Work out file permission (chmod) instantly with clear inputs, formula shown and shareable results.
Unix permissions encode read as 4, write as 2 and execute as 1, summed per class of user. Owner, group and other each get one octal digit, so 750 means rwx for the owner, r-x for the group and nothing for everyone else. The execute bit means something different on directories: it grants the right to traverse into them rather than to run them.
Permission arithmetic
digit = 4 (read) + 2 (write) + 1 (execute) per class; mode = owner digit, group digit, other digit
On a directory, execute means traverse. Without it you cannot access anything inside even if you can list the names, because listing needs read and entering needs execute.
It lets any local user or compromised process modify the file. It is almost never necessary; the correct fix is usually to set the right owner or group.