Issue with associative array in Bash script

Question

I was trying to demonstrate the use of an associative arrary to gather counts. It is working in the while loop, but it appears to have lost all its information after the while loop. What am I doing wrong? All echo statements except for the one in the for loop have been added for debugging.

#!/bin/bash

declare -A shell_type

shells=$(cut -d: -f7 /etc/passwd)
echo "$shells"|
while read sh
do
  if [ -z "${shell_type[$sh]}" ]
  then
    shell_type[$sh]=1
  else
    let "shell_type[$sh] += 1"
  fi
  echo "$sh ${shell_type[$sh]}"
  echo "${!shell_type[*]}"
  echo "${shell_type[*]}"
done

echo "${shell_type[*]}"

for var in "${!shell_type[*]}"
do
  echo "$var count is ${shell_type[$var]}"
done

If you are using 4.2 or later, you can use shopt -s last pipe to allow the last command of a pipeline to execute in the current shell (not a subshell), which eliminates your problem. — chepner
– chepner, Commented Dec 5, 2013 at 14:22

janos · Accepted Answer · 2013-12-05 11:47:22Z

1

Do not pipe to your while loop, write it like this:

while read sh
do
  ...
done < <(cut -d: -f7 /etc/passwd)

If you do cmd | while then the while is in a different process. It inherits your local variables but it cannot modify them. By using input redirection, the while stays within the current process.

Btw, the first < there is for input redirection, and the <(...) after that is process substitution. You need a space in between otherwise the shell cannot separate these operators. You can read more about process substitution in man bash in the Process Substitution section.

edited Dec 5, 2013 at 11:47

answered Dec 5, 2013 at 5:43

janos

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3 Comments

user2836927 Over a year ago

Thank you! I also found I needed to remove the quotes around ${!shell_type[*]}; otherwise it was treated as a single value with embedded spaces. One follow-up question: The two less than (<) signs must have a space between them. What is this syntax?

janos Over a year ago

Good question! I added the explanation to my answer.

user2836927 Over a year ago

I believe I answered my own question. The first < is redirection, the second is part of <(), which is process substitution. I found this is the Advanced Bash Scripting Guide by Mendell Cooper

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