Linux CLI 18 🐧 id, chmod and umask commands - KandZ

These three commands handle user identity, file permissions, and default permission rules. Together, they form the foundation of Linux access control.

Overview

Command	Purpose	Best For
`id`	Show user identity	Finding your UID, GID, and groups
`chmod`	Change permissions	Setting who can read/write/execute
`umask`	Set default permissions	Controlling defaults for new files

The `id` Command

Displays information about a user — their identity and group memberships.

id
id kronos
id -u kronos
id -g kronos
id -n kronos
id -G kronos

Command	Description
`id`	Info for the current user
`id kronos`	Info for a specific user
`id -u kronos`	Show the user ID (UID)
`id -g kronos`	Show the primary group ID (GID)
`id -n kronos`	Show the name instead of numeric ID
`id -G kronos`	Show all groups the user belongs to

Examples

Current user info:

$ id
uid=1000(kronos) gid=1000(users) groups=1000(users),4(adm),27(sudo),1001(developers)

Specific user:

$ id kronos
uid=1000(kronos) gid=1000(users) groups=1000(users),4(adm),27(sudo),1001(developers)

User ID only:

$ id -u kronos
1000

Group ID only:

$ id -g kronos
1000

Group name instead of number:

$ id -n -g kronos
users

All groups (numeric):

$ id -G kronos
1000 4 27 1001

All groups (names):

$ id -Gn kronos
users adm sudo developers

Understanding `id` Output

uid=1000(kronos) gid=1000(users) groups=1000(users),4(adm),27(sudo),1001(developers)
│              │ │             │ │          │      │       │
│              │ │             │ │          │      │       └── Additional group
│              │ │             │ │          │      └────────── Additional group
│              │ │             │ │          └───────────────── Additional group
│              │ │             │ └──────────────────────────── Primary group name
│              │ │             └────────────────────────────── Primary group ID
│              │ └──────────────────────────────────────────── Username
│              └────────────────────────────────────────────── User ID
└───────────────────────────────────────────────────────────── Identifier type

Key concepts:

Term	Meaning
UID	User ID — a number that identifies a user
GID	Group ID — a number that identifies a group
Primary group	The main group a user belongs to
Supplementary groups	Additional groups a user belongs to

Root User

$ sudo id
uid=0(root) gid=0(root) groups=0(root)
# UID 0 = root (superuser)

Common UIDs:

UID	User	Notes
0	root	Superuser
1–999	System users	Services, daemons
1000+	Regular users	Human users

The `chmod` Command

CHange MODe — changes permissions of a file or directory.

chmod u+rwx,g-wx,o=rx 1.txt
chmod 640 1.txt

Who can use it?

The owner of the file
The superuser (root)

Two Ways to Change Permissions

Method	Example	Style
Symbolic	`chmod u+rwx,g-wx,o=rx file`	More readable
Octal	`chmod 640 file`	More concise

Method 1: Symbolic Representation

Symbol	Meaning
`u`	User (owner)
`g`	Group
`o`	Others
`a`	All (u+g+o)
`+`	Add permission
`-`	Remove permission
`=`	Set exact permissions
`r`	Readable
`w`	Writable
`x`	Executable
`s`	Set user ID
`S`	Set group ID

Symbolic Examples

# Add read, write, execute to the owner
chmod u+rwx file.txt

# Remove write and execute from the group
chmod g-wx file.txt

# Set exact permissions for others (read + execute)
chmod o=rx file.txt

# Combine multiple changes
chmod u+rwx,g-wx,o=rx file.txt

# Add execute to everyone
chmod a+x script.sh
# Same as: chmod u+x,g+x,o+x script.sh

# Remove write from group and others
chmod go-w file.txt

# Set specific permissions for owner only
chmod u=rw file.txt

Step-by-Step: `chmod u+rwx,g-wx,o=rx 1.txt`

Starting state (assume 1.txt has rw-rw-rw-):

Change	Result
Start	`rw-rw-rw-`
`u+rwx` — owner gets read+write+execute	`rwxrw-rw-`
`g-wx` — group loses write+execute	`rwxr--rw-`
`o=rx` — others set to read+execute	`rwxr-xr-x`

Final: rwxr-xr-x (755)

Method 2: Octal Representation

Three digits, one per group:

640
│││
││└── Others (o)
│└─── Group (g)
└──── Owner (u)

Octal	Binary	Symbolic	Meaning
`0`	000	`---`	No permissions
`1`	001	`--x`	Execute only
`2`	010	`-w-`	Write only
`3`	011	`-wx`	Write + execute
`4`	100	`r--`	Read only
`5`	101	`r-x`	Read + execute
`6`	110	`rw-`	Read + write
`7`	111	`rwx`	Read + write + execute

Octal Examples

# Owner: read+write, group: read, others: none
chmod 640 file.txt
# Result: -rw-r-----

# Owner: read+write+execute, group: read+execute, others: read+execute
chmod 755 script.sh
# Result: -rwxr-xr-x

# Owner: read only, group: read, others: read
chmod 444 file.txt
# Result: -r--r--r--

# Owner: read+write+execute, everyone else: nothing
chmod 700 private/
# Result: drwx------

# Owner: read+write, group: read+write, others: read
chmod 664 shared.txt
# Result: -rw-rw-r--

Step-by-Step: `chmod 640 1.txt`

6    4    0
│    │    │
│    │    └── Others: 0 = --- (no access)
│    └─────── Group:  4 = r-- (read only)
└──────────── Owner:  6 = rw- (read + write)

Result: rw-r----- (640)

`chmod` Common Patterns

Octal	Symbolic	Use Case
`644`	`rw-r--r--`	Regular files
`755`	`rwxr-xr-x`	Directories, executables
`600`	`rw-------`	Private files (SSH keys)
`640`	`rw-r-----`	Shared within group
`664`	`rw-rw-r--`	Group-writable files
`700`	`rwx------`	Private directories
`777`	`rwxrwxrwx`	❌ Avoid (insecure)

Recursive `chmod`

# Apply to all files and directories in a tree
chmod -R 755 project/

# Apply only to directories
find project/ -type d -exec chmod 755 {} \;

# Apply only to files
find project/ -type f -exec chmod 644 {} \;

The `umask` Command

User MASK — determines the default permissions applied to new files and directories.

umask
umask 0000
umask
umask 0022

Command	Description
`umask`	Show current mask
`umask 0000`	Set no restrictions
`umask 0022`	Set common default

How `umask` Works

The mask is subtracted from default permissions:

Type	Base Permission	Subtract umask	Result
Files	666 (`rw-rw-rw-`)	022	644 (`rw-r--r--`)
Directories	777 (`rwxrwxrwx`)	022	755 (`rwxr-xr-x`)

Why base values?

Files start at 666 — no execute by default (files aren’t programs)
Directories start at 777 — need execute to enter

Examples

Show current mask:

$ umask
0022

Set umask to 0000:

$ umask 0000
$ umask
0000

# Now new files get: 666 - 000 = 666 (rw-rw-rw-)
# New directories:   777 - 000 = 777 (rwxrwxrwx)

Set umask back to 0022:

$ umask 0022
$ umask
0022

# New files get: 666 - 022 = 644 (rw-r--r--)
# New directories: 777 - 022 = 755 (rwxr-xr-x)

Common `umask` Values

Umask	Files	Directories	Use Case
`022`	`644`	`755`	Default — owner writes, others read
`002`	`664`	`775`	Group collaboration
`077`	`600`	`700`	Private — no group/other access
`027`	`640`	`750`	Group read, no other access
`000`	`666`	`777`	❌ Insecure — avoid

Making `umask` Permanent

Add to ~/.bashrc:

echo "umask 0022" >> ~/.bashrc

Or edit manually:

nano ~/.bashrc
# Add at the end:
umask 0022
# Save: Ctrl+X, then Y, then Enter
source ~/.bashrc

Verify:

$ umask
0022

Testing `umask`

# Set mask
$ umask 077

# Create a file
$ touch test.txt
$ ls -l test.txt
-rw-------  1 kronos users  0 Jan 15 10:30 test.txt
# Result: 600

# Create a directory
$ mkdir testdir
$ ls -ld testdir
drwx------  2 kronos users  4096 Jan 15 10:30 testdir
# Result: 700

# Reset
$ umask 022

Complete Example Session

# ============================================
# PART 1: ID COMMAND
# ============================================

# Current user
$ id
uid=1000(kronos) gid=1000(users) groups=1000(users),4(adm),27(sudo)

# Specific user
$ id kronos
uid=1000(kronos) gid=1000(users) groups=1000(users),4(adm),27(sudo)

# UID only
$ id -u kronos
1000

# GID only
$ id -g kronos
1000

# Group name
$ id -gn kronos
users

# All groups (names)
$ id -Gn kronos
users adm sudo

# All groups (IDs)
$ id -G kronos
1000 4 27

# Root user
$ sudo id
uid=0(root) gid=0(root) groups=0(root)

# ============================================
# PART 2: CHMOD — SYMBOLIC
# ============================================

# Starting state
$ ls -l 1.txt
-rw-rw-rw-  1 kronos users  1024 Jan 15 10:30 1.txt

# Add execute to owner
$ chmod u+x 1.txt
$ ls -l 1.txt
-rwxrw-rw-  1 kronos users  1024 Jan 15 10:30 1.txt

# Remove write from group
$ chmod g-w 1.txt
$ ls -l 1.txt
-rwxr--rw-  1 kronos users  1024 Jan 15 10:30 1.txt

# Set others to read+execute
$ chmod o=rx 1.txt
$ ls -l 1.txt
-rwxr--r-x  1 kronos users  1024 Jan 15 10:30 1.txt

# Combined command (from rw-rw-rw-)
$ chmod u+rwx,g-wx,o=rx 1.txt
$ ls -l 1.txt
-rwxr-xr-x  1 kronos users  1024 Jan 15 10:30 1.txt

# ============================================
# PART 3: CHMOD — OCTAL
# ============================================

# Set permissions to 640
$ chmod 640 1.txt
$ ls -l 1.txt
-rw-r-----  1 kronos users  1024 Jan 15 10:30 1.txt

# Set to 755 (common)
$ chmod 755 1.txt
$ ls -l 1.txt
-rwxr-xr-x  1 kronos users  1024 Jan 15 10:30 1.txt

# Set to 644 (default for files)
$ chmod 644 1.txt
$ ls -l 1.txt
-rw-r--r--  1 kronos users  1024 Jan 15 10:30 1.txt

# Set to 600 (private)
$ chmod 600 1.txt
$ ls -l 1.txt
-rw-------  1 kronos users  1024 Jan 15 10:30 1.txt

# Recursive
$ chmod -R 755 project/

# ============================================
# PART 4: UMASK
# ============================================

# Show current mask
$ umask
0022

# Set to 0000
$ umask 0000
$ umask
0000

# Create a file — see result
$ touch test.txt
$ ls -l test.txt
-rw-rw-rw-  1 kronos users  0 Jan 15 10:30 test.txt
# 666 - 000 = 666

# Create a directory — see result
$ mkdir testdir
$ ls -ld testdir
drwxrwxrwx  2 kronos users  4096 Jan 15 10:30 testdir
# 777 - 000 = 777

# Set to 0077 (private)
$ umask 0077

# Create a file
$ touch private.txt
$ ls -l private.txt
-rw-------  1 kronos users  0 Jan 15 10:30 private.txt
# 666 - 077 = 600

# Create a directory
$ mkdir privatedir
$ ls -ld privatedir
drwx------  2 kronos users  4096 Jan 15 10:30 privatedir
# 777 - 077 = 700

# Reset to default
$ umask 0022
$ umask
0022

# Make permanent
$ echo "umask 0022" >> ~/.bashrc
$ source ~/.bashrc

# ============================================
# PART 5: PRACTICAL SCENARIOS
# ============================================

# Scenario 1: Setup a web directory
$ mkdir website
$ chmod 755 website
$ cd website
$ touch index.html
$ chmod 644 index.html

# Scenario 2: Protect a private key
$ chmod 600 ~/.ssh/id_rsa
$ ls -l ~/.ssh/id_rsa
-rw-------  1 kronos users  1675 Jan 15 10:30 id_rsa
# SSH requires this permission!

# Scenario 3: Share a file with your group
$ chmod 640 shared.txt
$ ls -l shared.txt
-rw-r-----  1 kronos users  1024 Jan 15 10:30 shared.txt
# Owner can read+write, group can read

# Scenario 4: Group collaboration
$ umask 002
$ touch team.txt
$ ls -l team.txt
-rw-rw-r--  1 kronos users  0 Jan 15 10:30 team.txt
# Both owner and group can write

# Scenario 5: Set restrictive default for new files
$ umask 0077
$ touch secret.txt
$ ls -l secret.txt
-rw-------  1 kronos users  0 Jan 15 10:30 secret.txt
# Only owner can access

Quick Reference

`id` Options

Option	Description
`-u`	User ID
`-g`	Primary group ID
`-G`	All group IDs
`-n`	Show names instead of numbers
`-r`	Show real ID (vs effective)

`chmod` Symbolic

Symbol	Meaning
`u`	Owner (user)
`g`	Group
`o`	Others
`a`	All
`+`	Add
`-`	Remove
`=`	Set exactly
`r`	Read
`w`	Write
`x`	Execute

`chmod` Octal

Octal	Symbolic	Meaning
`0`	`---`	None
`1`	`--x`	Execute
`2`	`-w-`	Write
`3`	`-wx`	Write + execute
`4`	`r--`	Read
`5`	`r-x`	Read + execute
`6`	`rw-`	Read + write
`7`	`rwx`	Full

`umask` Common Values

Umask	Files	Dirs	Use Case
`022`	644	755	Default
`002`	664	775	Group collaboration
`077`	600	700	Private
`027`	640	750	Semi-private

Best Practices

✅ Do This:

# Files — 644
chmod 644 file.txt

# Directories — 755
chmod 755 mydir

# Private files — 600
chmod 600 ~/.ssh/id_rsa
chmod 600 .env

# Executables — 755
chmod 755 script.sh

# Private directory — 700
chmod 700 ~/.ssh

# Group share — 640 or 664
chmod 640 shared.txt

# Set umask in ~/.bashrc for consistency
echo "umask 022" >> ~/.bashrc

# Verify with id
id
id -Gn

❌ Don’t Do This:

# Don't use 777
chmod 777 file.txt          # ❌ Security risk
chmod -R 777 /var/www       # ❌ Very dangerous!

# Don't remove execute from directories
chmod -x mydir              # ❌ Can't cd in!

# Don't forget SSH key permissions
chmod 644 ~/.ssh/id_rsa     # ❌ SSH refuses to use it
chmod 600 ~/.ssh/id_rsa     # ✅ Required

# Don't set umask without understanding
umask 0000                  # ❌ Everyone can read/write new files

# Don't forget to source after changing .bashrc
nano ~/.bashrc
# ... add umask ...
source ~/.bashrc            # ✅ Required to apply

Common Pitfalls

Pitfall	Problem	Solution
SSH key too permissive	SSH refuses	`chmod 600 ~/.ssh/id_rsa`
`777` on web files	Security hole	`644` files, `755` dirs
Removed `x` from directory	Can’t `cd` in	Add `x` back
`umask` not applied	Forgot to source	`source ~/.bashrc`
Wrong `id` flag	Unexpected output	Check `id --help`
Chmod without permission	Owner or root only	`sudo chmod ...`

Real-World Examples

1. Setting Up an SSH Key

# Generate key
$ ssh-keygen -t rsa

# Fix permissions
$ chmod 700 ~/.ssh
$ chmod 600 ~/.ssh/id_rsa
$ chmod 644 ~/.ssh/id_rsa.pub
$ chmod 644 ~/.ssh/authorized_keys

2. Web Server Setup

# Directory
$ mkdir /var/www/mysite
$ chmod 755 /var/www/mysite

# Files
$ touch /var/www/mysite/index.html
$ chmod 644 /var/www/mysite/index.html

# Uploads directory (needs write)
$ mkdir /var/www/mysite/uploads
$ chmod 775 /var/www/mysite/uploads

3. Shared Team Directory

# Set group ownership
$ sudo chgrp developers /shared/project
$ chmod 775 /shared/project

# Set setgid so new files inherit group
$ chmod g+s /shared/project

# Set umask for team
$ echo "umask 002" >> ~/.bashrc

4. Private Workspace

# Restrict everything
$ umask 077
$ mkdir private
$ touch secret.txt
$ ls -la
drwx------  2 kronos users  4096  private
-rw-------  1 kronos users    0  secret.txt

5. Check Current Identity

$ id
uid=1000(kronos) gid=1000(users) groups=...

$ id -Gn
users adm sudo developers
# Shows all groups — useful for permission debugging

Visual: How `umask` Works

Files:  666 - umask = result
        666 - 022   = 644

Directories:  777 - umask = result
              777 - 022   = 755

umask 022:
    666 (rw-rw-rw-)      777 (rwxrwxrwx)
  - 022 (----w--w-)    - 022 (----w--w-)
  ────────────────     ────────────────
  = 644 (rw-r--r--)    = 755 (rwxr-xr-x)

umask 077:
    666 (rw-rw-rw-)      777 (rwxrwxrwx)
  - 077 (---rwxrwx)    - 077 (---rwxrwx)
  ────────────────     ────────────────
  = 600 (rw-------)    = 700 (rwx------)

Summary

Command	Purpose	Example
`id`	Show user identity	`id -Gn`
`chmod`	Change permissions	`chmod 644 file.txt`
`umask`	Set default permissions	`umask 022`

Key takeaways:

id shows UID, GID, and all groups — use -u, -g, -G, -n for specific info
chmod changes permissions in two ways:
- Symbolic — chmod u+rwx,g-wx,o=rx file (readable)
- Octal — chmod 640 file (concise)
Octal values: 4=read, 2=write, 1=execute (add them up per group)
umask sets default permissions for new files/directories
Files start at 666, directories at 777 — umask is subtracted
Common umask: 022 → files 644, dirs 755
Make umask permanent in ~/.bashrc

Remember: These three commands are the access control trio:

id — “Who am I?”
chmod — “What can I do?”
umask — “What will new files look like?”

Use chmod 644 for files, chmod 755 for directories, and chmod 600 for secrets. Set your umask in ~/.bashrc so every new file has safe defaults. And never, ever use chmod 777 — it’s the fast track to a security breach!

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Overview

The id Command

Examples

Understanding id Output

Root User

The chmod Command

Two Ways to Change Permissions

Method 1: Symbolic Representation

Symbolic Examples

Step-by-Step: chmod u+rwx,g-wx,o=rx 1.txt

Method 2: Octal Representation

Octal Examples

Step-by-Step: chmod 640 1.txt

chmod Common Patterns

Recursive chmod

The umask Command

How umask Works

Examples

Common umask Values

Making umask Permanent

Testing umask

Complete Example Session

Quick Reference

id Options

chmod Symbolic

chmod Octal

umask Common Values

Best Practices

Common Pitfalls

Real-World Examples

1. Setting Up an SSH Key

2. Web Server Setup

3. Shared Team Directory

4. Private Workspace

5. Check Current Identity

Visual: How umask Works

Summary

Ads Blocker Detected!!!

The `id` Command

Understanding `id` Output

The `chmod` Command

Step-by-Step: `chmod u+rwx,g-wx,o=rx 1.txt`

Step-by-Step: `chmod 640 1.txt`

`chmod` Common Patterns

Recursive `chmod`

The `umask` Command

How `umask` Works

Common `umask` Values

Making `umask` Permanent

Testing `umask`

`id` Options

`chmod` Symbolic

`chmod` Octal

`umask` Common Values

Visual: How `umask` Works