Is device mounted? Both UUID and device names accepted

Question

I am trying to write an is_device_mounted script, which in turn will serve a greater purpose in my home Linux system.

It does not even have an error reporting function included and as you can see, I have made it clean for readers. My intention here is to review the code for general Linux. But if you are on a *BSD, I would appreciate your feedback too!

---

The first version of the script follows:

#!/bin/sh

set -eu

is_empty_string() { [ -z "${1}" ]; }
sanitize_device_string() { printf '%s' "${1}" | grep '^/dev/' | head -n 1; }
is_block_device() { [ -b "${1}" ]; }
is_device_uuid_identified() { printf '%s' "${1}" | grep -F '/by-uuid/'; }
translate_uuid_to_device_name() { readlink -f -n /dev/disk/by-uuid/"${1}"; }

is_device_mounted()
{
    if [ -n "${device_name}" ]; then
        # 1. basic regex should be working across platforms
        #    tested on FreeBSD, OpenBSD, NetBSD with success
        #    I prefer the starting with (^) rather than filtering through all text
        # 2. /proc/mounts is not available on all *BSDs, needs revision
        proc_mounts=$( grep "^${device_name} " /proc/mounts )
        [ -n "${proc_mounts}" ]
    fi
}

[ "${#}" -ne 1 ] && { echo "Invalid number of arguments."; exit 1; }
readonly raw_input_string=${1}

is_empty_string "${raw_input_string}" && { echo "The given argument is empty."; exit 1; }
readonly device_string=$( sanitize_device_string "${raw_input_string}" )

is_empty_string "${device_string}" && { echo "The given argument is not a device path."; exit 1; }
! is_block_device "${device_string}" && { echo "The given argument is not a block device."; exit 1; }
readonly block_device=${device_string}

if is_device_uuid_identified "${block_device}"
then
    readonly device_name=$( translate_uuid_to_device_name "${block_device}" )
else
    readonly device_name=${block_device}
fi

if is_device_mounted "${device_name}"
then
    echo "The device: ${block_device} IS mounted."
else
    echo "The device: ${block_device} IS NOT mounted."
fi

Answer 1

Nice work. I approve of set -eu, and the script pleases Shellcheck.

Here's the things I'd consider changing.

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I think error messages should go to the error stream. Example:

[ "${#}" -ne 1 ] && { echo "Invalid number of arguments." >&2; exit 1; }
#                                                         ^^^ here

---

Instead of using the negation operator, I'd replace the form ! test && error with plain test || error like this:

is_block_device "${device_string}" || { echo "The given argument is not a block device." >&2; exit 1; }

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The script doesn't work when I use other links to block devices, such as those in /dev/disk/by-label. I'd fix that by abandoning the /by-uuid/ test, and instead following symlinks until a real file or dangling link is found:

resolve_symlink() {
    f="$1"
    while [ -h "$f" ]
    do f=$(readlink -f -n "$f")
    done
    printf '%s' "$f"
}

is_empty_string "${device_string}" && { echo "The given argument is not a device path." >&2; exit 1; }
is_block_device "${device_string}" || { echo "The given argument is not a block device." >&2; exit 1; }

readonly device_name=$(resolve_symlink "$device_string")

if is_device_mounted "$device_name"
then

---

Why does is_device_mounted ignore its argument and use $device_name instead?

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Minor issue: we assume that the block device name contains no regex metacharacters here:

grep "^${device_name} "

That's probably a fair assumption on a non-weird Linux system; I normally use Awk for robust versions such tests ($1 = $device_name, with a suitable -v option) but I don't know how well that meets your portability goals.

If using grep (without the non-standard -q option), then it's usual to discard the output, and use grep's exit status directly, rather than capturing the output and testing it's non-empty.

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Minor/style: I'm not a big fan of using braces for every variable expansion. I prefer to reserve them for when they are really needed, and that seems to be the usual idiom.