Unit 5: Unix File Management

This note assumes that you are familiar with navigation with the Unix directory structure.

We will now learn some commands to help us deal with files.

Let's recreate the workshop directory in case you have already deleted it with the rmdir at the end of the last unit.

$ mkdir -p workshop
$ cd workshop
$ ls

All of the above commands should complete successfully and silently. Let's populate the directory with a new file. Cut-and-paste the command below into the command prompt :

wget https://raw.githubusercontent.com/nus-unix-workshop/2021-s1/master/test.txt

You should see a file being downloaded and saved with an output similar to below: 

--2020-07-27 15:26:49--  https://raw.githubusercontent.com/nus-unix-workshop/2021-s1/master/test.txt
Resolving raw.githubusercontent.com (raw.githubusercontent.com)... 151.101.0.133, 151.101.64.133, 151.101.128.133, ...
Connecting to raw.githubusercontent.com (raw.githubusercontent.com)|151.101.0.133|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 64 [text/plain]
Saving to: ‘test.txt’

test.txt              100%[======================>]      64  --.-KB/s    in 0s

2020-07-27 15:26:50 (2.35 MB/s) - ‘test.txt’ saved [64/64]

Now run ls, and you should see that ls returns test.txt as the content of the current working directory.

$ ls
test.txt

cp: CoPy files

Now let's try to copy this file to another name. 

$ cp test.txt foo.txt
$ ls
test.txt foo.txt
The command above copies the file test.txt into foo.txt.

If you want to copy the whole directory, use -r flag, where r stands for copying recursively.

Now let's create another directory called copy. 

$ cd ..
$ mkdir copy
$ cd copy
$ ls

Run pwd to double-check that you are in the directory called copy that is at the same level as workshop.

Now, we are going to use cp with the -r flag, to copy recursively the whole of workshop directory over.

$ cp -r ../workshop .

The command cp takes in two arguments, the first is the source, and the second is the destination.

Note that we use . above to indicate that we wish to copy the whole sub-tree of workshop over the current directory. The command should complete without any message. Upon completion, you can run ls to double-check that the workshop directory exists under workshop.

!!! Warning: cp Overwrites If there is an existing file with the same name, cp will overwrite the existing file without warning.

mv: MoVe or rename files

Now, let's change directory back to workshop. 

$ cd ../workshop
and use the mv command to rename foo.txt into bar.txt.

$ ls
foo.txt test.txt
$ mv foo.txt bar.txt
$ ls
bar.txt test.txt

As you can see above, just like cp, mv takes in two arguments, the first is the source and the second is the destination.

If the destination of mv is a directory, however, instead of renaming, the mv commands move the source to the destination directory.

$ ls
bar.txt test.txt
$ mv ../copy/workshop/foo.txt .
$ ls
bar.txt foo.txt test.txt

Here, you can see that we have moved foo.txt over to the current directory.

!!! Warning: mv Overwrites If there is an existing file with the same name, mv will overwrite the existing file without warning. mv comes with a -i flag that interactively asks you if you are sure if you want to overwrite a file. It is a good idea to always run mv -i. Hit Y to continue overwriting the existing file. We will see how to make -i the default tomorrow (sneak peak here).

Use Tab for Name Completion

If you have a very long file name, you may use the bash auto-completion feature to reduce typing. For instance, you may type: 

$ mv t
and press the Tab key, bash will complete the filename for you if there is only one filename with the prefix "t". Otherwise, it will fill up the filename to the point where you need to type in more characters for disambiguation. The Tab key can also complete the name of a command.

rm: ReMove files

We can use rm to remove files. Be careful with this command -- files deleted cannot be restored. There is no trash or recycled bin like in Mac or Windows.

$ ls
bar.txt foo.txt test.txt
$ rm foo.txt
$ ls
bar.txt test.txt

rm -rf

While the Unix command line provides lots of flexibility and power, with great power comes great responsibility. Some of the commands are extremely dangerous. rm -rf * is the most famous one. The notation * refers to all files, and the flag -f means forceful deletion (no question asked!), and -r means remove recursively everything under the current directory tree. Accidentally running this command has ruined many lives. Read more here

rm comes with a -i flag that interactively asks you if you are sure if you want to delete a file. It is a good idea to always run rm -i.

$ rm -i bar.txt
rm: remove regular file 'bar.txt'?

Type y or n to answer yes or no respectively.

cat: CATenate file content to screen

To quickly take a look at the content of the file, use the cat command.

$ cat test.txt
This is a test file for learning Unix file management commands.

less is a variant of cat that includes features to read each page leisurely and is useful for long files. 

$ less test.txt

In less, use <space> to move down one page, b to move Back up one page, and q to Quit.

man: Online MANual

An online help facility is available in Unix via the man command (man stands for MANual). To look for more information about any Unix command, for example, ls, type man ls. Type man man and refer to Man Pages to find out more about the facility. To exit man, press q.

Other Useful Online Tools