mjhjmtu

i am having trouble to execute "|" command in root mode, the command is as follows

cat xyz.sh | chmod 744 xyz.sh | echo "hello" > xyz.sh

and when i run this command using ./xyz.sh, an error occurs

./xyz.sh: line 1: hello: command not found

please specify me. If, i am doing something wrong with reason!

First, you need to understand what a pipeline is:

A pipeline is a sequence of one or more commands separated by the control operator |. For each command but the last, the shell shall connect the standard output of the command to the standard input of the next command

What happens in your pipeline is the following:

1. 
   cat sends the content of xyz.sh to the standard input of the next command.


2. 
   chmod receives that data but, AFAIK, it doesn't do anything with it. Instead, it changes the permissions of xyz.sh and sends nothing to the standard input of the next command.


3. 
   echo receives nothing but it doesn't matter because echo doesn't read standard input in the first place. As steeldriver said, echo overwrites the content of xyz.sh with the word hello.

Then, if you try to run xyz.sh, the only statement your shell finds is hello and it will try to run hello as a command. You don't have any hello command installed, so that's why you get the command not found error.