how to sort files by their permissions using ls?

ls does not directly support sorting by permissions, but you can combine it with the sort command:

ls -l | sort

You can use the -k option to sort to start matching from a specific character, the format is -k FIELD.CHAR, the permissions are the first field in the ls output. So e.g. -k 1.2 will start from the second character of the permission string, which will ignore any directory / device / link etc. flag, or -k 1.5 for sorting by group permissions.

If you don't want the additional output of ls -l, you can remove it with awk:

 ls -l | sort | awk '{ print $1, $NF}'

This will print only the first field (the permissions) and the last one (the filename).

You can also sort by octal value.

for i in *; do stat --format="%a %n" "$i"; done | sort -n