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

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

1. "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.
2. 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 $?oneline0$ exactly_one_line < twolines; echo $?1

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)?

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

1. Try reading a line, if failed go to step 5
2. If variable $LINE is NOT empty, goto step 6
3. Save line inside variable $LINE
4. Goto step 1
5. If $LINE is NOT empty, goto step 7
6. Exit the program with status code 1
7. 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