ls command more usefulThe .. operator that we saw earlier can also be used with the ls command, e.g. you can list directories that are ‘above’ you:
ubuntu@:~/Learning_unix$ cd ~/Learning_unix/Outer_directory/
ubuntu@:~/Learning_unix/Outer_directory$ ls ../../
command_line_course Learning_unix linux_bootcamp
Time to learn another useful command-line option. If you add the letter ‘l’ to the ls command it will give you a longer output compared to the default:
ubuntu@:~$ ls -l /home
total 8
drwxr-xr-x 2 adminuser adminuser 4096 Nov 2 18:19 adminuser
drwxr-xr-x 20 ubuntu ubuntu 4096 Nov 3 21:27 ubuntu
For each file or directory we now see more information (including file ownership and modification times). The ‘d’ at the start of each line indicates that these are directories. There are many, many different options for the ls command. Try out the following (against any directory of your choice) to see how the output changes.
ls -l
ls -R
ls -l -t -r
ls -lh
Note that the last example combine multiple options but only use one dash. This is a very common way of specifying multiple command-line options. You may be wondering what some of these options are doing. It’s time to learn about Unix documentation….
If every Unix command has so many options, you might be wondering how you find out what they are and what they do. Well, thankfully every Unix command has an associated ‘manual’ that you can access by using the man command. E.g.
man ls
man cd
man man # yes even the man command has a manual page
When you are using the man command, press space to scroll down a page, b to go back a page, or q to quit. You can also use the up and down arrows to scroll a line at a time. The man command is actually using another Unix program, a text viewer called less, which we’ll come to later on.
We now have a few (empty) directories that we should remove. To do this use the rmdir command, this will only remove empty directories so it is quite safe to use. If you want to know more about this command (or any Unix command), then remember that you can just look at its man page.
ubuntu@:~$ cd ~/Learning_unix/Outer_directory/
ubuntu@:~/Learning_unix/Outer_directory$ rmdir Inner_directory/
ubuntu@:~/Learning_unix/Outer_directory$ cd ..
ubuntu@:~/Learning_unix$ rmdir Outer_directory/
ubuntu@:~/Learning_unix$ ls
ubuntu@:~/Learning_unix$
** Note, you have to be outside a directory before you can remove it with rmdir **
Saving keystrokes may not seem important, but the longer that you spend typing in a terminal window, the happier you will be if you can reduce the time you spend at the keyboard. Especially, as prolonged typing is not good for your body. So the best Unix tip to learn early on is that you can tab complete the names of files and programs on most Unix systems. Type enough letters that uniquely identify the name of a file, directory or program and press tab…Unix will do the rest. E.g. if you type ‘tou’ and then press tab, Unix should autocomplete the word to ‘touch’ (this is a command which we will learn more about in a minute). In this case, tab completion will occur because there are no other Unix commands that start with ‘tou’. If pressing tab doesn’t do anything, then you have not have typed enough unique characters. In this case pressing tab twice will show you all possible completions. This trick can save you a LOT of typing!
Navigate to your home directory, and then use the cd command to change to the Learning_unix directory. Use tab completion to complete directory name. If there are no other directories starting with ‘L’ in your home directory, then you should only need to type ‘cd’ + ‘L’ + ‘tab’.
Tab completion will make your life easier and make you more productive! If you are typing too much - you are doing it wrong!
Another great time-saver is that Unix stores a list of all the commands that you have typed in each login session. You can access this list by using the history command or more simply by using the up and down arrows to access anything from your history. So if you type a long command but make a mistake, press the up arrow and then you can use the left and right arrows to move the cursor in order to make a change.
The history command is very useful to dump all the commands you have used to perform an analysis.
The following sections will deal with Unix commands that help us to work with files, i.e. copy files to/from places, move files, rename files, remove files, and most importantly, look at files. First, we need to have some files to play with. The Unix command touch will let us create a new, empty file. The touch command does other things too, but for now we just want a couple of files to work with.
ubuntu@:~$ cd Learning_unix/
ubuntu@:~/Learning_unix$ touch heaven.txt
ubuntu@:~/Learning_unix$ touch earth.txt
ubuntu@:~/Learning_unix$ ls
earth.txt heaven.txt