Week 12-13: SSH and the Linux command line

We'll spend the next few weeks working on a server running a Linux-based operating system.

We're going to run through some examples using the Bash shell and some utility programs.

Here are a few of the commands we'll use:

• man: display help information
• cd: change directory
• pwd: print working directory
• ls: list directory contents
• wc: count letters, words, or lines
• cp, rm, mv, touch: copy, remove, move, create/update files
• nano: a simple text editor. vim is more capable but more complicated.

SSH

SSH is protocol for secure network communication, remote login, and remote command execution.

To log in to a server that you have an account on, use the command ssh USERNAME@HOSTNAME.

To run a command on the server, use the same command but also list the command: ssh USERNAME@HOSTNAME cat .bashrc will print the contents of your .bashrc file.

File system

Most file systems we'll use are based on the same ideas. The only differences we'll run into (compared to working with the Windows command line) are:

• / is the path separator rather than \.
• There are no C: or D: drives; rather all storage devices are mounted on a directory, which can simplify man filesystem operations.

Globbing

Globbing is the way that Bash refers to multiple files at once (using wildcards). It will often work the same as in the Windows command line, but in the Windows command line globbing is performed by each program (so it can vary from program to program), while the Bash shell performs globbing itself.

Exercises

1. Create a directory structure that looks like the hierarchy shown below (with all files empty). The touch command creates an empty file if the name doesn't already exist.

   folder1/
       a.txt
       b.txt
       c.txt
   folder2/
       a.txt
       b.txt
       c.txt
   folder3/
       a.txt
       b.txt
   

2. What command would you use to remove all files starting with a, b, c, or d in your current directory? What happens if there is a subdirectory starting with b in your current directory?

3. The find command is used to locate files (and list the found files). How can you use it in combination with wc to count the number of files with names ending in .so in the /usr/lib directory?

4. The grep command searches through files (or standard input), much like findstr that is usually available in the Windows command line. It uses regular expressions to define a pattern for the search. Suppose you want to print all the lines of a Java program that aren't comments. What command could you use to do that? Assume that commented lines always start with some number of spaces and then the single-line Java comment token (//). Hint: look at the man page for grep to see how to select all lines that don't match a pattern.

File permissions

After logging in, run the command ls -al to give a detailed list of your home directory. Here's a partial output for my directory:

drwx------ 15 jlepak jlepak   4096 Apr 24 01:58 .
drwxr-xr-x 15 root   root     4096 Apr 24 00:40 ..
-rw-r--r--  1 jlepak jlepak    270 Apr 24 01:31 add-csc185.sh
drwx------  2 jlepak jlepak   4096 Nov 22 00:39 .aptitude
-rw-r-----  1 jlepak root    27681 Apr  2 19:18 auth.log
-rw-------  1 jlepak jlepak   1648 Apr 24 01:20 .bash_history

The first field gives details of the file permissions for each entry. There are 10 columns: the first indicates if the entry is for a directory (or other possibilities); the next 9 indicate if read, write, or execute (allowing the file to be run as a command) permissions are enabled for the user (the primary owner of the file, shown in field 3), group (a group of one or more users, shown in field 4), and all others.

The chmod program can change file permissions to enable or disable any of those permissions.

More exercises

1. Type the whoami and groups commands. What is their output?
2. Type the who command. Who else is logged in?
3. Navigate to someone else's home directory: use the command cd /home to navigate to the folder that contains all the user directories, and then pick one. What happens when you try to create a file in someone else's home? (You can use the command touch my-file.txt to try).
4. Navigate to the /var/www/ilzd.org/csc185 folder. /var/www/ilzd.org holds the files served up by the web server, and csc185 is a folder that everyone in class has permissions to write to.

Create a text file (either a .txt file or a .html file if you are familiar with HTML) and put a short message in it. Name it something unique so I know it's from you. Verify that you can see it in a web browswer by navigating to http://ilzd.org/csc185/yyy.txt (where yyy.txt is whatever you named your file).

Readings

Take a look at the Software Carpentry website, and especially the chapter about the shell.