While-loop subshell dilemma in Bash

Question

i want to compute all *bin files inside a given directory. Initially I was working with a `for-loop`: ``` var=0 for i in *ls *bin do perform computations on $i .... var+=1 done echo $var ``` However, in some directories there are too many files resulting in an error: `Argument list too long` Therefore, I was trying it with a piped `while-loop`: ``` var=0 ls *.bin | while read i; do perform computations on $i var+=1 done echo $var ``` The problem now is by using the pipe subshells are created. Thus, `echo $var` returns `0`. How can I deal with this problem? The original Code: ``` #!/bin/bash function entropyImpl { if [[ -n "$1" ]] then if [[ -e "$1" ]] then echo "scale = 4; $(gzip -c ${1} | wc -c) / $(cat ${1} | wc -c)" | bc else echo "file ($1) not found" fi else datafile="$(mktemp entropy.XXXXX)" cat - > "$datafile" entropy "$datafile" rm "$datafile" fi return 1 } declare acc_entropy=0 declare count=0 ls *.bin | while read i ; do echo "Computing $i" | tee -a entropy.txt curr_entropy=…

Accepted Answer

The problem is that the while loop is part of a pipeline. In a bash pipeline, every element of the pipeline is executed in its own subshell [ref]. So after the while loop terminates, the while loop subshell's copy of `var` is discarded, and the original `var` of the parent (whose value is unchanged) is echoed.

One way to fix this is by using Process Substitution as shown below:

```
var=0
while read i;
do
  # perform computations on $i
  ((var++))
done < <(find . -type f -name "*.bin" -maxdepth 1)
```

Take a look at BashFAQ/024 for other workarounds.

Notice that I have also replaced `ls` with `find` because it is not good practice to parse `ls`.

While-loop subshell dilemma in Bash

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