Reading from file by lines instead of words (columns)

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This one was tricky. My colleague needed to process text file by lines instead of words. Lets say you have following text file:

word1 word2
word3 word4

If you use old “for i in `cat file`; do; echo $i; done” output will be like

word1
word2
word3
word4

But when you need to cut the file by lines, following code might come in handy:

while read i
do
echo "This is new line $i"
done < file

which will output

This is new line word1 word2
This is new line word3 word4

Unix if statement “Parameter not set.” error when test variable is empty

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Let’s say you want to test if variable is empty or not:

if [ -n $TOP ]
then
command
fi

This is perfectly normal if the variable $TOP is not empty, but otherwise this test stops with an error “Parameter not set“. The solution is quite easy:

if [ -n "$TOP" ]
then
command
fi

You enclose the variable in “” so that test can be always done against something and doesn’t display silly errors.