Accessing Array through for loop in Linux Shell script

Question

I have the following shell script. The problem I am facing is that in the first iteration ${NUM[${i}]} returns the whole content of the array and every following iteration returns empty. Why is that so?

NUM="2 2 2 0"
i=0
    for CLIENT in ${BAG_CLIENTS} ; do
        if [ "$BFT" = true ]; then
          SERVERS="-$i"
        fi
        echo "Starting ${NUM[${i}]} clients with $PERCWRITES writes and $SERVERS serv$
        ssh rneiheiser@$CLIENT "~/runclient.sh $i ${NUM[${i}]} $PERCWRITES $SERVERS &$
        ((i++))
    done

And that's a really buggy way to pass arguments to ssh, though addressing the issue would be out-of-scope of the individual question. Basically -- ssh munges all its arguments together into a single string, and passes that string for execution by a remote shell. If you want to maintain control over how things are parsed by that remote shell, the proper practice is to build an single eval-safe string locally and pass it as a single argument. — Charles Duffy
– Charles Duffy, Commented Sep 12, 2017 at 16:19
BTW, all-caps variable names are used for variables with meaning to the OS and shell -- your own variables should use names with at least one lowercase character to avoid conflicts. See relevant POSIX spec @ pubs.opengroup.org/onlinepubs/9699919799/basedefs/…, fourth paragraph, reserving lowercase names for application use. — Charles Duffy
– Charles Duffy, Commented Sep 12, 2017 at 16:25
(When I say "buggy", that includes security bugs -- if you're passing, say, a comment string, a password, a filename, &c. provided by a user as one of your arguments, you don't want any $(rm -rf ~) inside that comment to be parsed by the remote shell as code). — Charles Duffy
– Charles Duffy, Commented Sep 12, 2017 at 16:28

Charles Duffy · Accepted Answer · 2017-09-12 16:26:54Z

7

You aren't defining an array at all -- you're defining a string.

When an array is accessed as a string, only its first item is returned. That is to say:

array=( "first item" "second item" )
echo "$array" ## echos only "first item"

When a string is accessed as an array, it acts like a single-element array, with only its first element.

string="first item"
echo "${string[0]}" # also echos "first item"

If you want to define an array and access it as an array, the syntax looks like:

array=( "first item" "second item" "third item" )
echo "${array[0]}" # "first item", again.

# iterate over each element in turn
for item in "${array[@]}"; do
  echo "Iterating over array element $item"
done

...after which you can either expand to all the items with "${array[@]}", iterate over the indices with "${!array[@]}", or access an individual one with ${array[$idx]} (where $idx is your index number).

edited Sep 12, 2017 at 16:26

answered Sep 12, 2017 at 16:18

Charles Duffy

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5 Comments

raycons Over a year ago

is there also a way to retrieve the data from the string in a similar manner like that? I'd like to receive this string as a command line argument

Charles Duffy Over a year ago

You can read it into an array. read -r -a arrayname <<<"$string"

Charles Duffy Over a year ago

...that said, that's only a reasonable approach if the restrictions on your array's potential contents are such that you have separator character(s) that can't possibly appear inside the elements themselves. Otherwise, you'll want a separate command-line argument for each element. It's common practice to pass -n foo -n bar to specify two array elements, foo and bar, for instance.

KargWare Over a year ago

the array syntax does only work in bash (/bin/bash) not in sh (/bin/sh)

Charles Duffy Over a year ago

@KargWare, right; sh doesn't support arrays at all. If one wants arrays, it's obligatory to use a shell (like ksh, bash, or zsh) that actually supports arrays.

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