echo -n I | od -to2 | head -n1 | cut -f2 -d" " | cut -c6

I’ll go through it part by part:

• echo -n I – You’re already familiar with echo. The -n switch just suppresses the newline \n character that, by default, follows the output. This part just prints the letter I.
• od -to2 – If you look at man ascii, you’ll notice that in the 7-bit octal range (00 to 0177), the letter I is 0111, which has the convenient property of being all 1’s. od is a program that lets you inspect, in human-readable format, what may or may not be printable characters. The -to2 switch tells it to print 2 bytes in octal. (The -to1 switch would not work because both little-endian and big-endian machines would indistinguishably print 111.) Little-endian machines will give you 0000000 000111 while big-endian machines will give you 0000000 111000. Great! Let’s simplify this result down a bit.
• head -n1 – Here’s an easy one. We’re just grabbing the first line of stdin and spitting it back out.
• cut -f2 -d" " – Another good one to know. Cut is string.split(). The -f2 switch tells it that the second field is desired, and the -d" " switch gives it a single space character as a delimeter.
• cut -c6 – Finally, here’s another cut. This one just grabs the 6th character. The 4th or the 5th would also suffice.

You could take this one-liner a step further and add conditional printing of “big endian” or “little endian”, but by point #2, it’s pretty much there.