I don t believe this is any more elegant/efficient than your own version but I will put it up in case any of the techniques are useful for improving your own. This could obviously be done much more simply/elegantly with a script though

The basic premise is to use tr to translate the rwx to the relevant octal numbers, then use sed to split into groups of 3 add pluses and then generate an awk command string whicg gets passed to awk to add them up.

ls -ld / | 
cut -c2-10 | 
tr  rwx-t   42100  | 
sed -E -e  s/(...)(...)(...)/1 2 3/g  
-e  s/([0-9])([0-9])([0-9])/1+2+3/g  
-e  s/^(.*)$/BEGIN {print 1}/g |
awk -f -`

if you can use find then this looks better:

find FILENAME -prune -printf %m

found it here

If your system has bash, and ls -l shows file permissions (rwxrwxrwx), you may accomplish what you were looking for with the following example:

#!/bin/sh
[ ! -d $1 ] && echo "Error: "$1" is not a directory" && exit 1 
set -- `ls -ld $1`;P=${1:1:9};P=${P//r/4 };P=${P//w/2 };P=${P//x/1 };P=${P//-/0 }
set -- $P; echo $((($1+$2)+$3))$((($4+$5)+$6))$((($7+$8)+$9))

The advantage is that other utilities such as awk or sed are not needed.

stat should have what you re looking for. Try one of these:

stat -c "%a" <filename>

or

stat --printf="%a" <filename>

This is long-winded, but I think it s more maintainable than yours. You can call it like this: ls -ld / | cut -c 2-10 | ./perm.awk.

#!/usr/bin/gawk -f
# OR: #!/usr/bin/nawk -f

function conv(num, len, i, val) {
    # converts binary string to decimal (or octal if fed 3 "bits" at a time)
    # call with num as argument, others are local variables
    len = length(num)
    for(i = len; i > 0; i--) {
        if (substr(num, i, 1) == 1) {
            val = val + 2 ^ (len - i)
        }
    }
    return val
}

# main loop
{
    # TODO: handle [sStT]
    gsub("[rwx]", 1)    # make it look binary
    gsub("-", 0)
    for(i = 0; i < 3; i++) {
        perm = perm conv(substr($0, i * 3 + 1, 3))    # convert the "bits" 3 at a time
    }
}

END {
    print perm
}

Implementations of ls vary, so your ls is not really portable either.

I d solve this problem using Lua, which compiles on any ANSI Standard C platform, together with the Lua POSIX library, which should build out of the box on any POSIX platform. There you can make your stat call directly and write clear simple code to do the arithmetic. An excellent introduction to Lua Programming in Lua is available free online (for the previous version of Lua). Finally, if you care about efficiency, the results will be extremely efficient, partly because Lua is fast, but mostly because you will be able to do everything while forking only one process—the Lua interpreter.