Bash program that returns 1 on input and 0 otherwise

Question

Is there a standard linux terminal program that when given text input (in standard in) returns a 1 and 0 if no text is provided? (The reverse logic would also be fine).

Example

echo hello | unknown_program  # returns 1
echo | unknown_program        # returns 0

Edit:

My usecase, is for calling a program from a c++-program, that should be agnostic to where on the drive it resides. Thats why i do not prefer to create a script file, but using applications that i would assume exist on any linux (or ubuntu in my case) computer.

This is the c++-code but that is not part of the question.

auto isConditionMet = std::system("git status --porcelain | unknown_program");

And I got a working answer, so at least I am happy.

Answer 1

grep -q '.' will do this. . matches any character except newline. grep returns a 0 static code (success) if there are any matches, and 1 if there are no matches.

echo hello | grep -q '.'; echo $? # echoes 0
echo | grep -q '.'; echo $? # echoes 1

If you want to ignore lines with just spaces as well, change . to [^ ].

Answer 2

With bash:

#!/usr/bin/env bash

if [[ -t 0 ]]; then
    echo "stdin is the TTY, no input has been redirected to me"
    exit 0
fi

# grab all the piped input, may block
input=$(cat)
if [[ -z $input ]]; then
    echo "captured stdin is empty"
    exit 0
fi

echo "I captured ${#input} characters of data"
exit 1

If this is saved as ./test_input and made executable, then:

$ ./test_input; echo $?
stdin is the TTY, no input has been redirected to me
0

$ ./test_input < /dev/null; echo $?
captured stdin is empty
0

$ echo | ./test_input; echo $?
captured stdin is empty
0

$ ./test_input <<< "hello world"; echo $?
I captured 11 characters of data
1

$ echo foo | ./test_input; echo $?
I captured 3 characters of data
1

Note that the shell's command substitution $(...) removes all trailing newlines, which is why the echo | ./test_input case report no data has been captured.

Answer 3

Using wc -w to count words, and shell arithmetic to check condition and get 0 or 1 to echo.

echo | echo "$(("$(wc -w)" > 0))"                       # echoes 0
echo hello world | echo "$(("$(wc -w)" > 0))" # echoes 1