Returning a string comparison result - bash

Question

I'm writing a small function which compare md512sums of two files. I can't say that I'm good in bash, but I need to return a simple result commented part is long code. It asks for a reduction.

By the way, if someone have any ideas to improve my code, I'll be grateful.

function TestStage()
{
    local URL="distfiles.gentoo.org/releases/${1:7:5}/current-stage3/${1}"
    wget -q ${URL}.DIGESTS
    local SUM_WEB=`cat ${1}.DIGESTS | sed '2q;d'`
    local SUM_LOC=`openssl dgst -r -sha512 ${1}`
    ####### time to return
    return [[ "${SUM_WEB:0:128}" == "${SUM_LOC:0:128}" ]]

    #if [ "${SUM_WEB:0:128}" == "${SUM_LOC:0:128}" ]
    #then
    #    rm ${1}.DIGESTS
    #    return 0
    #else
    #    rm ${1}.DIGESTS
    #    return 1
    #fi
}

As you may have guessed $1 is stage3-<arch>-<release>.tar.bz2.

you can clarify the confusion by adding expected inputs, and correct format for your require output. If you're getting error messages, please include the actual message using the `{}` editing tool near the top-left of the input box (click `edit` at the bottom). Good luck. — shellter, Dec 28 '12 at 23:08

score 7 · Answer 1 · answered Dec 29 '12 at 11:10

Improvements:

TestStage() {
    local url sum_web sum_loc
    url="distfiles.gentoo.org/releases/${1:7:5}/current-stage3/${1}"
    wget -q "$url.DIGESTS"
    { read; read -r sum_web; } < "$1.DIGESTS"
    sum_loc=$(openssl dgst -r -sha512 "$1")
    ####### time to return
    [[ "${sum_web:0:128}" = "${sum_loc:0:128}" ]]
    return
}

Use of lower case variable names.
No use of the deprecated function keyword.
Use of $(...) instead of backticks.
Use of bash builtins instead of sed to get the second line of file "$1.DIGESTS". This saves a process spawn and a subshell (and a useless use of cat).
return on its own will return the return code of the previous statement, here the test statement.
Declare all local variables at once.

If you don't care about the file $1.DIGESTS that will be saved, you can also do:

TestStage() {
    local url sum_web sum_loc
    url="distfiles.gentoo.org/releases/${1:7:5}/current-stage3/${1}"
    { read; read -r sum_web; } < <(wget -q -O- "$url.DIGESTS")
    sum_loc=$(openssl dgst -r -sha512 "$1")
    ####### time to return
    [[ "${sum_web:0:128}" = "${sum_loc:0:128}" ]]
    return
}

Now, "${1:7:5}" will, as I understand it, expand to the second field of stage3-<arch>-<release>.tar.bz2 (where fields are separated by hyphens). You could also do:

IFS=- read _ arch _ <<< "$1"

In this case, your function would be:

TestStage() {
    local arch url sum_web sum_loc
    IFS=- read _ arch _ <<< "$1"
    url="distfiles.gentoo.org/releases/$arch/current-stage3/${1}"
    { read; read -r sum_web; } < <(wget -q -O- "$url.DIGESTS")
    sum_loc=$(openssl dgst -r -sha512 "$1")
    ####### time to return
    [[ "${sum_web:0:128}" = "${sum_loc:0:128}" ]]
    return
}

Hope this helps.

Then, use as:

if TestStage "stage3-<arch>-<release>.tar.bz2"; then
    # return value is true, proceed accordingly
else
    # return value is false, proceed accordingly
fi

+1 overall, but isn't `return` redundant in this case (but I'll grant you, self-documenting) ;-? Good luck to all. — shellter, Dec 29 '12 at 21:22
@shellter, sure it is redundant and useless, but maybe this will be inside another if statement or loop, I don't know, that's the way he asked the question (I thought). — gniourf_gniourf, Dec 29 '12 at 21:39
"Self-documenting"? Almost the reverse - until reading this answer, I would have assumed that a bare return would return 0, not "the return value of the previous line". — scubbo, Oct 30 '22 at 17:41

score 1 · Answer 2 · answered Dec 28 '12 at 23:45

I am not sure whether you could return comparison result like the way you done. You will most probably run into syntax error.

But you can try following syntax:

myfunction()
{
    [ "test" = "test" ]
}

myfunction
echo $?    # here if you get 0 that means strings are equal otherwise 1

In case you do decide to follow this approach, make sure comparison statement is the last statement in your function.

score 0 · Answer 3 · edited May 23 '17 at 11:46

0

You can't return anything other than integer values. You can either go with Ashish's suggestion and retrieve an exit code from $? after the function call or you could evaluate your string comparison expression by surrounding it with if ; then and then just echo any value indicating that the two hashes are either identical or not.

edited May 23 '17 at 11:46

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answered Dec 29 '12 at 00:29

J. Katzwinkel

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Returning a string comparison result - bash

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