125 chmod permission modes

What 755 and 644 actually open up, and which umask produces them — all worked out from the three digits.

The ones you actually type

Of the 125, about ten are the ones your fingers actually reach for.

400r--------A read-only key file — even the owner cannot edit it.600rw-------Private keys and config. Only the owner reads and writes.640rw-r-----Config the team needs to read.644rw-r--r--The default for ordinary files: everyone reads, owner writes.664rw-rw-r--Files several people edit together.666rw-rw-rw-Anyone can edit it, so it is almost never used.700rwx------A folder or script only you touch.755rwxr-xr-xThe default for executables and web folders.775rwxrwxr-xA folder a team shares.777rwxrwxrwxEverything open to everyone. Used in a hurry, regretted later.

All 125

Every combination of the five values that actually get used — 0, 4, 5, 6, 7. Write-without-read and the like are left out.

Owner 0 — 0xx---

000001004005006007010011014015016017040041044045046047050051054055056057060061064065066067070071074075076077

Owner 1 — 1xx--x

100101104105106107110111114115116117140141144145146147150151154155156157160161164165166167170171174175176177

Owner 4 — 4xxr--

400401404405406407410411414415416417440441444445446447450451454455456457460461464465466467470471474475476477

Owner 5 — 5xxr-x

500501504505506507510511514515516517540541544545546547550551554555556557560561564565566567570571574575576577

Owner 6 — 6xxrw-

600601604605606607610611614615616617640641644645646647650651654655656657660661664665666667670671674675676677

Owner 7 — 7xxrwx

700701704705706707710711714715716717740741744745746747750751754755756757760761764765766767770771774775776777

How to read this

Read is 4, write 2, execute 1; add them for one digit. 7 is 4+2+1, 5 is 4+1.
The digits are owner, group, others, left to right.
umask is what gets taken away: from 777 for directories, from 666 for files.
Special bits (setuid 4000, setgid 2000, sticky 1000) add a fourth digit in front — /tmp is 1777.

Frequently asked questions

QWhat is the difference between 755 and 644?

One execute bit. 755 lets anyone run the file or step into the folder; 644 is read-only for others. Folders get 755, ordinary files 644.

QWhy not just use 777?

Because anyone who reaches the machine can then change the contents. Opening 777 to get past a permission error is taking the door off its hinges.

QWhat does umask 022 mean?

It is what gets subtracted. Directories start at 777 and become 755; files start at 666 and become 644.

QWhat is 1777, with four digits?

The leading digit is the special bits. The 1 is the sticky bit: on /tmp everyone may write, but nobody may delete another’s files.

QWhat does execute mean on a folder?

The right to go inside. With read alone you can list the names but open nothing within.