Linux CLI 3 🐧 ls, file and less commands - KandZ

Three essential commands for exploring and viewing files: ls (list directory contents), file (identify file types), and less (view file contents page by page).

The `ls` Command

List the contents of a directory with detailed information about files.

ls /var
ls /var ~
ls ~ -l
ls ~ -la
ls ~ -lt
ls --help

Command	Description
`ls /var`	List contents of `/var`
`ls /var ~`	List both `/var` and your home directory
`ls ~ -l`	List home directory with long format
`ls ~ -la`	Long format + show hidden files
`ls ~ -lt`	Long format + sort by modification time
`ls --help`	Show help for the `ls` command

Commands, Options, and Arguments

Every command has three parts:

ls   -la   /var
│     │      │
│     │      └── Argument (what to operate on)
│     └──────── Option(s) — modify behavior
└────────────── Command

Part	Example	Notes
Command	`ls`	The program to run
Options	`-l`, `-lt`, `-la`	Modify behavior. Short options use `-`
Long options	`--help`	Use `--` before a word
Arguments	`/var`, `~`	What the command acts on

Rule: -- always comes before a word option (e.g., --help, --all, --human-readable).

Combining short options:

ls -la         # Same as: ls -l -a
ls -ltr        # Long + time + reverse

Understanding `ls -l` Output

-rw-r--r--  1 kronos users  4096 Jan 15 10:30 file.txt
│           │ │      │      │    │           │
│           │ │      │      │    │           └── Filename
│           │ │      │      │    └────────────── Last modification date
│           │ │      │      └─────────────────── File size (bytes)
│           │ │      └────────────────────────── Group owner
│           │ └───────────────────────────────── File owner
│           └─────────────────────────────────── Hard links count
└─────────────────────────────────────────────── Permissions (rights)

Column	Meaning	Example
1st	Permissions (rights to the file)	`-rw-r--r--`
2nd	Hard links count	`1`
3rd	File owner	`kronos`
4th	Group owner	`users`
5th	File size in bytes	`4096`
6th	Last modification date	`Jan 15 10:30`
7th	Filename	`file.txt`

First character — file type:

Character	Meaning
`-`	Regular file
`d`	Directory
`l`	Symbolic link
`c`	Character device
`b`	Block device
`s`	Socket
`p`	Pipe

Example output:

drwxr-xr-x  5 kronos users  4096 Jan 15 10:30 Documents
-rw-r--r--  1 kronos users  1024 Jan 15 10:30 notes.txt
lrwxrwxrwx  1 kronos users    10 Jan 15 10:30 link -> /etc

Documents → directory (d)
notes.txt → regular file (-)
link → symbolic link (l)

The `file` Command

Determine the type of a file (based on content, not extension).

file *
file Downloads
file file
file file -b
file [A-D]*
file file -i

Command	Description
`file *`	Show file types for all files
`file Downloads`	Show file type for `Downloads`
`file filename`	Show file type for `filename`
`file filename -b`	Brief mode — only the type, no filename
`file [A-D]*`	Files in range A to D
`file filename -i`	Show MIME type

Example:

$ file *
notes.txt:    ASCII text
photo.jpg:    JPEG image data, JFIF standard
script.sh:    Bourne-Again shell script
archive.tar:  POSIX tar archive
Documents:    directory

Brief mode:

$ file notes.txt -b
ASCII text

MIME type mode:

$ file notes.txt -i
text/plain; charset=us-ascii

$ file photo.jpg -i
image/jpeg

Wildcards for ranges:

$ file [A-D]*
apple.txt:   ASCII text
banana.txt:  ASCII text
cherry.txt:  ASCII text
date.txt:    ASCII text
# Only files starting with A, B, C, or D

Common File Types Detected

Output	Meaning
`ASCII text`	Plain text file
`UTF-8 Unicode text`	Unicode text file
`Bourne-Again shell script`	Bash script
`Python script`	Python script
`JPEG image data`	JPEG image
`PNG image data`	PNG image
`PDF document`	PDF file
`Zip archive data`	ZIP archive
`directory`	Folder
`empty`	Empty file

The `less` Command

Display file contents one page at a time — perfect for reading long files.

less /etc/passwd

Navigating in less:

Key	Action
`↑` / `↓`	Scroll one line up / down
`PgUp` / `PgDn`	Scroll one page up / down
`Home`	Go to the beginning of the file
`End`	Go to the end of the file
`25g` + `Enter`	Go to the 25th line
`q`	Quit less
`/pattern`	Search forward for pattern
`n`	Next search match
`N`	Previous search match

Example:

$ less /etc/passwd

Display:

root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
bin:x:2:2:bin:/bin:/usr/sbin/nologin
sys:x:3:3:sys:/dev:/usr/sbin/nologin
sync:x:4:65534:sync:/bin:/bin/sync
games:x:5:60:games:/usr/games:/usr/sbin/nologin
man:x:6:12:man:/var/cache/man:/usr/sbin/nologin
lp:x:7:7:lp:/var/spool/lpd:/usr/sbin/nologin
mail:x:8:8:mail:/var/mail:/usr/sbin/nologin
news:x:9:9:news:/var/spool/news:/usr/sbin/nologin
:

The : at the bottom means there’s more content below. Press ↓ or PgDn to continue.

Jumping to a specific line:

25g + Enter     →  Go to line 25

Quitting:

q               →  Exit less

Complete Example Session

Let’s walk through a real exploration:

# 1. List the /var directory
kronos@olympos:~$ ls /var
backups  cache  crash  lib  local  lock  log  mail  opt  run  spool  tmp

# 2. List both /var and home with details
kronos@olympos:~$ ls /var ~ -la
/var:
total 48
drwxr-xr-x 12 root root  4096 Jan 15 10:30 .
drwxr-xr-x 20 root root  4096 Jan 15 10:30 ..
drwxr-xr-x  3 root root  4096 Jan 15 10:30 backups
drwxr-xr-x 20 root root  4096 Jan 15 10:30 cache
...

/home/kronos:
total 32
drwxr-xr-x 5 kronos users 4096 Jan 15 10:30 .
drwxr-xr-x 3 root   root  4096 Jan 15 10:30 ..
-rw-r--r-- 1 kronos users  220 Jan 15 10:30 .bash_logout
-rw-r--r-- 1 kronos users 3771 Jan 15 10:30 .bashrc
drwxr-xr-x 2 kronos users 4096 Jan 15 10:30 Documents
...

# 3. Sort by modification time (newest first)
kronos@olympos:~$ ls ~ -lt
total 32
-rw-r--r-- 1 kronos users  1024 Jan 15 12:45 notes.txt
-rw-r--r-- 1 kronos users  4096 Jan 15 10:30 .bashrc
drwxr-xr-x 2 kronos users  4096 Jan 15 10:30 Documents

# 4. Check file types
kronos@olympos:~$ file *
.bashrc:     ASCII text
.bash_logout: ASCII text
Documents:   directory
notes.txt:   ASCII text

# 5. Check a specific file with MIME type
kronos@olympos:~$ file notes.txt -i
text/plain; charset=us-ascii

# 6. Check files in range A-D
kronos@olympos:~$ file [A-D]*
Documents:   directory
Desktop:     directory
Downloads:   directory

# 7. View a file with less
kronos@olympos:~$ less /etc/passwd
# Press ↓, ↑, PgUp, PgDn to navigate
# Press 25g + Enter to jump to line 25
# Press q to quit

# 8. Get help for ls
kronos@olympos:~$ ls --help
Usage: ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).
...

Quick Reference

`ls` Options

Option	Description
`-l`	Long format (detailed)
`-a`	All files (including hidden)
`-la`	Long + all
`-lt`	Long + sort by time
`-ltr`	Long + sort by time (reversed)
`-lh`	Long + human-readable sizes
`--help`	Show help

`ls -l` Columns

Column	Meaning
1	Permissions + file type
2	Hard links
3	Owner
4	Group
5	Size (bytes)
6	Modification date
7	Filename

File Type Characters

Char	Meaning
`-`	Regular file
`d`	Directory
`l`	Symbolic link
`c`	Character device
`b`	Block device
`s`	Socket
`p`	Pipe

`file` Options

Option	Description
`-b`	Brief mode (no filename)
`-i`	Show MIME type
`*`	All files
`[A-D]*`	Range wildcard

`less` Navigation

Key	Action
`↑` / `↓`	Line up / down
`PgUp` / `PgDn`	Page up / down
`Home` / `End`	Beginning / end
`25g` + Enter	Go to line 25
`/pattern`	Search
`n` / `N`	Next / previous match
`q`	Quit

Best Practices

✅ Do This:

# Use tab completion for paths
ls /v[Tab]              # Completes to /var

# Combine options for efficiency
ls -lah                 # Long, all, human-readable

# Use file to check before opening
file unknown.dat
# Then use the appropriate program

# Use less instead of cat for long files
less /var/log/syslog    # Navigate page by page

❌ Don’t Do This:

# Don't use cat for large files
cat /var/log/syslog     # ❌ Floods your screen
less /var/log/syslog    # ✅ Page-by-page

# Don't guess file types by extension
# Extensions are just conventions on Linux!
file script.txt         # Could be anything

# Don't forget to quit less
less file.txt
# Press q to exit — Ctrl+C also works

Common Pitfalls

Pitfall	Problem	Solution
Using `cat` on big files	Floods terminal	Use `less`
Missing hidden files	Can’t see `.bashrc`	Use `ls -a`
Guessing file type	Wrong program opens it	Use `file`
Forgeting `q` in less	Stuck in pager	Press `q`
Wildcards matching too much	Unexpected results	Quote or narrow pattern

Summary

Command	Purpose	Example
`ls`	List directory contents	`ls -la ~`
`file`	Identify file type	`file *`
`less`	View file page by page	`less /etc/passwd`

Key takeaways:

ls -l gives you detailed file info in 7 columns
First character of permissions = file type (- file, d directory)
file identifies actual content, not extensions
less is essential for reading long files without flooding the terminal
Options modify command behavior — -l, -a, -t, --help

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The ls Command

Commands, Options, and Arguments

Understanding ls -l Output

The file Command