Day 2 - Working with Directories and Files

📌 Key Takeaways

• Three commands for dealing with location: pwd (where am I) · cd (move) · ls (what's here).
• ls -a shows hidden files and ls -l shows detailed info. A hidden file in Linux is a file whose name starts with ..
• cp copies, and mv both moves and renames. There is no separate rename command for mv.
• To handle directories you need -r (recursive) — cp -r, rm -r.
• rm has no recycle bin. Once it's deleted, it's gone. rm -rf is especially dangerous.

Where Am I Now

pwd            # /home/user
cd /etc        # move using an absolute path
cd ..          # go up to /
cd ~           # return home

• cd accepts both absolute and relative paths. When you are in /home/user, cd docs and cd /home/user/docs go to the same place.
• cd - remembers the previous location and goes back to it, which is handy when you work by hopping between two directories.
• Adding a user name after ~ means that user's home — cd ~alice goes to /home/alice.

[user@localhost ~]$ cd /var/log
[user@localhost log]$ cd /etc
[user@localhost etc]$ cd -
/var/log
[user@localhost log]$ pwd
/var/log

What's Here — ls

ls -l          # in detail
ls -la         # in detail, including hidden files
ls -lh         # sizes in KB/MB
ls -ltr        # time order, reversed — most recent file at the bottom

A few more options are worth knowing, since they make it easier to tell apart the answer choices on exam questions.

⚠️ Different case means a different option. -r is reverse, -R is recursive. -a shows everything including . and .., while -A omits those two. -s shows the block count, and -S sorts by size.

• A plain ls with no options sorts by name. -t puts most recently modified first, -S puts largest first, and adding -r flips each of them.
• The . and .. that always appear with ls -a are entries pointing to the current directory and the parent directory. Since their names start with ., they are hidden too.

Hidden Files

• In Linux, a hidden file is a file whose name starts with .. It is not a separate attribute.
• Most configuration files, such as .bashrc, .ssh, and .profile, fall into this category.
• They don't show up with a plain ls; you need ls -a to see them.

Reading One Line of ls -l

   -rw-r--r--  1  user  group  1024  Sep  3 09:00  a.txt
   │        │  │   │      │      │        │          │
   │        │  │   │      │      │        │          └── name
   │        │  │   │      │      │        └───────────── last modified time
   │        │  │   │      │      └────────────────────── size (bytes)
   │        │  │   │      └───────────────────────────── group
   │        │  │   └──────────────────────────────────── owner
   │        │  └──────────────────────────────────────── hard link count
   │        └─────────────────────────────────────────── permissions (details next week)
   └──────────────────────────────────────────────────── file type

The very first character is the file type.

Let's look at how each type appears in real output, all at once.

$ ls -l /dev/sda /dev/tty /etc/passwd /bin /tmp -d
lrwxrwxrwx.  1 root root         7 Sep  3 09:00 /bin -> usr/bin
brw-rw----.  1 root disk    8,   0 Sep  3 09:00 /dev/sda
crw-rw-rw-.  1 root tty     5,   0 Sep  3 09:00 /dev/tty
-rw-r--r--.  1 root root      2104 Sep  3 09:00 /etc/passwd
drwxrwxrwt. 12 root root      4096 Sep  3 09:10 /tmp

• A symbolic link has -> original after its name, and its size is the number of characters in the path it points to (usr/bin is 7 characters).
• A device file shows two numbers (major number, minor number) in the size column. A directory's size is not the total of the files inside it but the size of the directory entry itself.
• The time that ls -l shows is the last modified time (mtime). A file carries three timestamps.

Pitfall: ctime is not the creation time (create). It is the time the status changed (change). Editing the contents changes both mtime and ctime, while just running chmod changes only ctime.

Creating and Deleting

⚠️ rm -rf is the most dangerous command in Linux. With no recycle bin, recovery is practically impossible. It is safer to get in the habit of checking with ls first before deleting a path.

If you try it yourself, the error messages show what is required.

$ mkdir project/src/lib
mkdir: cannot create directory 'project/src/lib': No such file or directory
$ mkdir -p project/src/lib
$ touch project/src/lib/a.c
$ rmdir project
rmdir: failed to remove 'project': Directory not empty
$ rm project
rm: cannot remove 'project': Is a directory
$ rm -r project

• When -i and -f are used together, the one written last wins. RedHat-family systems often alias rm to rm -i for root, which is why root gets a confirmation prompt when typing rm.
• If the file doesn't exist, touch creates an empty (0-byte) file; if it exists, it leaves the contents alone and just sets the time to now. You can also specify the time directly with -t, like touch -t 202609031200 a.txt.

Wildcards — Pointing at Many Files at Once

ls *.txt             # everything ending in .txt
rm report?.bak       # report1.bak, reportA.bak ...
cp [a-c]*.conf /tmp  # .conf files starting with a, b, or c

• Wildcards are expanded first by the shell, not the command, turned into a list of file names, and then passed to the command. That is why you can check what gets matched with ls * before running rm *.
• * does not match hidden files (those starting with .). Even after rm *, .bashrc remains.

Copying and Moving

• A single mv serves as both move and rename. Linux has no dedicated rename command by default.
• Running mv a.txt b.txt within the same directory renames it, and giving a different directory as the target moves it.
• mv doesn't need -r. Directories are simply moved.

cp a.txt b.txt          # copy a file
cp -r docs backup       # copy a directory (error without -r)
mv a.txt /tmp/          # move
mv old.txt new.txt      # rename

The Result Differs When the Target Already Exists

cp and mv behave differently depending on whether the last argument is an existing directory.

cp a.txt b.txt c.txt /tmp/     # with several sources, the last one must be a directory
mv *.log /var/backup/          # move many files at once

• If a file with the same name exists at the target, it is overwritten without asking. Use -i to prevent this.

The fundamental difference between cp and mv. cp creates a new file — so the copy's owner becomes the person who copied it and its time becomes now (use -p to keep the original's). Within the same file system, mv doesn't move the data; it just re-attaches the name tag. Because the inode stays the same, even a large directory moves instantly, which is why -r isn't needed.

Links

• A symbolic link shows l as the first character in ls -l and appears in the form link -> original.
• A hard link cannot be made across different file systems, and cannot be made to a directory.

A file name is really a name tag attached to a number called an inode. The actual data, along with permission, owner, and size information, is held by the inode. The difference between the two kinds of links comes from this.

$ echo hello > a.txt
$ ln a.txt hard.txt
$ ln -s a.txt soft.txt
$ ls -li
131075 -rw-r--r--. 2 user user 6 Sep  3 09:00 a.txt
131075 -rw-r--r--. 2 user user 6 Sep  3 09:00 hard.txt
131090 lrwxrwxrwx. 1 user user 5 Sep  3 09:01 soft.txt -> a.txt
$ rm a.txt
$ cat hard.txt
hello
$ cat soft.txt
cat: soft.txt: No such file or directory

• a.txt and hard.txt have the same inode number and a link count of 2. They are not an original and a copy but two equal names for the same file.
• rm removes one name tag. The data is erased only when the link count reaches 0, so even after deleting a.txt, you can still read the contents through hard.txt.
• soft.txt has its own inode, and its content is only the path text a.txt. When the original disappears, it becomes a broken link with nothing to point to.

Tomorrow we will learn the commands for looking inside files and finding what you want.

📖 Glossary

• Working directory : The directory the shell is currently in. Check it with pwd.
• Hidden file : A file whose name starts with .. Visible only with ls -a.
• Recursive (-r) : An option that walks through subdirectories as well. Needed to handle directories with cp and rm.
• Hard link : Another name that points to the same inode.
• Symbolic link : A shortcut file that holds the path of another file. Created with ln -s.
• touch : A command that creates an empty file or updates the time info of an existing file.
• mkdir -p : An option that also creates intermediate paths if they are missing.