Introduction

There are many, many, ways to create (pseudo)-random strings in Bash, but I found this approach utilizing shuf to be the most understandable. Also, it gives excellent control over the output (avoiding whitespace and special characters) without resorting to the use of the tr command to remove unwanted characters. And it seems to be available for many systems without requiring installation of additional packages.

The shuf command

When first learning about a command like shuf it is helpful to start by reading the tldr and info entries for the command. They provide excellent summaries of its basic functions.

According to:

tldr shuf

It “generate[s] random permutations."

And according to:

info shuf

"‘shuf’ shuffles its input by outputting a random permutation of its input lines. Each output permutation is equally likely."

Example 1: Generating a random number

To get started, let’s generate a random number in the range of 0-5.

There are short and long versions of the command that both do the exact same thing.

Long version:

shuf --echo --head-count=1 {0..5}

Short version:

shuf -e -n 1 {0..5}

Example 2: Generating a random letter

Likewise, we can generate a random letter with the following command.

Long version:

shuf --echo --head-count=1  {a..z}

Short version:

shuf -e -n 1  {a..z}

Example 3: Generating several random letters

Similarly, we can generate 5 random letters with the following command.

Long version:

shuf --echo --head-count=5  {a..z}

Short version:

shuf -e -n 5  {a..z}

It’s important to note that in this form the letters won’t repeat.

Example 4: Generating several random letters that might repeat

If, however, we want to generate 5 random letters with the possibility of the letters repeating, we can use the following command.

Long version:

shuf --echo --repeat --head-count=5  {a..z}

Short version:

shuf -er -n 5  {a..z}

Example 5: Generating a random string of letters

If we want to generate a random string of letters that might repeat (instead of separate letters), we can use the following command.

Long version:

shuf --echo --repeat --zero-terminated --head-count=5  {a..z}

Short version:

shuf -erz -n 5  {a..z}

This can be a little hard to read since it outputs the result on the same line as the next shell prompt.

Example 6: Generating a random string of letters with legible output

If we want to generate a random string of letters that might repeat (instead of separate letters) and output the result on it’s own line, we can append the echo command.

Long version:

shuf --echo --repeat --zero-terminated --head-count=5  {a..z}; echo

Short version:

shuf -erz -n 5  {a..z}; echo

Thanks to this StackOverflow answer for help solving this.

Example 7: Generating a random alphanumeric string

Additional ranges can be added to make a random alphanumeric string.

Long version:

shuf --echo --repeat --zero-terminated --head-count=5  {A..Z} {a..z} {0..9}; echo

Short version:

shuf -erz -n 5  {A..Z} {a..z} {0..9}; echo

Example 8: Generating a random alphanumeric string of random length

We can nest the shuf command within itself to generate a random alphanumeric string of random length.

Long version:

shuf --echo --repeat --zero-terminated --head-count=$(shuf --echo --head-count=1 {0..5})  {A..Z} {a..z} {0..9}; echo

Short version:

shuf -erz -n $(shuf -e -n 1 {0..5})  {A..Z} {a..z} {0..9}; echo

Conclusion

The shuf command has many other uses. This is just a brief introduction to some of its basic functions.