如何用POSIX兼容Shell工具生成8位字母数字字符串？

Great question—you’re absolutely right to prioritize POSIX compliance and security here. A lot of commonly shared random string scripts rely on non-standard tools or parameters (like head -c) and overlook byte vs. character stream differences, which can introduce both compatibility and security issues.

Let’s Break Down Your Current Command #

Your implementation is POSIX-compliant (excluding /dev/urandom, as you noted) and addresses the byte/character stream concern effectively:

strings -n 1 < /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 8 | head -n 1

• strings -n 1: POSIX specifies the -n flag to set the minimum length of printable character sequences, so this safely extracts printable bytes from /dev/urandom without relying on non-standard options.
• tr -dc 'a-zA-Z0-9': Filters out all non-alphanumeric characters, ensuring we only work with the desired character set.
• fold -w 8: Splits the filtered output into lines of exactly 8 characters (POSIX-compliant, as fold’s -w is standard).
• head -n 1: Grabs the first complete 8-character line—unlike head -c, -n is POSIX-standard.

This approach avoids truncating multi-byte characters (since strings and fold operate on character streams, not raw bytes) and uses only POSIX-defined flags, so it will work across all systems that support /dev/urandom (which you correctly noted covers most major Unix-like OSes).

A More Efficient POSIX-Compliant Alternative #

Your command works well, but strings can be inefficient because it skips large chunks of non-printable data. A more direct approach uses od to read raw bytes and map them to alphanumeric characters, reducing unnecessary processing:

od -An -tu1 < /dev/urandom | awk '{
    for (i = 1; i <= NF; i++) {
        if ($i < 62) {  # 62 = total alphanumeric chars (26+26+10)
            printf "%c", $i < 26 ? $i + 65 : $i < 52 ? $i + 71 : $i - 4
        }
    }
}' | head -n 8

Here’s how it works:

• od -An -tu1: Outputs raw bytes from /dev/urandom as decimal numbers, with no address prefix (POSIX-compliant flags).
• awk: Filters bytes to only those in the 0–61 range (matching our 62-character set), then maps each byte to:
  • 0–25 → A–Z (add 65 to get the corresponding ASCII value)
  • 26–51 → a–z (add 71: 26+71=97, the ASCII code for 'a')
  • 52–61 → 0–9 (subtract 4: 52-4=48, the ASCII code for '0')
• head -n 8: Takes the first 8 characters (since awk prints them sequentially with no newlines).

This method is faster because it doesn’t discard non-printable bytes—it directly converts valid bytes to the desired character set.

Generating Random Strings Without /dev/urandom #

POSIX does not define a standard system random source, so any alternative will be less secure (since it relies on predictable inputs) or system-specific. For non-security-critical use cases, you can combine timestamp, process IDs, and environment data:

echo "$(date +%s%N)$$${PPID}$$USER$HOME" | sha256sum | tr -dc 'a-zA-Z0-9' | head -n 8

• This uses a hash of predictable but variable data (current time in nanoseconds, process ID, parent process ID, username, home directory) to generate a pseudo-random string.
• Important: This is not cryptographically secure—use it only for non-sensitive purposes like temporary file names, not for passwords or authentication tokens.

For cryptographically secure random strings without /dev/urandom, you’d need to rely on system-specific tools (like arc4random on BSD/macOS), but these are not POSIX-standard.

内容的提问来源于stack exchange，提问作者Harold Fischer