UNIX for Dummies Questions & Answers

I have a text file called file1 which contains the text: "ls -l"
When I enter this command:

Code:

bash < file1 > file1

file1 gets erased. However if I enter this command:

Code:

bash < file1 > newfile

the output from "ls -l" is stored in newfile. My question is why doesn't file1's text ("ls -l") get replaced with the output of the ls -l command?

For that the OS would have to execute your instructions sequentially... This is what multiprocessing OS is all about (remember? UNIX is a preemptive multitasking, multiprocessing OS...)

I don't know what a preemptive, multitasking, multiprocessing OS is let alone how it relates to my question

if we look at how the second version works:
1. bash will create a empty file called newfile, ready to accept the output from your commands.
2. bash will open the file1 to get input.
3. bash will run the ls -al that it found in file1.
4. the output of ls -al is sent to the output file called newfile.


so if we then look at the "broken" first version:
1. bash will create an empty file called file1. (over writing your "ls -al" command)
2. bash will open the file1 (now empty)
3. bash will find nothing to do.
4. bash will send the output of nothing to the new file called file1