chmod calculator.
Tick boxes, type 755, or paste rwxr-xr-x. Every view stays in step, and the chmod command is ready to copy.
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Permissions
755
rwxr-xr-x
Owner can read, write and run. Group can read and run. Others can read and run.
Add -R to apply it to a folder and everything inside. Careful with that on folders: files rarely need the execute bit that folders do.
How it works.
- 1
Tick the permissions you want, or type an octal mode or an rwx string.
- 2
The other views update straight away, with a plain-English line of who can do what.
- 3
Type the file name and copy the chmod command.
How the octal digits work
Each digit is one audience: owner, then group, then everyone else. Inside a digit, read is 4, write is 2 and execute is 1, and you add up what you want. 7 is 4 + 2 + 1, everything. 5 is 4 + 1, read and run. 6 is 4 + 2, read and write. So 755 means the owner can do anything and the rest can read and run.
The rwx string says the same thing in letters, three per audience. rwxr-xr-x is 755. A dash means that permission is off.
setuid, setgid and the sticky bit
An optional fourth digit goes in front: 4 for setuid, 2 for setgid, 1 for sticky. setuid makes a program run as its owner, which is how passwd can edit a file only root can write. setgid does the same for the group, and on a folder makes new files inherit the folder's group. The sticky bit on a folder like /tmp lets people delete only their own files.
In the rwx string they replace the x: s for setuid and setgid, t for sticky. A capital S or T means the special bit is set but execute isn't, which is usually a mistake.
Questions, answered.
What does chmod 755 mean?
The owner can read, write and execute. The group and everyone else can read and execute, but not change the file. It's the usual mode for scripts, programs and folders. In letters it's rwxr-xr-x.
What does chmod 644 mean?
The owner can read and write, the group and everyone else can only read. It's the usual mode for ordinary files like HTML, images and config. In letters it's rw-r--r--.
Why is chmod 777 a bad idea?
777 lets every user on the system change or replace the file. On a server that means any compromised process can rewrite your code. If something only works at 777, the real fix is usually the file's owner or group, not wider permissions.
What's the difference between 600 and 400?
600 lets the owner read and write, 400 only read. SSH private keys need one of the two: ssh refuses a key that the group or others can read.
What does the s in rwsr-xr-x mean?
It's the setuid bit sitting in the owner's execute slot: the program runs with the owner's permissions. In octal that's 4755. A t in the last slot, as in rwxrwxrwt, is the sticky bit, 1777.
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