Q:

Syntax error: invalid arithmetic operator (error token is ".")

I have to search word occurrences in a plain text file using a script. The script that I wrote is:
#!/bin/bash

if [ $# -ne 1 ]
then
        echo "inserire il file che si vuole analizzare: "
        read
        fileIn=$REPLY
else
        fileIn=$1
fi

echo "analisi di $fileIn"

for parola in $( cat $fileIn )
do
        freq=${vet[$parola]}
        vet[$parola]=$(( freq+1 ))
done

for parola in ${!vet[*]}
do
        echo $parola ${vet[$parola]}
done

unset array

But when I run it I get this error:
./script02.sh: line 16: qua.: syntax error: invalid arithmetic operator (error token is ".")

What's wrong and how do I fix it? Thanks.

A:

In the first attempt, you don't get a value from ${vet[$parola]}. This causes the syntax error for the arithmethic operation Use a default value instead:
for parola in $(cat "$fileIn")
do
    freq=${vet[$parola]}
    [[ -z $freq ]] && freq=0
    vet[$parola]=$(( freq + 1 ))
done

You might also need to declare your array as associative if you want to store non-integer keys:
declare -A vet

for ...

Suggestion:
#!/bin/bash

if [[ $# -ne 1 ]]; then
    read -p "inserire il file che si vuole analizzare: " fileIn
else
    fileIn=$1
fi

echo "analisi di $fileIn"

declare -A vet  ## If you intend to use associative arrays. Would still work in any way.

for parola in $(<"$fileIn"); do
    freq=${vet[$parola]}
    if [[ -n $freq ]]; then
        vet[$parola]=$(( freq + 1 ))
    else
        vet[$parola]=1
    fi
done

for parola in "${!vet[@]}"; do
    echo "$parola ${vet[$parola]}"
done

unset vet  ## Optional

