shuf

shuf shuffles: it outputs its input lines in a random order. Every possible ordering is equally likely.

shuf [options] [file]
shuf -e [options] arg...
shuf -i lo-hi [options]

$ shuf playlist.txt

shuf is the opposite of sort — instead of imposing an order, it removes one. With no file, it reads standard input.

Where the lines come from

shuf has three ways of getting its input, one per usage form:

Option	Input is…
(a file, or stdin)	the lines of the file.
`-e`, `--echo`	the command-line arguments, each treated as one line.
`-i`, `--input-range=LO-HI`	the integers from `LO` to `HI`, each treated as one line.

$ shuf -e red green blue          # shuffle three given words
$ shuf -i 1-100                   # the numbers 1..100 in random order

Option	Effect
`-n`, `--head-count=COUNT`	Output at most `COUNT` lines — a random sample rather than the whole shuffle.
`-r`, `--repeat`	Allow lines to be repeated, so output can be longer than the input. Pair with `-n` to set how many.
`-o`, `--output=FILE`	Write to `FILE` instead of standard output.
`-z`, `--zero-terminated`	Treat the NUL character as the line delimiter.

shuf -n 1 is a common idiom — pick one random line.

A shuffle is random by default. To get the same shuffle every time — for a repeatable test, say — fix the randomness:

Option	Effect
`--random-seed=STRING`	Seed the shuffle with `STRING`. The same seed gives the same shuffle.
`--random-source=FILE`	Take the random bytes from `FILE`. The same file gives the same shuffle. Cannot be combined with `--random-seed`.

Code	Meaning
`0`	Success.
`1`	A failure — the input could not be read, or `-r` was used with no lines to repeat.