Unexpected behavior when processing input via stdin but file input works fine - bash

Question

I have a program which transposes a matrix. It works properly when passed a file as a parameter, but it gives strange output when given input via stdin.
This works:
$ cat m1
1 2 3 4
5 6 7 8
$ ./matrix transpose m1
1 5
2 6
3 7
4 8
This doesn't:
$ cat m1 | ./matrix transpose
5
[newline]
[newline]
[newline]
This is the code I'm using to transpose the matrix:
function transpose {
# Set file to be argument 1 or stdin
FILE="${1:-/dev/stdin}"
if [[ $# -gt 1 ]]; then
print_stderr "Too many arguments. Exiting."
exit 1
elif ! [[ -r $FILE ]]; then
print_stderr "File not found. Exiting."
exit 1
else
col=1
read -r line < $FILE
for num in $line; do
cut -f$col $FILE | tr '\n' '\t'
((col++))
echo
done
exit 0
fi
}
And this code handles the argument passing:
# Main
COMMAND=$1
if func_exists $COMMAND; then
$COMMAND "${#:2}"
else
print_stderr "Command \"$COMMAND\" not found. Exiting."
exit 1
fi
I'm aware of this answer but I can't figure out where I've gone wrong. Any ideas?

Answer 1

for num in $line; do
cut -f$col $FILE | tr '\n' '\t'
((col++))
echo
done
This loop reads $FILE over and over, once for each column. That works fine for a file but isn't suitable for stdin, which is a stream of data that can only be read once.
A quick fix would be to read the file into memory and use <<< to pass it to read and cut.
matrix=$(< "$FILE")
read -r line <<< "$matrix"
for num in $line; do
cut -f$col <<< "$matrix" | tr '\n' '\t'
((col++))
echo
done
See An efficient way to transpose a file in Bash for a variety of more efficient one-pass solutions.