Linux File Viewing, Permissions, and Environment Variables¶

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This section covers file viewing commands, Linux permissions, ownership, and environment variables — all heavily used by DevOps engineers for log analysis, access control, and runtime debugging.

Linux File Permissions Explained¶

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drwxrwxrwx

-  → file
d  → directory
l  → link

r → read  (4)
w → write (2)
x → execute (1)

Owner | Group | Others

Changing File Permissions Using `chmod`¶

Give Read & Write Permission to User Only¶

[opc@new-k8s ~]$ chmod 600 random.txt
[opc@new-k8s ~]$ ll
-rw-------. 1 opc opc 0 Apr 15 04:19 random.txt

Give Read & Write Permission to User and Group¶

[opc@new-k8s ~]$ chmod 660 random.txt
[opc@new-k8s ~]$ ll
-rw-rw----. 1 opc opc 0 Apr 15 04:19 random.txt

Changing File Owner and Group (`chown`)¶

[opc@new-k8s ~]$ sudo chown opc:vignesh random.txt
[opc@new-k8s ~]$ ll
-rw-rw----. 1 opc vignesh 0 Apr 15 04:19 random.txt

Changing Group of a Directory and All Files Recursively¶

[opc@new-k8s ~]$ sudo chown -R opc:vignesh test/

[opc@new-k8s ~]$ ll
drwxrwxr-x. 4 opc vignesh 100 Apr 13 12:46 test

Advanced Permissions: `umask`¶

The umask (user file-creation mode mask) automatically sets the default permissions for any newly created file or directory. - Default file permission is 666 - Default directory permission is 777

If the system umask is 0022, a new file will be initialized with 644 (666 - 022 = 644).

[opc@new-k8s ~]$ umask
0022

Special Permissions: SUID, SGID, and Sticky Bit¶

For advanced security architectures, DevOps engineers must be familiar with special permissions beyond the standard Read/Write/Execute bits.

SUID (Set User ID)¶

When the SUID bit is set on an executable file, any user running that file executes it with the privileges of the file's owner (e.g., executing a binary as root without actually being root). - Denoted by an s in the owner execute space (e.g., -rwsr-xr-x). - Applied via chmod u+s /path/to/file or chmod 4755.

SGID (Set Group ID)¶

When the SGID bit is set on a directory, any files created inside it forcefully inherit the group ownership of the directory itself, rather than the primary group of the user who created it. This is heavily used for shared collaboration folders. - Denoted by an s in the group execute space (e.g., drwxrwsr-x). - Applied via chmod g+s /path/to/folder or chmod 2775.

Sticky Bit¶

When the Sticky Bit is applied to a universally shared directory (like /tmp), it prevents users from deleting or renaming files owned by other users within that directory, even if the directory gives write w access to everyone. - Denoted by a t at the end of the permission block (e.g., drwxrwxrwt). - Applied via chmod +t /path/to/folder or chmod 1777.

Practice Tasks¶

Use head and tail to inspect a file
Change permissions of a file to 600 and 660
Change ownership of a file using chown
Print all environment variables
Print only the PATH variable

🧠 Quick Quiz — File Permissions & Environment Variables¶

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Which permission value represents read and write access for the file owner only?

📝 Want More Practice?¶

To strengthen your understanding and prepare for interviews, try the full 20-question practice quiz based on this chapter:

👉 Start File Permissions & Environment Variables Quiz (20 Questions)

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