Question 1

What does 'chmod' stand for?

Accepted Answer

`chmod` is short for 'change mode', Unix terminology for changing the access permissions on a filesystem object. It's been a core Unix command since the early 1970s. Modern Linux, macOS, BSD, WSL, and even some Windows-native filesystems via Cygwin/MSYS support it identically. The companion commands are `chown` (change owner) and `chgrp` (change group).

Question 2

How do I read the three digits in a chmod command?

Accepted Answer

Each digit applies to a different class: (1st) Owner, (2nd) Group, (3rd) Others. Each digit is a sum of bits: 4 (read), 2 (write), 1 (execute). So `755` is rwx for owner (4+2+1=7) and r-x for group and others (4+0+1=5). `644` is rw- for owner and r-- for everyone else. Sometimes you'll see a four-digit form like `4755`, that leading digit is the special bits (setuid, setgid, sticky).

Question 3

What are common chmod permission sets?

Accepted Answer

**755**, owner full access, others read+execute. Standard for executables and directories. **644**, owner read+write, others read-only. Standard for regular files (configs, HTML, source code). **700**, owner only, no access for anyone else. Use for private directories. **600**, owner read+write, no one else. Required for SSH private keys and many secrets. **400**, owner read-only, immutable to everyone else. **777**, universal write access. Almost always a security mistake.

Question 4

What is the difference between octal and symbolic notation?

Accepted Answer

Octal (numeric): `chmod 755 file` sets the entire mode at once. Symbolic: `chmod u+x file` modifies one bit without touching others. Symbolic syntax: `u/g/o/a` (user/group/other/all) + `+/-/=` (add/remove/set) + `r/w/x`. So `chmod go-w file` removes write from group and other; `chmod a=r file` sets read-only for everyone. Use octal when setting the whole permission set from scratch; symbolic for targeted changes.

Question 5

Is chmod 777 dangerous?

Accepted Answer

Almost always yes. `777` grants read, write, and execute to every user on the system, including service accounts, other tenants on shared hosting, and any process that's been compromised. The most common 'fix' for permission errors on the internet, and the most common security incident in shared environments. Real fixes: figure out which user the process runs as, set ownership with `chown`, give that user the permissions it needs. `777` is the duct tape of file permissions.

Question 6

What are setuid, setgid, and the sticky bit?

Accepted Answer

Special bits in the optional 4th leading digit. **setuid (4)**, when an executable runs, it runs as the file owner instead of the invoking user. How `sudo` and `passwd` work. **setgid (2)**, on executables, runs as the file's group; on directories, new files inherit the directory's group instead of the user's primary group. **sticky bit (1)**, on a directory, prevents users from deleting files they don't own, even if the directory is world-writable. Standard on `/tmp`.

Question 7

Why does SSH refuse my private key?

Accepted Answer

OpenSSH refuses to use a private key file that's readable by anyone other than the owner, too easy to leak. The fix: `chmod 600 ~/.ssh/id_rsa` (or 400 if you want to prevent accidental edits). The directory itself should be `chmod 700 ~/.ssh`. The `authorized_keys` file should be 600. The public key (`.pub`) doesn't matter security-wise but 644 is conventional. Same rule for AWS PEM files, GPG private keys, and most other credential files.

Question 8

What standards does Chmod Calculator validate against?

Accepted Answer

Chmod Calculator checks compliance with POSIX file permissions, Unix chmod specification.

Question 9

What are the limitations of Chmod Calculator?

Accepted Answer

Only covers standard Unix rwx permissions, does not handle ACLs Does not model SELinux or AppArmor contexts Numeric output assumes octal representation

Octal	Symbolic	Permission Meaning	Security Profile
7	`rwx`	Full Access	Administrative
6	`rw-`	Read + Write	Standard File
5	`r-x`	Read + Execute	Public Executable
4	`r--`	Read Only	Immutable
0	`---`	No Access	Hardened

Chmod Calculator

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What is Chmod Calculator?

Frequently Asked Questions

Technical Deep Dive

Chmod Calculator

Symbolic to octal

setuid / sticky bits

Preset shortcuts

01 Octal-Symbolic Reference Matrix

02 Permission Bit Pipeline

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