Redirections shell

Question

Can anyone explain the following?

:~$ ls macreave.sh fgvdvg > testo 2> testo
:~$ cat testo 
macreave.sh 
ccess fgvdvg: No such file or directory 

:~$ ls macreave.sh fgvdvg > testo 2>&1
:~$ cat testo 
ls: cannot access fgvdvg: No such file or directory 
macreave.sh 

Doing it the first way it eats the first few characters of the error message, the second way shows it just fine. Why is that?

Answer 1

With the second command

:~$ ls macreave.sh fgvdvg > testo 2>&1

you will cause the stderr ouput of ls to be written to the same filedescriptor than stdout (2>&1) and the stdout to the file testo: so you have a unique flux that finish in your file.

With the first command instead

ls macreave.sh fgvdvg > testo 2> testo

you are redirecting independently to the same file testo. The problem is pipings and redirections can be asyncronus (see this answer too); you have 2 fluxes that finish in your file both recreating it.

It seems that:

• the faster was the stderror (for few bytes) that creates a file testo (>) and write inside ls: cannot a
• then it is arrived the stdout that recreates the same file (>) erasing what inside and puts the output macreave.sh
• finally it is arrived the second part of stderr that writes in the file what remains ccess fgvdvg: No such file or directory.

Maybe if you execute again you will have a different result.

References

• Bash manual
• Pipes, how do data flow in a pipeline