[SOLVED] [Bash] Process lines from file one at a time.

Zixy · 01-06-2013, 11:08 PM

Currently, I have code which reads from a file. That bit works. I'd like to process each line on the file one at a time.

Currently my code (edited for secuirty) is

Code:

while read LINE 
 do
    if [ my file exists ] 
       then
		echo "Do action A or B"
		read ANSWER
		if [ $ANSWER = "A" ] 
       then
			do whatever
		elif [ $ANSWER = "B" ]; then
			do something else
		fi
	fi
done < "$FILENAME"

However, it checks all lines at once.

So I can't do A or B to the lines that need different options.

I need one line to be processed at a time.

Sorry if I am unclear! If you have any questions, I will try my best to answer them!

Stéphane Ascoët · 01-07-2013, 12:56 AM

Test the existence of the file first, and then, use "for l in file" instead of "while read line"

Zixy · 01-07-2013, 01:59 AM

Quote:

Originally Posted by Stéphane Ascoët

Test the existence of the file first, and then, use "for l in file" instead of "while read line"

Could you show me that with my code please, I am a bit confused to what you are saying.

Stéphane Ascoët · 01-07-2013, 02:12 AM

if [ my file exists ]
then
for l in `cat file`
echo "Do action A or B"
read ANSWER
if [ $ANSWER = "A" ]
then
do whatever
elif [ $ANSWER = "B" ]; then
do something else
fi

konsolebox · 01-07-2013, 03:07 AM

If you process $LINE won't that be one line at a time?

Zixy · 01-07-2013, 08:32 AM

Quote:

Originally Posted by Stéphane Ascoët

if [ my file exists ]
then
for l in `cat file`
echo "Do action A or B"
read ANSWER
if [ $ANSWER = "A" ]
then
do whatever
elif [ $ANSWER = "B" ]; then
do something else
fi

I have to check if the file exists using the line from the file. (It's a list of possible directory names.)

Zixy · 01-07-2013, 08:33 AM

Quote:

Originally Posted by konsolebox

If you process $LINE won't that be one line at a time?

I was originally, but the guy above said that doesn't work :P

colucix · 01-07-2013, 08:58 AM

Quote:

Originally Posted by Stéphane Ascoët

Test the existence of the file first, and then, use "for l in file" instead of "while read line"

This is not advisable: if a line contains spaces it will be split in fields and the loop will make an iteration over each field.

colucix · 01-07-2013, 09:03 AM

Quote:

Originally Posted by Zixy

I was originally, but the guy above said that doesn't work :P

Who said that? Actually your script should work. You can easily test it by adding a counter, as in:

Code:

while read line
do
  echo $((++c)): "$line"
done < "$FILENAME"

or put

Code:

#!/bin/bash -x

at the beginning of your script to see the execution flow. Useful for debugging in general.

Zixy · 01-07-2013, 09:09 AM

Quote:

Originally Posted by colucix

Who said that? Actually your script should work. You can easily test it by adding a counter, as in:

Code:

while read line
do
  echo $((++c)): "$line"
done < "$FILENAME"

or put

Code:

#!/bin/bash -x

at the beginning of your script to see the execution flow. Useful for debugging in general.

I thought I had read it. It turns out that I was that guy above!

I'm going to go work on it some more and I will get back to you with results. All help is appreciated.

grail · 01-07-2013, 10:53 AM

I would point out that a little gotcha is in your original code, which is that if you call read on stdin (look inside your "if") then it will draw the answer from the
open file descriptor which is already being read from, ie $FILENAME

colucix · 01-07-2013, 11:11 AM

Quote:

Originally Posted by grail

I would point out that a little gotcha is in your original code, which is that if you call read on stdin (look inside your "if") then it will draw the answer from the
open file descriptor which is already being read from, ie $FILENAME

Good catch, grail!

In this case, what do you think about redirecting the input from the file using an unused file descriptor and feed the while read loop from there? Example:

Code:

exec 3< $FILENAME
while read line <&3
do
  read answer
  echo $((++c)): $line $answer
done
exec 3>&-

grail · 01-07-2013, 12:11 PM

Yeah that would work, although I do quite like -u option for read to make it a little easier to follow:

Code:

exec 3< $FILENAME

while read -u3 line
do
...

Zixy · 01-07-2013, 02:27 PM

Quote:

Originally Posted by grail

Yeah that would work, although I do quite like -u option for read to make it a little easier to follow:

Code:

exec 3< $FILENAME

while read -u3 line
do
...

This did it!

Thank you for everyone's help. I really do appreciate it.

konsolebox · 01-07-2013, 08:48 PM

Quote:

Originally Posted by colucix

Code:

exec 3>&-

That should be:

Code:

exec 3<&-

And this should be similar:

Code:

{
    ... do other things

    while read -u 3 LINE; do
        echo "$LINE"
    done

    ... do other things
} 3< "$FILENAME"

Or just:

Code:

while read -u 3 LINE; do
    echo "$LINE"
done 3< "$FILENAME"

With that we no longer have to close file descriptors with exec N<&-.