How to loop through character in string and still detect null char in Bash

Question

I have this function:

function convert_ascii_string_to_decimal {
    ascii=$1
    unset converted_result

    while IFS="" read -r -n 1 char; do
        decimal=$(printf '%d' "'$char")
        echo $decimal
        converted_result="$converted_result $decimal"
    done < <(printf %s "$ascii")
    converted_result=$(echo $converted_result | xargs) #strip leading and trailing
}

It is meant to take an ascii string variable, loop through every character, and concatenate the ascii decimal representation to a string. However, this while loop seems to ignore null chars, ie characters with ascii 0. I want to be able to read every single ascii there is, including null.

oliv · Accepted Answer · 2018-11-14 08:56:06Z

3

To get all characters of a string as decimal number, you can use hexdump to parse a string:

 echo -e "hello \x00world" | hexdump -v -e '1/1 "%d "'
 104 101 108 108 111 32 0 119 111 114 108 100 10

This also works for parsing a file:

echo '05 04 03 02 01 00 ff' | xxd -r -ps  > file
hexdump --no-squeezing --format '1/1 "%d "' file 
5 4 3 2 1 0 255

hexdump explanation:

options -v and --no-squeezing prints all bytes (without skipping duplicated bytes)
options -e and --format allows giving a specific format
format is 1/1 "%d " which means
- Iteration count = 1 (process the byte only once)
- Byte count = 1 (apply this format for each byte)
- Format = "%d" (convert to decimal)

edited Nov 14, 2018 at 8:56

answered Nov 13, 2018 at 17:00

oliv

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5 Comments

Daily Memes Over a year ago

your first solution gives this error: hexdump: illegal option -- - usage: hexdump [-bcCdovx] [-e fmt] [-f fmt_file] [-n length] [-s skip] [file ...] hd [-bcdovx] [-e fmt] [-f fmt_file] [-n length] [-s skip] [file ...]

oliv Over a year ago

@DailyMemes I updated the answer with using short (instead of long) option.

stack0114106 Over a year ago

@olive.. could you pls add explanation

stack0114106 Over a year ago

the options -v-e and the format 1/1.. why specifically 1/1

Daily Memes Over a year ago

but there is a 10 at the end? I don't want the 10

max · Accepted Answer · 2018-11-13 16:49:29Z

1

You can't store the null character in a bash variable, which is happening in your script with the $char variable.

I suggest using xxd instead of writing your own script:

echo -ne "some ascii text" | xxd -p

If we echo a null charcter:

$ echo -ne "\0" | xxd -p
00

answered Nov 13, 2018 at 16:49

max

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2 Comments

Daily Memes Over a year ago

also the problem with this is if there is no stdout, it still reads it as null char. Is it possible to detect whether there was an stdout? or is the lack of stdout represented by null?

Gordon Davisson Over a year ago

@DailyMemes I don't understand your question about what happens "if there is no stdout". In that case, I get nothing, not a null. Try cat /dev/null | xxd -p -- that gives me no output at all. Same with @oliv's hexdump answer, BTW.

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