Export serialized array in bash

Question

I'm trying to serialize an array in bash and then export it:

function serialize
{
    for i in ${1[@]}; do
      ret+=$i" "
    done
    return ${ret::-1}
}

MEASUREMENT_OUTPUT_FILES=( t1DQ.txt t1CS.txt t2RXe.txt t2e.txt )
export MEASUREMENT_OUTPUT_FILES=${serialize MEASUREMENT_OUTPUT_FILES[@]}

The code produces the following error:

MEASUREMENT_OUTPUT_FILES=${serialize MEASUREMENT_OUTPUT_FILES[@]}: bad substitution

Any ideas what the correct syntax (error in the last line starting with export) would be?

That's not a safe serialization form for an array. Array elements (and filenames) are perfectly legal to contain spaces. — Charles Duffy
– Charles Duffy, Commented Jun 23, 2017 at 16:27
Also, if you did want to use that form, you could just use string=${array[*]}. — Charles Duffy
– Charles Duffy, Commented Jun 23, 2017 at 16:27
(By the way -- the function keyword here is a ksh-ism that isn't compatible with POSIX sh; bash supports it for compatibility, but the preferred syntax is the POSIX-compliant one, serialize() { with no function preceding). — Charles Duffy
– Charles Duffy, Commented Jun 23, 2017 at 16:29

ruakh · Accepted Answer · 2017-06-24 01:52:09Z

2

I believe you want:

export MEASUREMENT_OUTPUT_FILES=$(serialize "${MEASUREMENT_OUTPUT_FILES[@]}")

(where $(...) is a notation for command substitution).

That said, your command is actually equivalent to

export MEASUREMENT_OUTPUT_FILES="${MEASUREMENT_OUTPUT_FILES[*]}"

so you don't need the serialize function unless you want to improve your serialization logic. (Which you should consider doing, IMHO: just joining with a space is error-prone, because what if one of the arguments includes a space?)

Edited to add: Also, I don't know how we all missed this before, but this:

    return ${ret::-1}

actually needs to be this:

    echo "${ret::-1}"

or this:

    printf %s "${ret::-1}"

since return is for setting the exit status of a function, which must be an integer. (It's intended for indicating success, zero, vs. failure, nonzero, though some commands assign special meanings to multiple nonzero values.) What you want is for your function to "print" the files, so you can capture them.

edited Jun 24, 2017 at 1:52

answered Jun 23, 2017 at 16:26

ruakh

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3 Comments

Charles Duffy Over a year ago

Why call serialize or any other function to concatenate array elements with spaces? ${MEASUREMENT_OUTPUT_FILES[*]} will do, as will printf -v output_string '%s ' "${MEASUREMENT_OUTPUT_FILES[@]}" if one doesn't trust the value of IFS.

ruakh Over a year ago

@CharlesDuffy: I noticed that, too (thanks to your first comment to the OP), and was updating my answer just as you were commenting on it. :-)

chepner Over a year ago

Even if you don't trust IFS, you can override it in a function; serialize M a b c with serialize () { var=$1; shift; local IFS=" "; printf -v "$var" '%s' "$*"; } would set M="a b c" regardless of the global value of IFS.

Sam Liddicott · Accepted Answer · 2023-08-11 10:22:35Z

1

bash has built-in variable serialization into bash-code using declare -p, and eval de-serializes, including arrays.

For this array:

a=("a b" "c'd")

serialize a into A

A=$(declare -p a)
echo "$A"
declare -a a=([0]="a b" [1]="c'd")

to de-serialize:

eval "$A"

to de-serialize into a different array name b

eval "declare -a b=${A#*=}"

answered Aug 11, 2023 at 10:22

Sam Liddicott

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