Linux Fundamentals - Lesson 01 (ls)

Created: 2026-03-21

Lesson 01 - list files with `ls`

Goal

Learn ls deeply enough to:

list files and directories confidently,
reveal hidden files,
sort by time and size,
format output for human reading and scripts.

Quick Commands

ls
ls -l
ls -a
ls -al
ls -lt
ls -lS
ls -lh
ls -1
ls --format=commas
ls --help

Core Concepts

1) What is `ls`

ls is a core Linux/Unix command to list directory contents.

2) Arguments style

-x means short argument (single-letter style)
--word means long argument (full-word style)

Important clarification:

It is not "single vs multiple arguments".
It is mostly "short option name vs long option name".
Example: ls -l -a is same as ls -la (multiple short options can be combined).
Example: ls --color=auto uses one long option.

3) Long listing (`ls -l`) columns

permissions
hard link count
owner
group
size
last modification time
filename

4) Hidden files

dotfiles start with .
. means current directory
.. means parent directory

5) File times

atime: last access
mtime: last content modification
ctime: last metadata change

Guided Practice

Paste your real terminal output under each section.

Practice A - first look

ls
ls --color=no
ls --color=yes
ls --color=auto .

Output:

root@ubuntu:~$ ls
File-01.txt  file-01  file-01.txt  file-02  filesystem  notmyfile  notmyfile2  testDir  testdir

root@ubuntu:~$ ls --color=no
File-01.txt  file-01  file-01.txt  file-02  filesystem  notmyfile  notmyfile2  testDir  testdir

root@ubuntu:~$ ls --color=yes
File-01.txt  file-01  file-01.txt  file-02  filesystem  notmyfile  notmyfile2  testDir  testdir

root@ubuntu:~$ ls --color=auto .
File-01.txt  file-01  file-01.txt  file-02  filesystem  notmyfile  notmyfile2  testDir  testdir

Mentor note:

Great run. You proved color behavior correctly.
--color=no disables color.
--color=yes forces color.
--color=auto shows color when output is a terminal.

Image (your screenshot):

![practice-a](./images/lesson-01-practice-a.png)

Practice B - more details

clear
ls -l .
ls -n

What to observe:

names vs UID/GID output
each -l column meaning

Output:

root@ubuntu:~$ ls -l
total 168
-rw-r--r-- 1 root      root      149370 Mar 21 10:58 File-01.txt
-rw-r--r-- 1 root      root           0 Mar 21 10:58 file-01
-rw-r--r-- 1 root      root          18 Mar 21 10:58 file-01.txt
-rw-r--r-- 1 root      root           0 Mar 21 10:58 file-02
lrwxrwxrwx 1 root      root           1 Mar  4 09:06 filesystem -> /
-rw-r--r-- 1 testuser  testuser      18 Mar 21 10:58 notmyfile
-rw-r--r-- 1 otheruser otheruser     19 Mar 21 10:58 notmyfile2
drwxr-xr-x 2 root      root        4096 Mar 21 10:58 testDir
drwxr-xr-x 3 root      root        4096 Mar 21 10:58 testdir

root@ubuntu:~$ ls -n
total 168
-rw-r--r-- 1    0    0 149370 Mar 21 10:58 File-01.txt
-rw-r--r-- 1    0    0      0 Mar 21 10:58 file-01
-rw-r--r-- 1    0    0     18 Mar 21 10:58 file-01.txt
-rw-r--r-- 1    0    0      0 Mar 21 10:58 file-02
lrwxrwxrwx 1    0    0      1 Mar  4 09:06 filesystem -> /
-rw-r--r-- 1 1001 1001     18 Mar 21 10:58 notmyfile
-rw-r--r-- 1 1002 1002     19 Mar 21 10:58 notmyfile2
drwxr-xr-x 2    0    0   4096 Mar 21 10:58 testDir
drwxr-xr-x 3    0    0   4096 Mar 21 10:58 testdir

Mentor note:

Perfect observation.
clear only clears visible terminal screen; it does not delete command history.
-l = long format (human-friendly owner/group names).
-n = numeric owner/group IDs (UID/GID numbers).

Practice C - hidden files

ls
ls -a
ls .
ls ..
ls -A .
ls -al .

What to observe:

why -a reveals dotfiles
difference between -a and -A

Output:

root@ubuntu:~$ ls
File-01.txt  file-01  file-01.txt  file-02  filesystem  notmyfile  notmyfile2  testDir  testdir

root@ubuntu:~$ ls -a
.  .bash_history  .hidden-file  .ssh     .vimrc      File-01.txt  file-01.txt  filesystem  notmyfile2  testdir
.. .bashrc        .profile      .theia   .wget-hsts  file-01      file-02      notmyfile   testDir

root@ubuntu:~$ ls .
File-01.txt  file-01  file-01.txt  file-02  filesystem  notmyfile  notmyfile2  testDir  testdir

root@ubuntu:~$ ls ..
bin  bin.usr-is-merged  boot  dev  etc  home  lib  lib.usr-is-merged  lib64  lost+found  media  mnt  opt  proc  run  root  sbin  sbin.usr-is-merged  snap  srv  swapfile  sys  tmp  usr  var

root@ubuntu:~$ ls -A
.bash_history  .hidden-file  .ssh    .vimrc      File-01.txt  file-01.txt  filesystem  notmyfile2  testdir
.bashrc        .profile      .theia  .wget-hsts  file-01      file-02      notmyfile   testDir

root@ubuntu:~$ ls -al
total 204
drwx------  6 root      root       4096 Mar 21 10:58 .
drwxr-xr-x 22 root      root       4096 Mar  4 09:06 ..
-rw-------  1 root      root         10 Feb 10  2025 .bash_history
-rw-r--r--  1 root      root       3234 Mar  4 09:07 .bashrc
-rw-r--r--  1 root      root          0 Mar 21 10:58 .hidden-file
-rw-r--r--  1 root      root        161 Apr 22  2024 .profile
drwxr-xr-x  2 root      root       4096 Mar  4 09:05 .ssh
drwxr-xr-x  2 root      root       4096 Mar  4 09:07 .theia
-rw-r--r--  1 root      root        109 Mar  4 09:07 .vimrc
-rw-r--r--  1 root      root        165 Mar  4 09:07 .wget-hsts
-rw-r--r--  1 root      root     149370 Mar 21 10:58 File-01.txt
-rw-r--r--  1 root      root          0 Mar 21 10:58 file-01
-rw-r--r--  1 root      root         18 Mar 21 10:58 file-01.txt
-rw-r--r--  1 root      root          0 Mar 21 10:58 file-02
lrwxrwxrwx  1 root      root          1 Mar  4 09:06 filesystem -> /
-rw-r--r--  1 testuser  testuser     18 Mar 21 10:58 notmyfile
-rw-r--r--  1 otheruser otheruser    19 Mar 21 10:58 notmyfile2
drwxr-xr-x  2 root      root       4096 Mar 21 10:58 testDir
drwxr-xr-x  3 root      root       4096 Mar 21 10:58 testdir

Learner interpretation:

ls -a shows all files including hidden files.
. is current directory and .. is parent directory.
ls -A is almost all (excludes . and ..).
ls -al combines all files with long listing details.

Mentor feedback:

Excellent interpretation.
"Parent directory" is the best term (more precise than parent folder).
Your command-combination explanation is correct: -al is combined short options.

Practice D - sort by time

date +%s
date
ls -lt
ls -ltu
ls -ltc
touch theNewestFile
ls -ltu
ls -ltc
echo "hello world!" > file-02
ls -ltu
ls -ltc
chmod 444 file-01
ls -ltu
ls -ltc

What to observe:

when order changes for -u
when order changes for -c

Learner interpretation (checkpoint before running):

ctime is metadata change time (permissions, moving/renaming, ownership, etc.).
atime is last access time.
mtime is last content modification time.

Mentor feedback:

Excellent. That model is correct and ready for the next exercise.

Output:

root@ubuntu:~$ date +%s
1774094082

root@ubuntu:~$ date
Sat Mar 21 11:54:42 UTC 2026

root@ubuntu:~$ ls -ltu
...
drwxr-xr-x 2 root root 4096 Mar 21 11:54 testDir
drwxr-xr-x 3 root root 4096 Mar 21 11:54 testdir
...

root@ubuntu:~$ touch theNewestFile
root@ubuntu:~$ ls -ltu
...
-rw-r--r-- 1 root root 0 Mar 21 11:54 theNewestFile
...

root@ubuntu:~$ echo "hello world!" > file-02
root@ubuntu:~$ ls -ltc
...
-rw-r--r-- 1 root root 13 Mar 21 11:54 file-02
...

root@ubuntu:~$ chmod 444 file-01
root@ubuntu:~$ ls -ltc
...
-r--r--r-- 1 root root 0 Mar 21 11:54 file-01
...

Learner interpretation:

theNewestFile went to the top after touch because it was just created.
ls -ltu is about access time (atime).
ls -ltc moved files after metadata changes, like chmod.

Mentor feedback:

Great analysis.
Important correction: ls -ltc uses ctime for sorting/display, not both mtime and ctime.
Baseline behavior is ls -lt (mtime). Then -u switches to atime, and -c switches to ctime.

Checkpoint Q/A:

Q: If you run echo "x" >> file-01.txt, which one should reflect it most directly: -lt, -ltu, or -ltc?
Learner answer: -lt because it modifies file content.
Mentor validation: Correct. Content append updates mtime, so ls -lt is the primary one to watch.

Practice E - sort by size

ls -s
ls -ls
ls -lS
ls -lh
ls -l --si
ls -lSh

What to observe:

extra blocks column from -s
case-sensitive behavior of -s vs -S

Output:

root@ubuntu:~$ ls -s
total 168
144 File-01.txt  4 file-01.txt  0 filesystem  4 notmyfile2  4 testdir
0   file-01      4 file-02      4 notmyfile   4 testDir     0 theNewestFile

root@ubuntu:~$ ls -ls
total 168
144 -rw-r--r-- 1 root      root      144907 Mar 21 11:53 File-01.txt
  0 -r--r--r-- 1 root      root           0 Mar 21 11:53 file-01
  4 -rw-r--r-- 1 root      root          18 Mar 21 11:53 file-01.txt
  4 -rw-r--r-- 1 root      root          13 Mar 21 11:54 file-02
  0 lrwxrwxrwx 1 root      root           1 Mar  4 09:06 filesystem -> /
  4 -rw-r--r-- 1 testuser  testuser      18 Mar 21 11:53 notmyfile
  4 -rw-r--r-- 1 otheruser otheruser     19 Mar 21 11:53 notmyfile2
  4 drwxr-xr-x 2 root      root        4096 Mar 21 11:53 testDir
  4 drwxr-xr-x 3 root      root        4096 Mar 21 11:53 testdir
  0 -rw-r--r-- 1 root      root           0 Mar 21 11:54 theNewestFile

root@ubuntu:~$ ls -lS
total 168
-rw-r--r-- 1 root      root      144907 Mar 21 11:53 File-01.txt
drwxr-xr-x 2 root      root        4096 Mar 21 11:53 testDir
drwxr-xr-x 3 root      root        4096 Mar 21 11:53 testdir
-rw-r--r-- 1 otheruser otheruser     19 Mar 21 11:53 notmyfile2
-rw-r--r-- 1 root      root          18 Mar 21 11:53 file-01.txt
-rw-r--r-- 1 testuser  testuser      18 Mar 21 11:53 notmyfile
-rw-r--r-- 1 root      root          13 Mar 21 11:54 file-02
lrwxrwxrwx 1 root      root           1 Mar  4 09:06 filesystem -> /
-r--r--r-- 1 root      root           0 Mar 21 11:53 file-01
-rw-r--r-- 1 root      root           0 Mar 21 11:54 theNewestFile

root@ubuntu:~$ ls -lh
total 168K
-rw-r--r-- 1 root      root      142K Mar 21 11:53 File-01.txt
...

root@ubuntu:~$ ls -l --si
total 173k
-rw-r--r-- 1 root      root      145k Mar 21 11:53 File-01.txt
...

root@ubuntu:~$ ls -lSh
total 168K
-rw-r--r-- 1 root      root      142K Mar 21 11:53 File-01.txt
drwxr-xr-x 2 root      root      4.0K Mar 21 11:53 testDir
drwxr-xr-x 3 root      root      4.0K Mar 21 11:53 testdir
...

Learner interpretation:

-s shows size blocks.
-S sorts by size.
-h makes output easier to read.
Preferred readable command: ls -lSh.

Mentor feedback:

Correct and solid.
Precision note: -s shows allocated disk blocks, while -l size column shows byte size (or human-readable with -h).
-lSh is a strong practical default when quickly finding large files.

Practice F - formatting

ls -1
ls --format=commas
ls -m
ls --format=long
ls -lQ
ls -l --time-style=locale
ls -l --time-style=iso
ls -l --time-style=full-iso

Output:

root@ubuntu:~$ ls -1
File-01.txt
file-01
file-01.txt
file-02
filesystem
notmyfile
notmyfile2
testDir
testdir
theNewestFile

root@ubuntu:~$ ls --format=commas
File-01.txt, file-01, file-01.txt, file-02, filesystem, notmyfile, notmyfile2, testDir, testdir, theNewestFile

root@ubuntu:~$ ls -m
File-01.txt, file-01, file-01.txt, file-02, filesystem, notmyfile, notmyfile2, testDir, testdir, theNewestFile

root@ubuntu:~$ ls --format=long
total 168
-rw-r--r-- 1 root      root      144907 Mar 21 11:53 File-01.txt
...

root@ubuntu:~$ ls -lQ
total 168
-rw-r--r-- 1 root      root      144907 Mar 21 11:53 "File-01.txt"
...

root@ubuntu:~$ ls -l --time-style=locale
total 168
-rw-r--r-- 1 root      root      144907 Mar 21 11:53 File-01.txt
...

root@ubuntu:~$ ls -l --time-style=iso
total 168
-rw-r--r-- 1 root      root      144907 03-21 11:53 File-01.txt
...

root@ubuntu:~$ ls -l --time-style=full-iso
total 168
-rw-r--r-- 1 root      root      144907 2026-03-21 11:53:47.235283758 +0000 File-01.txt
...

Learner interpretation:

ls -1 prints one entry per line; with -l this is usually unnecessary.
ls --format=commas and ls -m look the same.
-Q quotes file names.
locale, iso, and full-iso change date/time formatting.

Mentor feedback:

Excellent read.
Correction: -m is not modification-related; it is a formatting alias for comma-separated output.
locale means system locale format, iso is compact ISO-style, and full-iso includes full date, seconds precision, and timezone.

Practice G - extra useful options

ls -al --author
ls -ald
ls -ali
ls -alR
ls -alr
ls -alSr
ls --version
ls --help

Output:

[paste output here]

Quiz + Answers

Q1. Whole command for long list in current directory?

Answer: ls -l .

Q2. Argument for sorting by time?

Answer: -t

Q3. For long color argument, use --, -, or .?

Answer: -- (example: ls --color=auto)

Q4. UID represents?

Answer: User identifier

Q5. How to list parent directory?

Answer: ls ..

Q6. I have ls -lt. What to add to see content modification order?

Answer: nothing; -t with default long list already uses mtime

Q7. Full command for long + human readable sizes?

Answer: ls -lh

Q8. Full command for comma-separated short list (not -m)?

Answer: ls --format=commas

Q9. Short list, one per line, with quotes?

Answer: ls -1Q

Reflection

Easiest part:
Most confusing part:
3 commands I will keep using:
What I will practice tomorrow:

Lesson 01 - list files with ls

Goal

Quick Commands

Core Concepts

1) What is ls

2) Arguments style

3) Long listing (ls -l) columns

4) Hidden files

5) File times

Guided Practice

Practice A - first look

Practice B - more details

Practice C - hidden files

Practice D - sort by time

Practice E - sort by size

Practice F - formatting

Practice G - extra useful options

Quiz + Answers

Reflection

Backlinks

Lesson 01 - list files with `ls`

1) What is `ls`

3) Long listing (`ls -l`) columns