Bash split a space-delimited string into a variable number of substrings

Question

Suppose I have two space-delimited strings in my bash script, which are

permitted_hosts="node1 node2 node3"

and

runs_list="run1 run2 run3 run4 run5"

These respectively represent the list of permitted hosts and the list of runs to execute. So, I need to run each of the runs in $runs_list on 1 of the hosts in $permitted_hosts.

What I'd like to do is divide $runs_list into $N substrings, where $N is the number of elements in $permitted_hosts and where each of the $N substrings is mapped to a different element in $permitted_hosts.

If that's confusing, then consider this concrete workaround solution. For the exact given values of $permitted_hosts and $runs_list above, the following bash script checks the current host, and if the current host is in $permitted_hosts, then it launches the runs in $runs_list that are associated with the current host. Of course, this script doesn't use the variables $permitted_hosts and $runs_list, but it achieves the desired effect for the given example. What I'm really trying to do is modify the code below so that I can modify the values of variables $permitted_hosts and $runs_list and it will work appropriately.

#!/bin/bash
hostname=$(hostname)
if [ "$hostname" == "node1" ]; then
    runs="run1 run2"
elif [ "$hostname" == "node2" ]; then
    runs="run3 run4"
elif [ "$hostname" == "node3" ]; then
    runs="run5"
else
    echo "ERROR: Invoked on invalid host ('$hostname')! Aborting." 
    exit 0
fi

for run in $runs; do
    ./launch $run
done

ruakh · Accepted Answer · 2014-11-12 01:01:24Z

1

So, firstly — instead of space-delimited strings, you should probably use arrays:

permitted_hosts=(node1 node2 node3)
runs_list=(run1 run2 run3 run4 run5)

If you have to start out with space-delimited strings, you can at least convert them to arrays:

permitted_hosts=($permitted_hosts_str)
runs_list=($runs_list_str)

That out of the way . . . basically you have two steps: (1) convert the hostname into an integer representing its position in permitted_hosts:

hostname="$(hostname)"
num_hosts="${#permitted_hosts[@]}"      # for convenience

host_index=0
while true ; do
    if [[ "${permitted_hosts[host_index]}" = "$hostname" ]] ; then
        break
    fi
    (( ++host_index ))
    if (( host_index > num_hosts )) ; then
        printf 'ERROR: Invoked on invalid host ('%s')! Aborting.\n' "$hostname" >&2
        exit 1
    fi
done
# host_index is now an integer index into permitted_hosts

and (2) convert this integer into an appropriate subset of runs_list:

num_runs="${#runs_list[@]}"      # for convenience
for (( run_index = host_index ; run_index < num_runs ; run_index += num_hosts )) ; do
    ./launch "${runs_list[run_index]}"
done

So, for example, if you have H hosts, then host #0 will launch run #0, run #H, run #2H, etc.; host #1 will launch run #1, run #H+1, run #2H+1, etc.; and so on.

answered Nov 12, 2014 at 1:01

ruakh

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1 Comment

synaptik Over a year ago

Thanks a lot. I knew there had to be a better way :)

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