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

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

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}

Single bash line:

sed -n $((1+$RANDOM%`wc -l test.txt | cut -f 1 -d ' '`))p test.txt

Slight problem: duplicate filename.

Here's a simple Python script that will do the job:

import random, sys
lines = open(sys.argv[1]).readlines()
print(lines[random.randrange(len(lines))])

Usage:

python randline.py file_to_get_random_line_from

Another way using 'awk'

awk NR==$((${RANDOM} % `wc -l < file.name` + 1)) file.name

A solution that also works on MacOSX, and should also works on Linux(?):

N=5
awk 'NR==FNR {lineN[$1]; next}(FNR in lineN)' <(jot -r $N 1 $(wc -l < $file)) $file 

Where:

• N is the number of random lines you want

• NR==FNR {lineN[$1]; next}(FNR in lineN) file1 file2 --> save line numbers written in file1 and then print corresponding line in file2

• jot -r $N 1 $(wc -l < $file) --> draw N numbers randomly (-r) in range (1, number_of_line_in_file) with jot. The process substitution <() will make it look like a file for the interpreter, so file1 in previous example.

Using only vanilla sed and awk, and without using $RANDOM, a simple, space-efficient and reasonably fast "one-liner" for selecting a single line pseudo-randomly from a file named FILENAME is as follows:

sed -n $(awk 'END {srand(); r=rand()*NR; if (r<NR) {sub(/\..*/,"",r); r++;}; print r}' FILENAME)p FILENAME

(This works even if FILENAME is empty, in which case no line is emitted.)

One possible advantage of this approach is that it only calls rand() once.

As pointed out by @AdamKatz in the comments, another possibility would be to call rand() for each line:

awk 'rand() * NR < 1 { line = $0 } END { print line }' FILENAME

(A simple proof of correctness can be given based on induction.)

Caveat about rand()

"In most awk implementations, including gawk, rand() starts generating numbers from the same starting number, or seed, each time you run awk."

-- https://www.gnu.org/software/gawk/manual/html_node/Numeric-Functions.html

#!/bin/bash

IFS=$'\n' wordsArray=($(<$1))

numWords=${#wordsArray[@]}
sizeOfNumWords=${#numWords}

while [ True ]
do
	for ((i=0; i<$sizeOfNumWords; i++))
	do
		let ranNumArray[$i]=$(( ( $RANDOM % 10 )  + 1 ))-1
		ranNumStr="$ranNumStr${ranNumArray[$i]}"
	done
	if [ $ranNumStr -le $numWords ]
	then
		break
	fi
	ranNumStr=""
done

noLeadZeroStr=$((10#$ranNumStr))
echo ${wordsArray[$noLeadZeroStr]}

Here is what I discovery since my Mac OS doesn't use all the easy answers. I used the jot command to generate a number since the $RANDOM variable solutions seems not to be very random in my test. When testing my solution I had a wide variance in the solutions provided in the output.

  RANDOM1=`jot -r 1 1 235886`
   #range of jot ( 1 235886 ) found from earlier wc -w /usr/share/dict/web2
   echo $RANDOM1
   head -n $RANDOM1 /usr/share/dict/web2 | tail -n 1

The echo of the variable is to get a visual of the generated random number.