Watch file output in bash terminal as it is being written to the file

Question

I am asking this question because I remember very clearly having seen someone I worked with do this before I had a very good understanding of linux and the command line.

I am looking for a way to have the content of a file that is being updated sent directly to a terminal rather than simply logged. The closest approximation I have to this is using watch with tail. What I would like is to have the updates written directly to the terminal at the same time the file is updated.

Anyone seen something like this?

Answer 1

I often redirect both stdout and stderr of some command, e.g (in abatch job)

make >& _make.out

then in another terminal, I can run

tail -f _make.out

it will continuously display the last lines of _make.out, so I'll get it shown in the terminal.

Answer 2

Use the command tee - from it's man page,

Synopsis

  tee [OPTION]... [FILE]...

Description

  Copy standard input to each FILE, and also to standard output.

Or, you could run tail -f on the written file to watch as it is being written (-f is follow).

Answer 3

The tee program does exactly what you want. It reads from stdin and displays the data on the terminal while redirecting it to a file a the same time. It is an old UNIX tool and also available from GNU coretutils.

Redirect output of process to a file and display it on terminal at the same time:

process | tee output.file

If you want to append to output.file use the option -a:

process | tee -a output.file