Regex for Linux file permissions (numeric notation)

Question

I can't for the life of me figure out the proper regex for this.

What I'm looking for is a regex to match a valid numeric representation of Linux file permissions (e.g. 740 for all-read-none, 777 for all-all-all). So far I've tried the following:

strtotest=740
echo "$strtotest" | grep -q "[(0|1|2|3|4|5|7){3}]"
if [ $? -eq 0 ]; then
    echo "found it"
fi

The problem with the above is that the regex matches anything with 1-5 or 7 in it, regardless of any other characters. For example, if strtotest were to be changed to 709, the conditional would be true. I've also tried [0|1|2|3|4|5|7{3}] and [(0|1|2|3|4|5|7{3})] but those don't work as well.

Is the regex I'm using wrong, or am I missing something that has to deal with grep?

Answer 1

what you want is probably

grep -Eq "[0-7]{3}"

edit: if you're using this for finding files with certain permissions, you should rather have a look at find (-perm) or stat

Answer 2

The simplest and most obvious regexp which is going to work for you is:

grep -q '(0|1|2|3|4|5|7)(0|1|2|3|4|5|7)(0|1|2|3|4|5|7)'

Here is an optimized version:

grep -Eq '(0|1|2|3|4|5|7){3}'

since 6 can represent permissions as well, we could optimize it further:

grep -Eq '[0-7]{3}'