What command lists all files beginning with "a" and having 1 or 2 digits that follow?

Question

What command lists all files beginning with "a" and having 1 or 2 digits that follow? (The output might display a0, a1, a2, a3, a00, a01, a99,... but not a333, not b12, not art53,...)

Answer 1

In the zsh shell,

setopt EXTENDED_GLOB
printf '%s\n' a[[:digit:]](#c1,2)(.)

The pattern a[[:digit:]](#c1,2)(.) would match any regular file whose name starts with an a followed by one or two digits. The (.) at the end makes sure we only get names of regular files, while [[:digit:]](#c1,2) matches one or two digits.

In sh or bash etc.,

printf '%s\n' a[[:digit:]] a[[:digit:]][[:digit:]]

This would list all names that start with an a and then has either one or two digits ofter that (we use one pattern for names with one digit, and another for names with two digits). It is not certain that you'd only get names of regular files though (you may also get names of directories etc.), and if one or the other pattern (or both) does not match any name, the pattern will remain unexpanded (use shopt -s nullglob in bash to make the non-matching patterns be removed instead).

To weed out any non-regular file, use a loop:

for name in a[[:digit:]] a[[:digit:]][[:digit:]]; do
    if [ -f "$name" ]; then
        printf '%s\n' "$name"
    fi
done

Here, we apply an explicit -f test to each name. This test will be true if the name refers to a regular file (or to a symbolic link to a regular file). If the test is true, we print the name.

Answer 2

A more general solution, and one not dependent on which shell you're using, might be something like

Helios:t larry$ ls -1
a0
a10
a100
ax
axe
b4
Helios:t larry$ find . -mindepth 1 -maxdepth 1 -type f -regextype egrep -regex '\./a[[:digit:]]{,2}'
./a10
./a0

But that spawns an additional process.