CODEWITHSUNDEEP
shellrandom
Newbie Prog
Newbie Prog

Reputation:

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?

Upvotes: 268

Views: 112436

Answers (13)

Tracker1
Tracker1

Reputation: 19334

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.

Upvotes: 20

Yokai
Yokai

Reputation: 1220

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.

Upvotes: 31

peak
peak

Reputation: 116710

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

Upvotes: 0

dreday13
dreday13

Reputation: 1

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.

Upvotes: 0

Ken Roy
Ken Roy

Reputation: 1261

#!/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]}

Upvotes: 0

jrjc
jrjc

Reputation: 21873

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.

Upvotes: 1

unbeknown
unbeknown

Reputation:

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

Upvotes: 396

Thomas Vander Stichele
Thomas Vander Stichele

Reputation: 36549

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)

Upvotes: 74

Adam Rosenfield
Adam Rosenfield

Reputation: 400224

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

Upvotes: 3

Baskar
Baskar

Reputation: 1647

Another way using 'awk'

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

Upvotes: 3

PolyThinker
PolyThinker

Reputation: 5218

Another alternative:

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

Upvotes: 72

Paolo Tedesco
Paolo Tedesco

Reputation: 57202

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}

Upvotes: 11

asalamon74
asalamon74

Reputation: 6170

Single bash line:

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

Slight problem: duplicate filename.

Upvotes: 4

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