What's the difference between 755 and 644?

755 (rwxr-xr-x) — owner can read/write/execute, group and others can read/execute. Used for executable files and directories. 644 (rw-r--r--) — owner can read/write, group and others can only read. Used for regular files (HTML, CSS, images, configs).

Why does 777 work but everyone says don't use it?

777 grants read/write/execute to everyone. It works in the sense that the file becomes accessible by all processes, but it's a security catastrophe — any compromised user or process can modify the file. Use 644 for files, 755 for executables, 700 for private data, 600 for SSH keys. Reserve 777 only for /tmp-style world-writable directories that intentionally need it.

What's the difference between numeric and symbolic chmod?

Numeric (chmod 755) sets all 9 permission bits in one shot. Symbolic (chmod u+x file) modifies specific bits without touching others. Use numeric for absolute permissions, symbolic for relative changes. Both produce the same final state — pick whichever reads cleaner for your case.

Why doesn't chmod -R work as expected on directories?

Recursive chmod applies the SAME permissions to all files and directories — but executable bit on a regular file is rarely what you want. Use find: 'find . -type f -exec chmod 644 {} +' for files, 'find . -type d -exec chmod 755 {} +' for directories. Or chmod's symbolic +X (capital X) which only sets execute on directories and already-executable files.

Why does sudo chmod fail to change /tmp?

Some directories have the sticky bit set (last digit 1, e.g., 1777). The sticky bit on /tmp means only the owner of a file can delete it, regardless of directory permissions. This prevents users from deleting each other's temp files. To remove sticky bit: chmod -t /path. To set sticky: chmod +t /path or chmod 1755 /path.

Chmod चीट शीट

Unix file permissions के लिए त्वरित संदर्भ: octal notation, symbolic notation, special bits और recursive command पैटर्न।

Permission values

Symbol	Value	Effect
`r`	4	Read — view file contents or list directory
`w`	2	Write — modify file or add/remove directory entries
`x`	1	Execute — run file or enter (cd into) directory
`-`	0	Permission denied

Common octal modes

Octal	Symbolic	Use case
`700`	`rwx------`	Private executable (only owner)
`755`	`rwxr-xr-x`	Public executable / directory
`644`	`rw-r--r--`	Public file (HTML, CSS, configs)
`600`	`rw-------`	Private file (SSH keys, secrets)
`666`	`rw-rw-rw-`	World-writable file (rare, risky)
`777`	`rwxrwxrwx`	World-everything (almost never correct)
`400`	`r--------`	Read-only for owner (locked file)

Symbolic notation

Pattern	Effect
`chmod u+x file`	Add execute for owner
`chmod g-w file`	Remove write for group
`chmod o=r file`	Set others to read-only
`chmod a+r file`	Add read for all (a = u+g+o)
`chmod ug+w file`	Add write for owner and group
`chmod -R u+w dir/`	Recursive — apply to all contents
`chmod +X dir/`	Add execute only on directories and already-executable files

Special bits (4-digit octal)

Bit	Octal	Effect
setuid	`4xxx`	Run executable with owner's privileges (e.g., 4755)
setgid	`2xxx`	Run with group's privileges, OR new files in directory inherit group
sticky	`1xxx`	Only file owner can delete (used on /tmp, e.g., 1777)

In ls -l output, special bits show as s, S, t, or T in the execute column.

Reading ls -l output

-rwxr-xr-x  1 alice  staff  4096 May  6 12:34 script.sh
└┬┘└┬┘└┬┘└┬┘
 │  │  │  └─ others permissions (r-x = read, execute)
 │  │  └──── group permissions (r-x = read, execute)
 │  └─────── owner permissions (rwx = read, write, execute)
 └────────── file type: - regular file, d directory, l symlink, b/c device

Recursive permission fixes

Goal	Command
Files 644, dirs 755 (websites)	`find . -type f -exec chmod 644 {} +` then `find . -type d -exec chmod 755 {} +`
Add x only on directories	`chmod -R +X .`
Lock everything down	`chmod -R go= .`
SSH config dir	`chmod 700 ~/.ssh && chmod 600 ~/.ssh/*`

Visual chmod calculator: Stax Chmod Calculator.

How chmod permissions work

Every Unix file has three permission classes: owner (u), group (g), and others (o). Each class can be granted three permissions: read (r=4), write (w=2), and execute (x=1). Add the values to combine: rwx = 7, rw- = 6, r-x = 5, r-- = 4, --- = 0.

Octal vs symbolic notation

The three-digit octal notation (e.g., 755) sets owner, group, others in that order. Each digit is the sum of permissions for that class. 755 = owner rwx (4+2+1=7), group r-x (4+1=5), others r-x (5). Symbolic notation lets you adjust permissions selectively without resetting the whole bitmask.

अक्सर पूछे जाने वाले प्रश्न

What's the difference between 755 and 644?: 755 (rwxr-xr-x) — owner can read/write/execute, group and others can read/execute. Used for executable files and directories. 644 (rw-r--r--) — owner can read/write, group and others can only read. Used for regular files (HTML, CSS, images, configs).
Why does 777 work but everyone says don't use it?: 777 grants read/write/execute to everyone. It works in the sense that the file becomes accessible by all processes, but it's a security catastrophe — any compromised user or process can modify the file. Use 644 for files, 755 for executables, 700 for private data, 600 for SSH keys. Reserve 777 only for /tmp-style world-writable directories that intentionally need it.
What's the difference between numeric and symbolic chmod?: Numeric (chmod 755) sets all 9 permission bits in one shot. Symbolic (chmod u+x file) modifies specific bits without touching others. Use numeric for absolute permissions, symbolic for relative changes. Both produce the same final state — pick whichever reads cleaner for your case.
Why doesn't chmod -R work as expected on directories?: Recursive chmod applies the SAME permissions to all files and directories — but executable bit on a regular file is rarely what you want. Use find: 'find . -type f -exec chmod 644 {} +' for files, 'find . -type d -exec chmod 755 {} +' for directories. Or chmod's symbolic +X (capital X) which only sets execute on directories and already-executable files.
Why does sudo chmod fail to change /tmp?: Some directories have the sticky bit set (last digit 1, e.g., 1777). The sticky bit on /tmp means only the owner of a file can delete it, regardless of directory permissions. This prevents users from deleting each other's temp files. To remove sticky bit: chmod -t /path. To set sticky: chmod +t /path or chmod 1755 /path.