Generate strings of varying in boundaries with bash script

Question

I want to generate strings with lengths between 1-4 with my bash script. I know how to create strings with set length, but want to know if there is a slight alteration to the following command to make it choose between 1-4 length.

cat /dev/urandom | tr -dc A-Za-z0-9 | head -c 4

This will create strings with numbers/letters with standard length 4. Any way to tweek it to fit my needs?

tpschmidt · Accepted Answer · 2020-05-25 15:57:25Z

2

Not the smartest solution but this should work I guess

cat /dev/urandom | tr -dc A-Za-z0-9 | head -c "$(shuf -i 1-4 -n 1)"

answered May 25, 2020 at 15:57

tpschmidt

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2 Comments

JamesTheProg Over a year ago

Should work, but because my program already takes about 10 seconds for what I want it to do, I was afraid that adding another random function here might increase the execution time further than I want it. Was really hoping there was a fix for it that I wasn't aware of, based on altering the "head.." part.

Léa Gris Over a year ago

Useless cat. Also use character class and switch locale for consistent results across systems: LC_ALL=POSIX tr -dc '[:alnum:]' </dev/urandom | head -c "$(shuf -i 1-4 -n 1)"

Socowi · Accepted Answer · 2020-05-25 15:58:07Z

2

That's simple: Use a random number/character generator to generate the argument to head -c too.

In bash you can use the built-in variable $RANDOM together with an arithmetic expression. The random numbers are not that good (especially when using % someBigNumber) but sufficient for most scenarios:

< /dev/urandom tr -dc A-Za-z0-9 | head -c "$((RANDOM % 4 + 1))"

If you need better randomness or a portable script use /dev/urandom again:

< /dev/urandom tr -dc A-Za-z0-9 | head -c "$(< /dev/urandom tr -dc 1-4 | head -c 1)"

answered May 25, 2020 at 15:58

Socowi

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Comments

Léa Gris · Accepted Answer · 2020-05-26 08:03:31Z

Here a method to generate n random strings of random min length to max length:

It reads a stream of random [:alnum:] alphanumeric characters; each time with a different random amount, and store it in an array.

This method maximizes efficiency with:

The use of Bash's built-in functions.
A single call to external command tr.
Reading random unsigned bytes from the /dev/urandom fast random generator device.

#!/usr/bin/env bash

# count: Number of strings to generate
# l_min: Minimum length of string
# l_max: Maximum length of string

declare -i count=10 l_min=4 l_max=8

# Array containing random strings

declare -a rand_strings=()

declare -i i=0
while [ $i -lt $count ]; do
  declare -i l=$((RANDOM%(l_max-l_min+1)+l_min))
  read -r -N $l rand_strings[i]
  i=$((i+1))
done < <(
  LC_ALL=POSIX
  tr -dc '[:alnum:]' </dev/urandom
)

printf '%d ranndom strings of %d to %d characters\n' $count $l_min $l_max
printf '%q\n' "${rand_strings[@]}"

Sample output:

10 ranndom strings of 4 to 8 characters
4QeeD9oT
GZQ7dD0
r3wb5D
17HBZhp
SP1i
t1OHq
1L4qpz7T
F5zG
Dw9HQ
ysG2chf

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