Random Passwords from the Shell: A Small Tool with Security Boundaries

A shell command can generate a strong random string in seconds. Its usefulness ends there.

That distinction matters. Randomness addresses guessing resistance; it does not address storage, rotation, recovery, access control, or accidental disclosure. For durable credentials, I use a password manager or a secrets-management platform. For a disposable lab secret, test token, or one-time bootstrap value, the shell can still be the right tool.

A compact generator

LC_ALL=C tr -dc 'A-Za-z0-9!@#%^_=+.,:;?-' </dev/urandom \
  | head -c 32
printf '\n'

/dev/urandom supplies operating-system randomness. LC_ALL=C keeps character matching predictable across environments, tr limits the output alphabet, and head fixes the length.

Adjust the character set to the system accepting the credential. Some applications reject particular symbols or silently transform them. A password that cannot survive the complete provisioning path is not operationally useful, regardless of its entropy.

Make the safe path reusable

random_password() {
  local length="${1:-32}"

  LC_ALL=C tr -dc 'A-Za-z0-9!@#%^_=+.,:;?-' </dev/urandom \
    | head -c "$length"
  printf '\n'
}

random_password 40

Quoting the length variable and keeping configuration local make the function easier to reuse without unexpected shell behavior.

The controls around the command

The command is the easy part. Before using its output, consider where the secret can leak:

• Terminal scrollback, screen recordings, and shared sessions
• Clipboard history and synchronization
• Shell history if the value is pasted into a later command
• Process arguments visible to other users
• CI logs, configuration files, and source control

For production use, generate the value as close as possible to the destination and write it directly to an approved secrets store. Avoid printing it when the surrounding tool can accept standard input or a protected file descriptor.

Generation is only the first place a secret can leak. Pasting it into another command may leave a durable copy in Bash history, even when the original command was sound.

Lessons learned

The random string is the easy part. This command says nothing about who can retrieve the value, how it reaches the destination, when it expires, or how access will be recovered and audited.

I use it only when that narrow boundary is acceptable. A durable or shared credential needs evidence across its whole lifecycle: protected delivery, storage, use, rotation, revocation, recovery, and deletion. A complicated-looking password cannot make up for weak handling.

Sources and disclosures

• GNU Coreutils: sources of random data
• OWASP Secrets Management Cheat Sheet

This historical note reflects a general shell pattern and the author’s operating guidance. It is not a substitute for an organization’s approved cryptographic, identity, or secrets-management standard.