getlines

#!/usr/bin/env bash
set -euo pipefail

# get a single line or slice of lines from stdin/files, similar to head + tail together


usage() {
    local base
    base="$(basename "$0")"
    cat >&2 <<EOM
usage: $base LINES [FILE...]

Print LINES from FILE(s) or stdin.

LINES may be delimited with hyphens for slices or commas for separate lines.

For example:

    Print line 3 from foo.txt:
    $base 3 foo.txt

    Print lines 5 and 10 from stdin:
    $base 5,10

    Print lines 20-30 and 100 from stdin:
    $base 20-30,100

EOM
}

if [ $# -eq 0 ]; then
    usage
    exit 1
fi

if [[ "$1" == "-h" || "$1" == "--help"  ]]; then
    usage
    exit
fi

spec="$1"
shift

if [ -z "$spec" ]; then
    usage
    exit 1
fi

sedspec=

mapfile -td, lines < <(echo -n "$spec")
for line in "${lines[@]}"; do
    if [[ $line == *-* ]]; then
        mapfile -td- slice < <(echo -n "$line")

        if [ "${#slice[@]}" -ne 2 ]; then
            usage
            exit 2
        fi

        sedspec="${sedspec}${slice[0]},${slice[1]}p;"
    else
        sedspec="${sedspec}${line}p;"
    fi
done

# sed print 5-10: 5,10p
# sed print 1 and 3: 1p;3p

cat -- "$@" | sed -n "$sedspec"