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I wanted to know if there is any way of reading from two input files in a nested while loop one line at a time. For example, lets say I have two files FileA and FileB.

FileA:

[jaypal:~/Temp] cat filea
this is File A line1
this is File A line2
this is File A line3

FileB:

[jaypal:~/Temp] cat fileb
this is File B line1
this is File B line2
this is File B line3

Current Sample Script:

[jaypal:~/Temp] cat read.sh 
#!/bin/bash
while read lineA
    do echo $lineA 
    while read lineB
        do echo $lineB 
        done < fileb
done < filea

Execution:

[jaypal:~/Temp] ./read.sh 
this is File A line1
this is File B line1
this is File B line2
this is File B line3
this is File A line2
this is File B line1
this is File B line2
this is File B line3
this is File A line3
this is File B line1
this is File B line2
this is File B line3

Problem and desired output:

This loops over FileB completely for each line in FileA. I tried using continue, break, exit but none of them are meant for achieving the output I am looking for. I would like the script to read just one line from File A and then one line from FileB and exit the loop and continue with second line of File A and second line of File B. Something similar to the following script -

[jaypal:~/Temp] cat read1.sh 
#!/bin/bash
count=1
while read lineA
    do echo $lineA 
        lineB=`sed -n "$count"p fileb`
        echo $lineB
        count=`expr $count + 1`
done < filea

[jaypal:~/Temp] ./read1.sh 
this is File A line1
this is File B line1
this is File A line2
this is File B line2
this is File A line3
this is File B line3

Is this possible to achieve with while loop?

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4 Answers 4

up vote 16 down vote accepted

If you know for sure that some character will never occur in the first file then you can use paste.

Example of paste using default delimiter tab:

paste file1 file2 | while IFS="$(printf '\t')" read -r f1 f2
do
  printf 'f1: %s\n' "$f1"
  printf 'f2: %s\n' "$f2"
done

Example of paste using @:

paste -d@ file1 file2 | while IFS="@" read -r f1 f2
do
  printf 'f1: %s\n' "$f1"
  printf 'f2: %s\n' "$f2"
done

Note that it is enough if the character is guaranteed to not occur in the first file. This is because read will ignore IFS when filling the last variable. So even if @ occurs in the second file it will not be split.

Example of paste using some bash features for arguably cleaner code:

while IFS=$'\t' read -r f1 f2
do
  printf 'f1: %s\n' "$f1"
  printf 'f2: %s\n' "$f2"
done < <(paste file1 file2)

Bash features used: ansi c string ($'\t') and process substitution (<(...)) to avoid the while loop in a subshell problem.

If you cannot be certain that any character will never occur in both files then you can use file descriptors.

while true
do
  read -r f1 <&3 || break
  read -r f2 <&4 || break
  printf 'f1: %s\n' "$f1"
  printf 'f2: %s\n' "$f2"
done 3<file1 4<file2

Not tested much. Might break on empty lines.

File descriptors number 0, 1, and 2 are already used for stdin, stdout, and stderr, respectively. File descriptors from 3 and up are (usually) free. The bash manual warns from using file descriptors greater than 9, because they are "used internally".

Note that open file descriptors are inherited to shell functions and external programs. Functions and programs inheriting an open file descriptor can read from (and write to) the file descriptor. You should take care to close all file descriptors which are not required before calling a function or external program.

Here is the same program as above with the actual work (the printing) separated from the meta-work (reading line by line from two files in parallel).

work() {
  printf 'f1: %s\n' "$1"
  printf 'f2: %s\n' "$2"
}

while true
do
  read -r f1 <&3 || break
  read -r f2 <&4 || break
  work "$f1" "$f2"
done 3<file1 4<file2

Now we pretend that we have no control over the work code and that code, for whatever reason, tries to read from file descriptor 3.

unknowncode() {
  printf 'f1: %s\n' "$1"
  printf 'f2: %s\n' "$2"
  read -r yoink <&3 && printf 'yoink: %s\n' "$yoink"
}

while true
do
  read -r f1 <&3 || break
  read -r f2 <&4 || break
  unknowncode "$f1" "$f2"
done 3<file1 4<file2

Here is an example output. Note that the second line from the first file is "stolen" from the loop.

f1: file1 line1
f2: file2 line1
yoink: file1 line2
f1: file1 line3
f2: file2 line2

Here is how you should close the file descriptors before calling external code (or any code for that matter).

while true
do
  read -r f1 <&3 || break
  read -r f2 <&4 || break
  # this will close fd3 and fd4 before executing anycode
  anycode "$f1" "$f2" 3<&- 4<&-
  # note that fd3 and fd4 are still open in the loop
done 3<file1 4<file2
share|improve this answer
    
Thanks @lesmana. It's much clearer now. –  jaypal singh Dec 12 '11 at 1:30

Open the two files on different file descriptors. Redirect the input of the read built-in to the descriptor that the file you want is connected to. In bash/ksh/zsh, you can write read -u 3 instead of read <&3.

while IFS= read -r lineA && IFS= read -r lineB <&3; do
  echo "$lineA"; echo "$lineB"
done <fileA 3<fileB

This snippet stops when the shortest file has been processed. See Reading two files into an IFS while loop -- Is there a way to get a zero diff result in this case? if you want to keep processing until the end of both files.

See also When would you use an additional file descriptor? for additional information on file descriptors, and Why is `while IFS= read` used so often, instead of `IFS=; while read..`? for an explanation of IFS= read -r.

share|improve this answer
    
Thanks @Gilles for the additional links on file descriptor. –  jaypal singh Dec 12 '11 at 1:31
    
@Gilles perhaps I misunderstood you, but I could not make the loop process the longest file entirely (which is always $fileA in my case), so I made that into a separate question, being: is there a way to write the loop so that diff doesn't notice any difference between input and output? unix.stackexchange.com/questions/26780/… the closest i could get was diff only finding one line of difference. –  ixtmixilix Dec 14 '11 at 11:36

I know you want a shell script, but you might want to take a look at the paste command.

share|improve this answer
    
Thanks @lutzky. paste is cool too. –  jaypal singh Dec 19 '11 at 20:37

Alternatively, I suppose you could slurp the file into an array variable tying each line of the file into array[line_of_file_index] using bash's mapfile command. However, I am not sure if it is only for Bash3 higher or Bash4.

http://wiki.bash-hackers.org/commands/builtin/mapfile

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