What does 755 actually mean?

The owner gets read, write and execute (7 = 4+2+1); the group and others get read and execute (5 = 4+1). It is the typical permission for directories and for scripts that others may run but not modify.

Is 777 ever a good idea?

Almost never. It lets every user on the machine write to and execute the file — the quickest way to get a web script modified or a directory trashed. Use the narrowest permission that works.

Numeric or symbolic form — which should I use?

They describe the same thing. Numeric (644) is compact and convenient in scripts; symbolic (u=rw,go=r) changes only the bits you mention, which is safer for small tweaks on shared files.

Chmod Calculator — Guide

Compute your chmod permissions and commands with this online chmod calculator.

What it is and when to use it

chmod controls who can read, write and execute a file on Linux and macOS. Permissions are grouped for the owner, the group and everyone else, and each group is a sum of read (4), write (2) and execute (1) — which is why 755 reads as rwxr-xr-x.

Works out chmod permission values and commands by ticking boxes, so you do not have to remember what rwx maps to.

How to use it

Tick read/write/execute for owner, group and others
The numeric form (e.g. 755) and symbolic form update above
Copy the generated chmod command

Things that trip people up

Be careful granting execute permission, and especially careful with 777

Questions people ask

What does 755 actually mean?: The owner gets read, write and execute (7 = 4+2+1); the group and others get read and execute (5 = 4+1). It is the typical permission for directories and for scripts that others may run but not modify.
Is 777 ever a good idea?: Almost never. It lets every user on the machine write to and execute the file — the quickest way to get a web script modified or a directory trashed. Use the narrowest permission that works.
Numeric or symbolic form — which should I use?: They describe the same thing. Numeric (644) is compact and convenient in scripts; symbolic (u=rw,go=r) changes only the bits you mention, which is safer for small tweaks on shared files.

Next steps

Open the Chmod Calculator tool

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