filter

#!/bin/bash
# Send matching lines to be processed, pass non-matching lines thru unprocessed.
# Example: </etc/fstab filter "^[^#]" column -t

if test $# -lt 2; then
    printf "Usage: filter <pattern> <command> [<command args>]\n" >&2
    exit 2
fi

# Decorate, sort, then undecorate lines to ensure line order is preserved, even
# if the command uses different buffering (e.g. only produces output after
# reading all input).
# Undecorate using whitespace rather than just tabs so we can use "column" as
# the command (column converts the tab separator into spaces).
pattern=$1
shift
i=0
{
while read -r line; do
    i=$((i+1))
    if [[ "$line" =~ $pattern ]]; then
        printf "%d\t%s\n" "$i" "$line" >&3
    else
        printf "%d\t%s\n" "$i" "$line"
    fi
done 3> >("$@")
} |
sort -k1n |
sed 's/^[0-9][0-9]*[[:space:]]*//'