# CLI Part II: Files and Directories

**CHEG 667-013 — Chemical Engineering with Computers**  
Department of Chemical and Biomolecular Engineering, University of Delaware

---

## Key idea

Create, move, copy, and delete files and directories from the command line.

## Key goals

- Create directories with `mkdir` and remove them with `rmdir`
- Create files with `touch` and `cat`
- Move, copy, and remove files with `mv`, `cp`, and `rm`
- Understand that these operations are permanent — there is no trash can or undo

---


## 1. Making and removing directories

If we want to add or remove folders to our file hierarchy, we use the following commands:

- `mkdir` — make a directory
- `rmdir` — remove a directory (it needs to be empty)

**WARNING!** Removing a directory is *permanent*. **There is no trash can or undo!** This will apply to files, too.

Now go back to your home directory (`cd ~`) and make a new folder called `cheg667`:

```
$ mkdir cheg667
```

Verify that you made the directory by using `ls`. You can remove this directory with the `rmdir` command.


## 2. Moving, copying, and removing files

Now we'll learn how to move, copy, or remove files.

- `mv` — move (or rename) a file
- `cp` — copy a file
- `rm` — delete or remove a file

**WARNING!** All of these file operations are permanent! There is no undo! Moving or copying a file on top of another will erase or *clobber* the original. Be careful!

We need some files to work on. Do this in your home directory:

```
$ touch foobar
```

The `touch` command is a way of updating the modification time of a file. If the file doesn't exist, `touch` will create one. If we use `ls -lt`, we should see at the top that we created the file `foobar`, which has zero bytes.

```
-rw-rw-r-- 1 ef1j ef1j 0 Mar 30 06:57 foobar
```

Notice the file permissions. The file can be read by the user, group, and other users. Only the user and the group have permission to write to or otherwise modify the file or erase it.

Next, move `foobar` to the directory `~/cheg667`:

```
$ mv foobar cheg667
```

If you type `ls` now, you will not see it. But if you change directory to `cheg667` or type `ls cheg667`, you will find that it was moved to that directory.

Change directories to `cheg667` and type the following:

```
$ cp foobar foo
```

This copies the file `foobar` to a new file, `foo`:

```
$ ls
foo  foobar
```

Notice that the file modification times are different:

```
$ ls -l
total 1
-rw-rw-r-- 1 ef1j ef1j 0 Mar 30 07:08 foo
-rw-rw-r-- 1 ef1j ef1j 0 Mar 30 06:57 foobar
```


## 3. Creating files with cat

Create another file, but this time with text using the `cat` command:

```
$ cat > bar
Terminal whispers,
cat > file.txt begins
empty lines take shape
^D
```

When you type `cat > bar`, it is telling the terminal to redirect the *standard input* of the terminal to a file. (More on this later.) When you hit enter, the input that you type will be written to the file. You can stop by typing `control-D`. Verify that the text was written to the file, again using the `cat` command:

```
$ cat bar
Terminal whispers,
cat > file.txt begins
empty lines take shape.
```

Using `ls`, you will also see that the file has a non-zero size:

```
-rw-rw-r-- 1 ef1j ef1j 67 Mar 30 07:18 bar
```

Now copy the file `bar`. Call it `bar2`:

```
$ cp bar bar2
```

Do you see it in the directory listing?

You can remove `bar2` with the command:

```
$ rm bar2
```

Check the directory listing again. The file should be gone.

## 4. Creating files with echo

The `echo` command prints text to the terminal. It's useful for quick output and for writing short content to files using a redirect:

```
$ echo "Hello, world!"
Hello, world!
$ echo "This is a test" > test.txt
$ cat test.txt
This is a test
```

Use `>>` to append instead of overwriting:

```
$ echo "Another line" >> test.txt
$ cat test.txt
This is a test
Another line
```


## 5. Changing file permissions with chmod

In the previous section, we saw that files have permissions (`r`, `w`, `x`) for the owner, group, and others. The `chmod` command lets you change these permissions. You'll need this when you want to make a file executable, like a script.

The most common use for us is adding execute permission:

```
$ chmod u+x myscript.sh
```

Here, `u` means the user (owner), `+x` means add execute permission. You can also set permissions for the group (`g`) or others (`o`):

- `chmod g+r file` — give the group read access
- `chmod o-w file` — remove write access from others
- `chmod a+x file` — give everyone execute access (`a` = all)

> **Exercise 1:** Practice making new text files with the `cat` and `echo` commands. Rename a file using `mv`. Practice copying files. Use `man` to learn more about `mv`, `cp`, `rm`, and `chmod`.