Recently I needed to create a script that processed two input files. By processed I mean that the script needed to get a line from one file, then get a line from the second file, and then do something with them. Sounds easy enough, but it's not that easy unless you know about some of bash's extended redirection capabilities.

For the sake of this example, let's say that we want to implement a simple version of the paste command as a bash script. The paste command reads a line from each of its input files and then pastes them together and writes the combined result to stdout as a single line. Our example version will only do this for two input files. Plus it won't do any error checking and it will assume that the files contain the same number of lines.

Our input files, file1 and file2 are:

  $ cat file1
  f1 1
  f1 2
  f1 3
  f1 4
  $ cat file2
  f2 1
  f2 2
  f2 3
  f2 4

Your first thought might be something like this:

#!/bin/bash

while read f1 <$1
do
    read f2 <$2
    echo $f1 $f2
done

  $ sh paste-bad.sh file1 file2
  f1 1 f2 1
  f1 1 f2 1
  f1 1 f2 1
  f1 1 f2 1
  f1 1 f2 1
  f1 1 f2 1
  f1 1 f2 1
  ...
  Ctrl-C

Your next thought might be to read the files in one by one and then take the buffered data and paste it together afterwards:

#!/bin/bash

i=0
while read line
do
    f1[$i]="$line"
    let i++
done <$1

i=0
while read line
do
    f2[$i]="$line"
    let i++
done <$2

i=0
while [[ "${f1[$i]}" ]]
do
    echo ${f1[$i]} ${f2[$i]}
    let i++
done

  $ sh paste-ok.sh file1 file2
  f1 1 f2 1
  f1 2 f2 2
  f1 3 f2 3
  f1 4 f2 4

The other solution, is to use some more advanced redirection:

#!/bin/bash

while read f1 <&7
do
    read f2 <&8
    echo $f1 $f2
done \
    7<$1 \
    8<$2

In this version, at the end of the loop we specify multiple input redirections using the full general form of bash's input redirection: [n]<word. If no leading [n] is specified the default is 0, which is normal stdin redirection. However, by specifying a small integer in front of a redirection we can redirect multiple input files to the command, in this case the command is the while loop:

  ...
  done \
        7<$1 \
        8<$2

To make the read commands work we need to use a another form of bash's redirection, in this case we use bash's ability to duplicate a file descriptor (like the C library function dup2()). File descriptor duplication allows two file descriptors to refer to the same open file. Since read normally reads from stdin and not file descriptor 7 or 8 we need a way to duplicate file descriptor 7 (or 8) on stdin, bash's file descriptor duplication does just that:

  while read f1 <&7
  ...
      read f2 <&8
  ...

Bash contains similar forms of redirection for output files as well. See the bash man page for more information.