If you can guarantee that some character will never occur in the first file then you can use paste.
For example you know for sure that @
will never occur:
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 using some bash features for arguably cleaner code and paste using default delimiter tab:
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 two 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
The simplest way is to use /dev/tty
as the read for keyboard input.
For example:
#!/bin/bash
echo hello | while read line
do
echo We read the line: $line
echo is this correct?
read answer < /dev/tty
echo You responded $answer
done
This breaks if you don't run this on a terminal, and wouldn't allow for input to be redirected into the program, but otherwise works pretty well.
More generally, you could take a new file handle based off the original stdin, and then read from that. Note the exec
line and the read
#!/bin/bash
exec 3<&0
echo hello | while read line
do
echo We read the line: $line
echo is this correct?
read answer <&3
echo You responded $answer
done
In both cases the program looks a bit like:
% ./y
We read the line: hello
is this correct?
yes
You responded yes
The second variation allows for input to also be redirected
% echo yes | ./y
We read the line: hello
is this correct?
You responded yes
Best Answer
You can't do that in your
while
. You need to use another file descriptorTry the following version :
See How to keep other commands from "eating" the input
NOTES
bash
is notC
orPerl
, no need to put;
on each end lines