Bash scripting: filling up an Array inside a for loop

Question

I try to create an array that is filled up with some values. But i don't know how to do it. I tried something below but it didn't work.

My code:

i=0
for c in colors; do
array[$i]=$c
echo {$c[$i]}
i=`expr $i + 1`
done

note: "colors" is some kind of "ps -ef" command that returns a list of values. it has "blue,red,yellow" values for example.

colors= 'ps -ef | grep colors'

Exactly how the colors list is returned is important. If they're returned one-to-a-line, there are more reliable ways to get them out than string-splitting (which is what you're doing, and which is what your accepted answer does, which will change, for instance, a color * to a list of files in the current directory, or a color light magenta to two entries, light and magenta). — Charles Duffy
– Charles Duffy, Commented Feb 14, 2014 at 13:33
By the way, in bash, (( ++i )) is the modern way to write i=$(expr $i + 1). — Charles Duffy
– Charles Duffy, Commented Feb 14, 2014 at 13:35
...and the only thing completely nonfunctional with your code (as opposed to best-practices violations) was writing {$c[$i]} rather than ${c[$i]} and in colors rather than in $colors. — Charles Duffy
– Charles Duffy, Commented Feb 14, 2014 at 13:37
...also, as mywiki.wooledge.org/ProcessManagement documents, you should prefer the pgrep command rather than grepping the output of ps. — Charles Duffy
– Charles Duffy, Commented Feb 14, 2014 at 13:43

anubhava · Accepted Answer · 2014-02-14 13:29:51Z

5

You can use this script to fill up the array in a loop:

array=()
for c in $colors; do
    array+=( "$c" )
done

OR even simpler:

array=( $(command) )

edited Feb 14, 2014 at 13:29

answered Feb 14, 2014 at 13:21

anubhava

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4 Comments

Charles Duffy Over a year ago

If we assume that string-splitting is a given, yes. The best-practices approach if the input is coming in one-to-a-line would be to use a while/read loop to split out the elements -- that way we don't have glob expansion, IFS-based splitting, or other such side effects.

AloneInTheDark Over a year ago

is there a example about your "simpler" command? is it like "array=($("ps -ef|grep pmon"))"?

Charles Duffy Over a year ago

@AloneInTheDark, no quotes; just array=( $(ps -ef | grep ...) ) -- but it's a bad practice for several reasons.

anubhava Over a year ago

@AloneInTheDark: When you use array=( $(command) ) then each element is delimited by space or new lines. So if your element itself contains spaces then that will be broken into multiple array elements. There are ways to handle this; e.g. using IFS appropriately as in Charles's answer.

Charles Duffy · Accepted Answer · 2014-02-14 13:40:07Z

5

If your colors are coming in one-to-a-line, you're potentially losing information (dropping the distinction between spaces and newlines, expanding glob characters, etc) as soon as you expand the variable that stores them unquoted.

Don't do that. Instead, follow BashFAQ #1:

colors=()
while IFS='' read -r color; do
    colors+=( "$color" )
done < <(get-color-list)

That's even more true if you only want, say, a specific column out of each line; read will do the column-splitting for you, making it easier to assign.

answered Feb 14, 2014 at 13:40

Charles Duffy

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