Unable to add element to array in bash

Question

I have the following problem. Let´s assume that $@ contains only valid files. Variable file contains the name of the current file (the file I'm currently "on"). Then variable element contains data in the format file:function.

Now, when variable element is not empty, it should be put into the array. And that's the problem. If I echo element, it contains exactly what I want, although it is not stored in array, so for cycle doesn't print out anything.

I have written two ways I try to insert element into array, but neither works. Can you tell me, What am I doing wrong, please?

I'm using Linux Mint 16.

#!/bin/bash

nm $@ | while read line
do
  pattern="`echo \"$line\" | sed -n \"s/^\(.*\):$/\1/p\"`"
  if [ -n "$pattern" ]; then
    file="$pattern"  
  fi
  element="`echo \"$line\" | sed -n \"s/^U \([0-9a-zA-Z_]*\).*/$file:\1/p\"`"
  if [ -n "$element" ]; then
    array+=("$element")
    #array[$[${#array[@]}+1]]="$element"
    echo element - "$element"
  fi
done

for j in "${array[@]}"
do
  echo "$j"
done

There are options to nm that print the file name on each line (usually -r or -R, IIRC). That would save you most of the manipulation work you're doing. — Jonathan Leffler
– Jonathan Leffler, Commented Mar 27, 2014 at 15:04
use shell trace debug feature set -vx (or simplified output) with set -x. You'll see how the variables are being evaluated, and then often very easy to fix. Also, join the 90's and stop using back-tics for command substitution. var=$(cmd_produces_output), is not 100% portable, but much easier to read and nest. Finally, only use dbl-quotes on sed cmds when you need variable expansion inside the the cmd. use single-quotes normally, and then avoid having to escape them. Good luck. — shellter
– shellter, Commented Mar 27, 2014 at 15:05
In any case, you are setting the value of array in a subshell, so it is not defined for the following for loop. — chepner
– chepner, Commented Mar 27, 2014 at 15:06
Yep, i know about some options. Actually I used option -o, which does, as you say, prints filename on every line. But I found out it is forbidden to use any option to nm. And I'm sorry, but I dont understand $(...). Do you mean I should replace line 5 for pattern=$(echo "$line"| ...) ? The same with line 9. — user1967718
– user1967718, Commented Mar 27, 2014 at 15:09

Jonathan Leffler · Accepted Answer · 2024-01-20 15:54:33Z

24

Your problem is that the while loop runs in a subshell because it is the second command in a pipeline, so any changes made in that loop are not available after the loop exits.

You have a few options. I often use { and } for command grouping:

nm "$@" |
{
while read line
do
    …
done
for j in "${array[@]}"
do
    echo "$j"
done
}

In bash, you can also use process substitution:

while read line
do
    …
done < <(nm "$@")

Bash also supports an option shopt -s lastpipe which runs the last process in a pipeline in the current shell environment so that changes made by the last part of the pipeline are available to the rest of the script. Then you do not need the command grouping notation or process substitution.

Also, it is better to use $(…) in place of back-quotes `…` (and not just because it is hard work getting back quotes into markdown text!).

Your line:

element="`echo \"$line\" | sed -n \"s/^U \([0-9a-zA-Z_]*\).*/$file:\1/p\"`"

could be written:

element="$(echo "$line" | sed -n "s/^U \([0-9a-zA-Z_]*\).*/$file:\1/p")"

or even:

element=$(echo "$line" | sed -n "s/^U \([0-9a-zA-Z_]*\).*/$file:\1/p")

It really helps when you need to nest command substitution operations. For example, to list the lib directory adjacent to where gcc is found:

ls -l $(dirname $(dirname $(which gcc)))/lib

vs

ls -l `dirname \`dirname \\\`which gcc\\\`\``/lib

I know which I find easier!

edited Jan 20, 2024 at 15:54

answered Mar 27, 2014 at 15:08

Jonathan Leffler

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1 Comment

user1967718 Over a year ago

Oh, thank you very much, I didn't realize subshell. Actually this is my very first script. So now I'll know. Thank you again.

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