What's an easy way to read random line from a file?

Question

You can use shuf:

shuf -n 1 $FILE

There is also a utility called rl. In Debian it's in the randomize-lines package that does exactly what you want, though not available in all distros. On its home page it actually recommends the use of shuf instead (which didn't exist when it was created, I believe). shuf is part of the GNU coreutils, rl is not.

rl -c 1 $FILE

Another alternative:

head -$((${RANDOM} % `wc -l < file` + 1)) file | tail -1

sort --random-sort $FILE | head -n 1

(I like the shuf approach above even better though - I didn't even know that existed and I would have never found that tool on my own)

This is simple.

cat file.txt | shuf -n 1

Granted this is just a tad slower than the "shuf -n 1 file.txt" on its own.

perlfaq5: How do I select a random line from a file? Here's a reservoir-sampling algorithm from the Camel Book:

perl -e 'srand; rand($.) < 1 && ($line = $_) while <>; print $line;' file

This has a significant advantage in space over reading the whole file in. You can find a proof of this method in The Art of Computer Programming, Volume 2, Section 3.4.2, by Donald E. Knuth.

using a bash script:

#!/bin/bash
# replace with file to read
FILE=tmp.txt
# count number of lines
NUM=$(wc - l < ${FILE})
# generate random number in range 0-NUM
let X=${RANDOM} % ${NUM} + 1
# extract X-th line
sed -n ${X}p ${FILE}