How to add elements to an array in Bash on each iteration of a loop?

Question

I have this depends file in my $PATH:

#!/bin/bash
k=0
for i in "$@"
do
  DP[k]="nodejs-$i"
  k=$((k+1))
done

echo $DP

I ran depends js kd and it returned:

nodejs-js

this surprised me as I thought that the result I would get would be:

nodejs-js nodejs-kd

as the loop was meant to be adding new elements to the DP array of form nodejs-$i where $i is the input I provided to the depends script when I ran it. I have tried using this depends script instead:

#!/bin/bash
DP=()
for i in "$@"
do
  DP+=("nodejs-$i")
done

echo $DP

but it returned the exact same result, with the js kd inputs (i.e., the output was nodejs-js).

Jay jargot · Accepted Answer · 2016-04-06 00:36:40Z

7

Your script is correctly adding elements into the array, check how to read the DP array below.

Give this a try:

#!/bin/bash
k=0
for i
do
  DP[k]="nodejs-$i"
  k=$((k+1))
done

printf "DP array size is %d\n" "${#DP[@]}"
printf "%s " "${DP[@]}"
printf "\n"

--edited-- Note that new applications are encouraged to use printf instead of echo.

Funny => By default the for statement loops over the script's arguments.

The test:

$ ./depends js kd
DP array size is 2
nodejs-js nodejs-kd

edited Apr 6, 2016 at 0:36

answered Apr 6, 2016 at 0:03

Jay jargot

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4 Comments

Mark Reed Over a year ago

It should be "${DP[@]}" - with the quotation marks - not ${DP[*]}, in general. The former preserves any internal whitespace, while the latter does not. Also, echo is not deprecated. If you are explicitly using bash, as here, its behavior is consistent. When using unspecified varieties of /bin/sh, echo has inconsistencies not present with printf - but then, printf itself may not exist at all in such shells, so it's not really an improvement.

Jay jargot Over a year ago

@MarkReed thx! array notation fixed! According to the APPLICATION USAGE section of the POSIX standard for echo, the printf built-in command is a preferred replacement for echo .

Mark Reed Over a year ago

That guideline is for POSIX. The code given is explicitly for #!/bin/bash, which is already not POSIX compliant - its echo takes options, including -n.

chepner Over a year ago

It's actually simpler to use the += operator to append to the array, rather than manually updating the index k. DP+=( "nodejs-$i" ). (Assuming you don't need to support bash 3.0 or earlier.)

pjh · Accepted Answer · 2016-04-06 00:03:17Z

3

The problem is that Bash treats $DP as if it was ${DP[0]}. You need

echo "${DP[@]}"

answered Apr 6, 2016 at 0:03

pjh

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