unable to get expected result shell array

Question

#!/bin/sh
NR=0
getent passwd | while IFS=':' read USERNAME PSSWD MY_UID MYGID GECOS HOMEDIRS SHELLS
do
users[$NR]=$USERNAME
((NR++))
done
echo "${users[*]}"

if I execute above with ksh replacing sebang it works, but with bash it does not.

Answer 1

Both sh and bash create a subshell after the pipe.

In other words, while block inherits the variables from the parent shell but modification won't affect the variables in the parent shell.

ksh is not creating a subshell, so it is working.

In my tests, the script provided does not run in pure sh.

A simple sh solution would be to use /etc/passwd directly:

#!/bin/sh --
NR=0
users=""
while IFS=':' read USERNAME PSSWD MY_UID MYGID GECOS HOMEDIRS SHELLS ; do 
  users="${users} ${USERNAME}" 
  NR=$(expr "${NR}" + 1) 
done < /etc/passwd
printf "%s\n" ${users} 

There is below a solution using bash (with @pjh hint):

#!/bin/bash --
NR=0
passwdentries="$(getent passwd)"
if [ $? -ne 0 ] ; then
    exit 1
fi
while IFS=':' read USERNAME PSSWD MY_UID MYGID GECOS HOMEDIRS SHELLS ; do 
   users[$NR]=$USERNAME 
   ((NR++)) 
done <<<"${passwdentries}"
printf "%s\n" ${users[*]} 

In order to illustrate this subshell situation, the version below prints all the users, but still the values of the variables are unchanged in parent shell (with @glennjackman hint):

#!/bin/bash --
NR=0
getent passwd | (
  while IFS=':' read USERNAME PSSWD MY_UID MYGID GECOS HOMEDIRS SHELLS ; do
    users[$NR]=$USERNAME
    ((NR++))
  done
  printf "%s\n" ${users[*]}
  )