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Accepted
Sergiy Kolodyazhnyy
Asked: 2015-12-01 01:05:27 +0800 CST

Shell variable doesn't change with pipe-while-read combination

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Consider case 1:

$ COUNT=0 ; while [ $COUNT -ne 3  ]; do echo $COUNT; COUNT=$(expr $COUNT + 1 );done
0
1
2
$ echo $COUNT
3

By the end of the loop COUNT variable is 3, as expected

Consider case 2:

$ COUNT=1; find . -print0 | while IFS= read -r -d '' FILE; do echo "$FILE"; expr $COUNT + 1; COUNT=$(expr $COUNT + 1 )  ;done 
.
2
./file name
3
./file
4
./a
b
5
$ echo $COUNT
1

As you can see, in this case COUNT remained the same. Why ? It can be seen that it's changing inside the while loop, but not once it's out of the loop.

What exactly I am missing here ?

command-line

1 Answers

  1. Best Answer

    In your first case, all commands executed in the same shell. The content of COUNT is changed.

    In your second case a subshell is started with piping |, and changes in the subshell have no effect in the current shell. But he subshell knows the variable COUNT and the first output is 2.

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