Bash: pipe |

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Commands used with a pipe | are run in a sub-shell, with the same pitfalls as using parenthesis might have:
http://linux.overshoot.tv/wiki/bash_parenthesis

This will not give the expected result, which is to abort at the first wrong line:

#!/bin/bash

cat file.txt | while read line
do 
    if [ there_is_something_wrong_with_the_line ]; then
       exit 1
    fi
    do_something
done

Use input redirection instead:

#!/bin/bash

while read -r line
do 
    if [ there_is_something_wrong_with_the_line ]; then
       exit 1
    fi
    do_something
done < file.txt