avoid-subshell.sh

Shall高级编程

#!/bin/bash
# avoid-subshell.sh
# Suggested by Matthew Walker.

Lines=0

echo

cat myfile.txt | while read line;
                 do {
                   echo $line
                   (( Lines++ ));  #  Incremented values of this variable
                                   #+ inaccessible outside loop.
                                   #  Subshell problem.
                 }
                 done

echo "Number of lines read = $Lines"     # 0
                                         # Wrong!

echo "------------------------"


exec 3<> myfile.txt
while read line <&3
do {
  echo "$line"
  (( Lines++ ));                   #  Incremented values of this variable
                                   #+ accessible outside loop.
                                   #  No subshell, no problem.
}
done
exec 3>&-

echo "Number of lines read = $Lines"     # 8

echo

exit 0

# Lines below not seen by script.

$ cat myfile.txt

Line 1.
Line 2.
Line 3.
Line 4.
Line 5.
Line 6.
Line 7.
Line 8.