How to create a list and append values in bash script

Question

I'm having a while loop and need to append the outcome of the loop to the list(like in python) in bash script. How to do that. I created an array but there are no commas in it. It has only space but I want commas so that I can use them in the .env file

arr=()
while [ ]
do 
  ........
  ........
  ........
  val='some value'
  arr+=$val
done

echo ${arr}

output:

('some value1' 'some value2' 'some value3')

Expected Outcome:

['some value1','some value2','some value3']

score 3 · Accepted Answer · answered Nov 05 '21 at 10:44

3

Here one idea/approach.

#!/usr/bin/env bash

arr=()

while IFS= read -r val; do
  arr+=("'$val'")
done < <(printf '%s\n' 'foo bar' 'baz more' 'qux lux')

(IFS=,; printf '[%s]' "${arr[*]}")

Output

['foo bar','baz more','qux lux']

answered Nov 05 '21 at 10:44

Jetchisel

7,493
2
19
18

In last line, you may remove `()` and `;` : `IFS=, printf...` – Edouard Thiel Nov 05 '21 at 10:58
1

@EdouardThiel, thank you for that wonderful comment. but have you tried what you suggested though? – Jetchisel Nov 05 '21 at 11:04

Edouard Thiel · Answer 2 · 2021-11-05T10:52:16.120

2

Here is a solution using echo :

arr=()
while [ ... ]
do 
  ........
  ........
  ........
  val='some value'
  arr+=("$val")              # () and "" added
done

# print each value protected by "" and separated by ,
first=true
echo -n "arr values = [ "
for v in "${arr[@]}" ; do
    if $first ; then first=false ; else echo -n ", " ; fi
    echo -n "\"$v\""
done
echo " ]"

# Another way to print arr values
declare -p arr

To write the array values in a python file, you may try this :

dest="somefile.py"
{
    first=true
    echo -n "arr = [ "
    for v in "${arr[@]}" ; do
        if $first ; then first=false ; else echo -n ", " ; fi
        echo -n "\"$v\""
    done
    echo " ]"
} >| "$dest"

Note that it is fragile, it won't work if v contains ". A solution is given here with operator @Q (search Parameter transformation" in man bash):

dest="somefile.py"
{
    first=true
    echo -n "arr = [ "
    for v in "${arr[@]}" ; do
        if $first ; then first=false ; else echo -n ", " ; fi
        echo -n "${v@Q}"    # operator @Q
    done
    echo " ]"
} >| "$dest"

edited Nov 05 '21 at 10:52

answered Nov 05 '21 at 10:06

Edouard Thiel

5,878
25
33

Hi @Thiel, how to write array so that it will appear VARIABLE_VALUE=['some value1','some value2','some value3'] format in .env file. Thanks for the answer. – swarna Nov 05 '21 at 10:14
@swarna I have added a snippet for that; you may update the original question. – Edouard Thiel Nov 05 '21 at 10:27
`echo -n` works fine in Bash, but the preferred and recommended solution is really to use `printf` which is coincidentally also portable. – tripleee Nov 05 '21 at 10:45
@treplee echo is simpler to use – Edouard Thiel Nov 05 '21 at 10:47

tripleee · Answer 3 · 2021-11-05T10:44:29.777

1

You have a number of issues here, starting from your expected output. The command echo ${arr} suffers from broken quoting and only prints the first element of the array anyway; the output you expect looks like how e.g. Python represents a list, but Bash would simply print the values themselves in whichever format you specify, without any explicit formatting you didn't put there yourself.

To print an array, try

printf '%s\n' "${arr[@]}"

To print an array with explicit formatting around each value, try e.g.

printf "['%s'" "${arr[0]}"
printf ",'%s'" "${arr[@]:1}"
printf ']\n'

Now, to build the array properly in the first place, you have to add to the array, not to the first element.

arr=()
while true;  # not while [ ]
do 
  ........
  arr+=('some value')  # use parentheses
done

To add VARIABLE_VALUE= before the output, just ... do that.

printf "VARIABLE_VALUE=['%s'" "${arr[0]}"
printf ",'%s'" "${arr[@]:1}"
printf ']\n'

edited Nov 05 '21 at 10:44

answered Nov 05 '21 at 09:46

tripleee

175,061
34
275
318

Hi @triplee, how to write array so that it will appear **VARIABLE_VALUES=['some value1','some value2','some value3']** format in .env file. Thanks for the answer. – swarna Nov 05 '21 at 09:52
Your question seems to imply that you already figured out that part. In brief, `VARIABLE_VALUES=('some value1' 'some value2' 'some value3')` and then use the code in this answer, obviously replacing `arr` with `VARIABLE_VALUES`. (Though you'll generally want to [avoid upper case for your private variables.)](https://stackoverflow.com/questions/673055/correct-bash-and-shell-script-variable-capitalization) – tripleee Nov 05 '21 at 09:55
Unless this `.env` file is consumed by Python, the format you say you want is probably not going to work. If it's going to be consumed by a Bourne shell (not Bash) you simply do not have access to array variables. – tripleee Nov 05 '21 at 09:56
This smacks of an [XY Problem](https://en.wikipedia.org/wiki/XY_problem) anyway. Probably (accept this answer, or post one of your own and accept that instead if you prefer, and) ask a new question with your _actual_ requirements if the problem you are trying to solve is actually different. – tripleee Nov 05 '21 at 09:57

How to create a list and append values in bash script

3 Answers3