Bash: Head & Tail behavior with bash script

Question

Suppose I have following script:-

test.sh

#!/bin/bash
command1  #prints 5 lines
command2  #prints 3 lines

I run th

Answer 1

This is more of a comment then an answer but it is too big for a comment.

I tried following script:

#!/usr/bin/env bash

rm -f "test_head.log"
echo "1 line"
echo "1 line" >> "test_head.log"
echo "2 line"
echo "2 line" >> "test_head.log"
echo "3 line"
echo "3 line" >> "test_head.log"
echo "4 line"
echo "4 line" >> "test_head.log"
echo "5 line"
echo "5 line" >> "test_head.log"
echo "6 line"
echo "6 line" >> "test_head.log"
echo "7 line"
echo "7 line" >> "test_head.log"
echo "8 line"   
echo "8 line" >> "test_head.log"

Then i ran the script with:

./test_head.sh | head -n1

The cat output is (to my surprise):

1 line

I have no idea what is going on.

After reading @ymonad comment i tried it out and replace echo with /bin/echo and that solved the problem. I hope he can explain more about this behaviour.