Return an error if input doesn't have exactly 1 line, otherwise pipe input to next step - bash

Question

I have a series of commands chained together with pipes:
should_create_one_line | expects_one_line
The first command should_create_one_line should produce an output that only has one line, but under strange circumstances it is possible for the output to be multiline or empty.
I would like to add a step in between these two, validate_one_line:
should_create_one_line | validate_one_line | expects_one_line
If its input contains exactly 1 line then validate_one_line will simply output its input. If its input contains more than 1 line or is empty then validate_one_line should cause the whole sequence of steps to stop and return an error code.
What command can I use for validate_one_line?

Answer 1

Use read. Here's a shell function that meets your specs:
exactly_one_line() {
local line # Use to echo the line
read -r line || return # Guarantee at least one line is read
read && return 1 # Indicate failure if another line is successfully read
echo "$line"
}
Notes
"One line" assumes a single line followed by a newline. If your input could be like, a file with contents but no newlines, then this will fail.
Given a pipeline like a|b, a cannot prevent b from running. At a minimum, b needs to handle when a produces no output.
Demo:
$ wc -l empty oneline twolines
0 empty
1 oneline
2 twolines
3 total
$ exactly_one_line < empty; echo $?
1
$ exactly_one_line < oneline; echo $?
oneline
0
$ exactly_one_line < twolines; echo $?
1

Answer 2

First off, you should seriously consider adding the validation code to expects_one_line. According to this post, each process starts in its own subshell, meaning that even if validate_one_line fails, you will get an error in expects_one_line because it will try to run with no input (or a blank line). That being said, here is a bash one-liner that you can insert into your pipe to validate:
should_create_one_line.sh | ( var="$(cat)"; [ $(echo "$var" | wc -l) -ne 1 ] && exit 1 || echo "$var") | expects_one_line.sh
The problem here is that when the validation subshell returns in the exit 1 case, expects_one_line.sh will still get a single blank line. If this works for you, then great. If not, it would be better to just put the following into the beginning of expects_one_line.sh:
input="$(cat)"
[ $(echo "$var" | wc -l) -ne 1 ] && exit 1
This would guarantee that expects_one_line.sh fails properly when getting a single line without having to wonder about what the empty line that the validation outputs will do to the script.
You may find this post helpful: How to read mutliline input from stdin into variable and how to print one out in shell(sh,bash)?

Answer 3

You can use a bash script to check the incoming data and call the other command when the input is only 1 line
The following code starts cat when it is ONLY fet in 1 line
sh -c 'while read CMD; do [ ! -z "$LINE" ] && exit 1; LINE=$CMD; done; [ -z "$LINE" ] && exit 1; printf "%s\n" $LINE | "$0" "$#"' cat
How this works
Try reading a line, if failed go to step 5
If variable $LINE is NOT empty, goto step 6
Save line inside variable $LINE
Goto step 1
If $LINE is NOT empty, goto step 7
Exit the program with status code 1
Call our program and pass our $line to it using printf
Example usage:
Printing out only if grep found 1 match:
grep .... | sh -c 'while read CMD; do [ ! -z "$LINE" ] && exit 1; LINE=$CMD; done; [ -z "$LINE" ] && exit 1; printf "%s\n" $LINE | "$0" "$#"' cat
Example of the question poster:
should_create_one_line | sh -c 'while read CMD; do [ ! -z "$LINE" ] && exit 1; LINE=$CMD; done; [ -z "$LINE" ] && exit 1; printf "%s\n" $LINE | "$0" "$#"' expects_one_line