chmod Numbers Explained: 600, 644, 700, and 755
Decode numeric chmod permissions, choose common file and directory modes, verify the result, and avoid unsafe recursive changes.
Start Interactive Lesson
A three-digit chmod mode sets permissions for the owner, group, and everyone else, in that order. Each digit adds read (4), write (2), and execute or directory-search (1). For example, chmod 640 report.txt gives the owner read and write access, the group read access, and everyone else no access. The numeric-permissions lesson provides a safe place to practise decoding the digits.
Build one permission digit
Each audience receives a value from 0 to 7:
| Number | Permission | Meaning |
|---|---|---|
| 0 | --- | no access |
| 1 | --x | execute a file or search a directory |
| 2 | -w- | write |
| 3 | -wx | write + execute/search |
| 4 | r-- | read |
| 5 | r-x | read + execute/search |
| 6 | rw- | read + write |
| 7 | rwx | read + write + execute/search |
Put three digits together as owner-group-others. Mode 754 therefore means rwxr-xr--.
Choose a mode for the target, not from a memorised list
These modes are common starting points, but the correct choice depends on who should use the path:
| Mode | Typical meaning |
|---|---|
600 | private file that only its owner reads and writes |
640 | owner reads/writes; group reads; others have no access |
644 | owner reads/writes; everyone else reads |
700 | private directory or executable available only to its owner |
750 | owner has full access; group can read and enter; others have none |
755 | owner can modify; everyone can read and enter or execute |
Apply a mode to one known target:
chmod 640 report.txt
chmod 750 scripts
Directories need the x bit to be entered and to access named items inside them. A directory with read permission but no search permission can list names in some situations while still preventing normal access to those entries. That is why copying a regular-file mode onto a directory often causes confusing failures.
Verify the result in symbolic form
Check the mode after changing it:
ls -l report.txt
ls -ld scripts
For 640, the permission string should begin with -rw-r----- for a regular file. For a 750 directory, it should begin with drwxr-x---. The first character describes the file type; the next nine characters represent owner, group, and other permissions.
Numeric and symbolic modes answer different questions
A numeric mode states the complete regular permission set you want. A symbolic change adjusts selected bits relative to the current mode:
chmod g+w shared.txt
chmod o-r private.txt
Use symbolic form when the goal is “add group write access” without resetting unrelated permissions. Use numeric form when you have deliberately chosen the full owner, group, and other mode. Special bits such as setuid, setgid, and the sticky bit add another digit and deserve a separate review before use.
Avoid chmod -R 777 as a troubleshooting shortcut. It grants every audience every regular permission and applies the decision to paths you may not have inspected. Start with one target, verify it, and expand scope only when all descendants need the same policy.
For the SSH-specific 600 versus 700 decision, read how to protect SSH files. GNU's numeric modes reference defines the octal values and special-mode behavior.
References
These documentation links provide authoritative details for the commands used in this article.
Up Next
Use chmod 600 for a private SSH key and 700 for its directory, then verify both permissions and ownership before reconnecting.