(Shell script) while loop: read two files at same time

[agnes]

I have this code, it reads a file line by line, and for every line it calls another script with the currently-read line as parameter.



#!/bin/sh

echo enter file name
read fname

exec<$fname

while read line
do
./script4b.sh $line
done


But what I want is it to call script4b.sh with two lines as parameter, where the first line comes from one file, and the other from another file.

So I need it to loop through (read) two files (both the same length) at once.

Does anybody know how to do this?

[ghostdog74]

give the second file another file descriptor number. see here (http://tldp.org/LDP/abs/html/x17434.html)

[agnes]

Ok your link was also about using stdin/stdout which was a bit confusing.
But searching on "exec" I found the solution, for the people who came here searching:


#!/bin/sh

echo enter file name
read fname

echo enter filename two
read fname2

exec 3<$fname
exec 4<$fname2

while IFS= read -r line1 <&3
IFS= read -r line2 <&4
do

echo $line1
echo $line2

sleep 1
done

[Arndt]

Ok your link was also about using stdin/stdout which was a bit confusing.
But searching on "exec" I found the solution, for the people who came here searching:


#!/bin/sh

echo enter file name
read fname

echo enter filename two
read fname2

exec 3<$fname
exec 4<$fname2

while IFS= read -r line1 <&3
IFS= read -r line2 <&4
do

echo $line1
echo $line2

sleep 1
done


You will need to decide what to do if the files don't contain the same number of lines, too.

[agnes]

You're right.
I'm not that far with shell scripts yet.
But I assume you do this by checking every iteration if the file is (still) readable? (Within the while loop.)

Like


if [ -r $fname -a -r $fname2 ]
then
blabla..
else
exit $E_FILE_ACCESS
fi

?

[geirha]

When read reads a line from the file, it will return true, and thus the while loop will do an iteration. When there's nothing more to read, it will return false, and the while loop will end. That's easy with just one file.

while read -r line; do echo "$line"; done < "$file"

Now, with two files like you have (this is the same loop as yours, I'm just using bash instead of dash) it gets a little more complicated

fname=/tmp/file1 fname2=/tmp/file2
echo -e 'a\nb\nc' > "$fname"
echo -e 'd\ne\nf\ng' > "$fname2"
while read -r -u3 line1; read -r -u4 line2; do echo "$line1:$line2"; done 3< "$fname" 4< "$fname2"
a:d
b:e
c:f
:g

When you have two commands between the "while" and "do", it will evaluate the return value of the last command, so in this case, it will read until it reaches the end of the second file. If the first file ends before the second, "$line1" will be empty in those iterations.

If you want it to stop reading when either of the files reaches the end, then use logical and (&&)

while read -r -u3 line1 && read -r -u4 line2; do echo "$line1:$line2"; done 3< "$fname" 4< "$fname2"
a:d
b:e
c:f

If either of the reads return false now, the loop will end.

Oh, and I recommend http://mywiki.wooledge.org/BashGuide for learning bash. I recommend against ABS as it doesn't teach good coding practice.

[agnes]

I don't fully understand,
I've read logical AND is "-a" in bash.

So it could be



while IFS= read -r line1 <&3 -a IFS= read -r line2 <&4
do
stuff
done

?

Anyway thanks for the Bash guide link.

[geirha]

No, -a is an argument to the test and [ commands (type «help [» and «help test» in a bash shell). With bash's [[ and ((, and between commands, you use && and || instead.


line="foo bar"
if [[ $line = *bar* || $line = *baz* ]]; then
echo "Line contains 'bar' or 'baz'";
fi

a=2
if (( a >= 0 && a < 5 )); then
echo "a is between 0 and 5"
fi


if grep -Fq 'foo' file1 && grep -Fq 'bar' file2; then
echo "file1 contains 'foo' and file2 contains 'bar'"
fi


[ is something still hanging around from bourne, and should only be used if you need your script to be able to run in other posix shells.