in all files found anywhere under the directory dir/.