Extract substrings with certain length randomly from a file with Bash - bash

Question

I have multiple text files, and from each file I need to extract random contiguous substrings with a certain length.
For example, I need to extract Five random substrings that consist of 3 contiguous characters each, or 4 random substrings that consist of 20 characters each.
In practice, let's assume this is the content of one of the files
Welcome to stackoverflow the best technical resource ever
so if I want Five random substrings that consist of 3 characters each, I expect an output that looks like this for example:
elc
sta
tec
res
rce
Your help would be much appreciated.

Answer 1

awk to the rescue!
awk -v n=5 -v s=3 'BEGIN {srand()}
{len=length($0);
for(i=1;i<=n;i++)
{k=rand()*(len-s)+1; printf "%s\t", substr($0,k,s)}
print ""}' file
there may be spaces in the extracted substrings

Answer 2

Create a function to pick a random substring:
random_string() {
line=$1
length=$2
# make sure we start at a random position that guarantees a substring of given length
start=$((RANDOM % ((${#line} - $length))))
# use Bash brace expansion to extract substring
printf '%s' "${line:$start:$length}"
}
Use the function in a loop:
#!/bin/bash
while IFS= read -r line; do
random1=$(random_string "$line" 3)
random2=$(random_string "$line" 20)
printf 'random1=[%s], random2=[%s]\n' "$random1" "$random2"
done < file
Sample output with the content Welcome to stackoverflow the best technical resource ever in file:
random1=[hni], random2=[low the best technic]
random1=[sta], random2=[e best technical res]
random1=[ove], random2=[ackoverflow the best]
random1=[rfl], random2=[echnical resource ev]
random1=[ech], random2=[est technical resour]
random1=[cal], random2=[ome to stackoverflow]
random1=[tec], random2=[o stackoverflow the ]
random1=[l r], random2=[come to stackoverflo]
random1=[erf], random2=[ stackoverflow the b]
random1=[me ], random2=[ the best technical ]
random1=[est], random2=[ckoverflow the best ]
random1=[tac], random2=[tackoverflow the bes]
random1=[e t], random2=[o stackoverflow the ]
random1=[al ], random2=[come to stackoverflo]