How to read from two input files using while loop
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
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
.
I know you want a shell script, but you might want to take a look at the paste
command.