37 lines (37 with data), 867 Bytes

#! /bin/bash
if [ "$#" -lt 1 ];then
    echo "Usage: $0 file1 [file2 ...]" >&2
    exit 1
fi
# detect format
format=""
for filename in "$@";do
    if [ "${filename/%.gz}" != "$filename" ];then
        new_format="gzip"
    elif [ "${filename/%.xz}" != "$filename" ];then
        new_format="xz"
    elif [ "${filename/%.bz2}" != "$filename" ];then
        new_format="bzip2"
    else
        new_format="raw"
    fi
    if [ -z "$format" ];then
        format="$new_format"
    fi
    if [ "$new_format" != "$format" ];then
        echo "Error: different format found in input files" >&2
        exit 1
    else
        format="$new_format"
    fi
done
# uncompress files
if [ "$format" = "gzip" ];then
    exec gzip -d -c "$@"
elif [ "$format" = "bzip2" ];then
    exec bzip2 -d -c "$@"
elif [ "$format" = "xz" ];then
    exec xz -d -c "$@"
else
    exec cat "$@"
fi