Portable way to find the number of processors/CPU's in a shell script?

Question

Since it's common to write shell scripts which pass the number of jobs to a command, I'm interested to know what a good, portable way is to get the number of processors on mainstream * Unix systems.

Something like this but not depending on Python.

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* By mainstream, I mean that will work on popular Unix system used in production today (Linux/BSD's/Darwin? But the more portable the better).

Answer 1

Here is a fairly portable function to get the number of processors that works in sh, bash and compatible shells:

• Uses nproc on Linux.
• Use getconf as a fallback, it's and part of coreutils.
• Tested to work on:
• Linux
• Darwin (macOS)
• FreeBSD, NetBSD, OpenBSD

... probably others, feel free to test :)

Feel free to suggest additions:

#!/bin/sh
portable_nproc() {
    OS="$(uname -s)"
    if [ "$OS" = "Linux" ]; then
        NPROCS="$(nproc --all)"
    elif [ "$OS" = "Darwin" ] || \
         [ "$(echo "$OS" | grep -q BSD)" = "BSD" ]; then
        NPROCS="$(sysctl -n hw.ncpu)"
    else
        NPROCS="$(getconf _NPROCESSORS_ONLN)"  # glibc/coreutils fallback
    fi
    echo "$NPROCS"
}

# test
portable_nproc

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A more terse command that covers many systems is to check getconf for glibc, then sysctl for BSD family of Unixes: eg:

getconf _NPROCESSORS_ONLN 2>/dev/null || sysctl -n hw.ncpu

I have a slight preference for checking each platform since it allows others to be added more easily, but in practice the single line works in many cases.

Answer 2

I'll throw in my one-liner heavily influenced by ideasman42's answer above:

nproc 2>/dev/null || sysctl -n hw.ncpu 2>/dev/null || getconf _NPROCESSORS_ONLN 2>/dev/null