How to select a random item from an array in shell

Question

I'm creating a bot in Shell Script:

# Array with expressions
expressions=("Ploink Poink" "I Need Oil" "Some Bytes are Missing!" "Poink Poink" "Piiiip Beeeep!!" "Hello" "Whoops! I'm out of memmory!")

# Seed random generator
RANDOM=$$$(date +%s)

# Loop loop loop loop loop loop ...
while [ 1 ]
do
    # Get random expression...
    selectedexpression=${expressions[$RANDOM % ${#RANDOM[*]}]}

    # Write to Shell
    echo $selectedexpression


    # Wait an half hour
    sleep 1 # It's one second for debugging, dear SOers
done

I want that it prints a random item from the expressions every second. I tried this but it does not work. It only prints the first one (Ploink Poink) every time. Can anyone help me out? Thanks

The tags should be fixed -- there are no arrays in POSIX shell. — David C. Rankin, Feb 26 '20 at 09:29

score 129 · Accepted Answer · edited Jan 04 '23 at 10:11

129

Change the line where you define selectedexpression to

selectedexpression=${expressions[ $RANDOM % ${#expressions[@]} ]}

You want your index into expression to be a random number from 0 to the length of the expression array. This will do that.

edited Jan 04 '23 at 10:11

Waldir Leoncio

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answered Mar 05 '10 at 17:13

Jacob Mattison

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Note that that code will still be biased towards the lower array indexes. – Joey Mar 06 '10 at 10:14
16

True. However, unless your list of expressions is very long, the effect is minor. $RANDOM is a number between 0 and 32767. Say you had 100 items in your list. The first 67 items would have a 328/32767 (.01001) chance, while the last 33 would have a 327/32767 (.00998) chance. For a shorter list the difference would be even less. Still, you make a good point, and the shell RANDOM function is not suitable for situations where you must have truly random numbers, such as cryptography. – Jacob Mattison Mar 07 '10 at 01:20
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For anyone that comes to this answer, and applies it in ZSH: Array index in ZSH starts at 1 (for some reason), so change the statement to: `selectedexpression=${expressions[$(($RANDOM % ${#expressions[@]} + 1 ))]}` – loklaan Jan 17 '19 at 08:27
Thank you @loklaan, you are my saviour – Lucat Mar 03 '20 at 19:42
1

To get a uniform distribution you can generate the random numbers by scaling a random floating point number between 0.0 and 1.0. Example for the array `a`: `echo "${a[$(awk '{srand($2); print int(rand()*$1)}' <<< "${#a[@]} $RANDOM")]}"`. Or use a program like `shuf -n1 -i0-...` or `jot -r 1 0 ...` which does the job for you. – Socowi Apr 20 '20 at 11:33

score 30 · Answer 2 · edited Oct 28 '17 at 19:50

30

arr[0]="Ploink Poink"
arr[1]="I Need Oil"
arr[2]="Some Bytes are Missing!"
arr[3]="Poink Poink"
arr[4]="Piiiip Beeeep!!"
arr[5]="Hello"
arr[6]="Whoops! I'm out of memmory!"
rand=$[$RANDOM % ${#arr[@]}]
echo $(date)
echo ${arr[$rand]}

edited Oct 28 '17 at 19:50

Mohsen Abasi

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answered May 08 '16 at 04:56

noobninja

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this gives - `Mon Mar 13 10:04:55 IST 2017 Whoops! I'm out of memmory!` – Abhinav Gauniyal Mar 13 '17 at 04:35
Additional items can be added to array, e.g. `arr[7]="Zip, ding, another line."` – noobninja Oct 29 '17 at 10:24

score 5 · Answer 3 · answered Jul 29 '14 at 02:38

5

Here's another solution that may be a bit more random than Jacob Mattison's solution (hard to say from the jot manpages):

declare -a expressions=('Ploink' 'I Need Oil' 'Some Bytes are Missing' 'Poink Poink' 'Piiiip Beeeep' 'Hello' 'Whoops I am out of memory')
index=$( jot -r 1  0 $((${#expressions[@]} - 1)) )
selected_expression=${expressions[index]}

answered Jul 29 '14 at 02:38

Powers

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[This `jot` implementation](https://opensource.apple.com/source/shell_cmds/shell_cmds-162/jot/jot.c.auto.html) uses floating point calculations to generate the random number. The system's `random()` function is scaled down to a number between `0.0` and `1.0` which is then scaled up to the range given in the arguments, therefore I consider the distribution to be uniform. – Socowi Apr 20 '20 at 11:14

presto8 · Answer 4 · 2021-09-17T18:51:01.117

Solution using shuf:

expressions=("Ploink Poink" "I Need Oil" "Some Bytes are Missing!" "Poink Poink" "Piiiip Beeeep!!" "Hello" "Whoops! I'm out of memmory!")
selectedexpression=$(printf "%s\n" "${expressions[@]}" | shuf -n1)
echo $selectedexpression

Or probably better:

select_random() {
    printf "%s\0" "$@" | shuf -z -n1 | tr -d '\0'
}

expressions=("Ploink Poink" "I Need Oil" "Some Bytes are Missing!" "Poink Poink" "Piiiip Beeeep!!" "Hello" "Whoops! I'm out of memmory!")
selectedexpression=$(select_random "${expressions[@]}")
echo "$selectedexpression"

score 2 · Answer 5 · edited Jun 29 '23 at 06:00

2

for random

1.

rand=("q" "w") 
r=$(shuf -i 0-${#rand[@]} -n 1)
echo ${rand[$r]}

echo `$((1 + $RAND % 5))` // **for number between 1..5**

edited Jun 29 '23 at 06:00

Baptiste Mille-Mathias

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answered Dec 14 '21 at 18:51

Netwons

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How to select a random item from an array in shell

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