Autocomplete file name in a bash script - bash

Question

I'm trying to imitate the bash file completion.
Suppose I have the following files:
test1
test2
With an input string of "te" I would like to get the output "test"
This is my current attempt ($c is the input string):
l=1
q="$c"
for j in $(ls -A | grep "^$c"); do
if [ "${j/$c}" != "$j" ]; then
n=$(ls -A | grep ^$j | wc -l)
if [ $n -gt $l ]; then
q="$j"
fi
fi
done
c="$q"
echo $c
Thanks for any help

Answer 1

I tend to think there is no a way to get this from completion engine since it’s not a part of GNU Bash but Readline. But at least we can get list of possible completions with compgen. And an inmplementaion of finding longest common prefix should not be problem. So...
#!/bin/bash
SCRIPTNAME="${0##*/}"
USAGE="Usage: $SCRIPTNAME <prefix>
Print common prefix of possible file name completions. Like <TAB> but to
stdout."
(( $# == 1 )) || { printf >&2 '%s\n' "$USAGE"; exit 1; }
PREFIX="$1"
commonprefix() {
(( $# >= 2 )) || {
echo "$1"
return 0
}
local -i i N M
for ((i=0; i<=${#1}; i++)); do
for ((N=1; N<=$#-1; N++)); do
let M=$N+1
[[ ${!N:i:1} == ${!M:i:1} ]] || break 2
done
done
echo "${1:0:i}"
}
readarray -t COMPLETIONS < <(compgen -f "$PREFIX")
commonprefix "${COMPLETIONS[#]}"

Answer 2

Although Dmitry Alexandrov already provided a better solution, I still would like to post my own one which I made while waiting for the answers:
l=1
while [ -n $l ]; do
l=${#c}
a=$(ls -A | grep "^$c" | wc -l)
q=$c
for i in $(ls -A | grep "^$q"); do
if [ $i == $q ]; then
unset l
break
else
v=$(ls -A | grep "^$q${i:$l:1}" | wc -l)
if [ $v == $a ]; then
q="$c${i:$l:1}"
break
fi
fi
done
if [ $c == $q ]; then break; fi
c=$q
done
echo $c
It works with all of my tests, but it's slow (although it could be optimized).

Answer 3

Just to show that you were thinking in correct direction, I made your code work:
#!/bin/bash
c=$1
q="$c"
for j in $c*; do
if [ "${j/$c}" != "$j" ]; then
startn=$(ls -1A | grep -c "^${j:0:$((${#c} + 1))}")
for (( i=${#c}; i <= ${#j}; i++ )); do
n=$(ls -1A | grep -c "^${j:0:$i}")
if [ "$n" -lt "$startn" ]; then
q="${j:0:$((i - 1))}"
elif [ "$n" -le "$startn" ]; then
q="${j:0:$i}"
fi
done
fi
done
c="$q"
echo "$c"
But, it's just a proof of concept, don't use it. See answer by Dmitry Alexandrov for a good solution.