Truncating a File While It's Being Used (Linux)

Truncating a file while it's being used (Linux)

Take a look at the utility split(1), part of GNU Coreutils.

How to delete older contents of file that is being continuously written to?

As Carl mentioned in the comments, you cannot really do this on an actively written log file. However, if the initial data is not relevant to you, you can do the following (though beware that you will loose all data)

> out.txt

For future, you can use a utility called logrotate(8)

How to empty ( truncate ) a file on linux that already exists and is protected in someway?

You have the noclobber option set. The error looks like it's from csh, so you would do:

cat /dev/null >! file

If I'm wrong and you are using bash, you should do:

cat /dev/null >| file

in bash, you can also shorten that to:

>| file

Truncate file at front

Truncate files at front seems not too hard to implement at system level.

But there are issues.

• The first one is at programming level. When opening file in random access the current paradigm is to use offset from the beginning of the file to point out different places in the file. If we truncate at beginning of file (or perform insertion or removal from the middle of the file) that is not any more a stable property. (While appendind or truncating from the end is not a problem).

In other words truncating the beginning would change the only reference point and that is bad.

• At a system level uses exist as you pointed out, but are quite rare. I believe most uses of files are of the write once read many kind, so even truncate is not a critical feature and we could probably do without it (well some things would become more difficult, but nothing would become impossible).

If we want more complex accesses (and there are indeed needs) we open files in random mode and add some internal data structure. Theses informations can also be shared between several files. This leads us to the last issue I see, probably the most important.

• In a sense when we using random access files with some internal structure... we are still using files but we are not any more using files paradigm. Typical such cases are the databases where we want to perform insertion or removal of records without caring at all about their physical place. Databases can use files as low level implementation but for optimisation purposes some database editors choose to completely bypass filesystem (think about Oracle partitions).

I see no technical reason why we couldn't do everything that is currently done in an operating system with files using a database as data storage layer. I even heard that NTFS has many common points with databases in it's internals. An operating system can (and probably will in some not so far future) use another paradigm than files one.

Summarily i believe that's not a technical problem at all, just a change of paradigm and that removing the beginning is definitely not part of the current "files paradigm", but not a big and useful enough change to compell changing anything at all.

Writing to the same file with awk and truncate

This probably isn't an answer but it's too much to go in a comment. I don't know the details of most of the tools you mention, nor do I really understand what it is you're trying to do but:

A shell is a tool to manipulate files and processes and schedule calls to other tools. Awk is a tool to manipulate text. You're trying to use awk like a shell - you have it sequencing calls to truncate and pkill and calling system to spawn a subshell each time you want to execute either of them. What you should be doing, for example, is just:

shell { truncate }

but what you're actually doing is:

shell { awk { system { shell { truncate } } } }

Can you take that role away from awk and give it back to your shell? It should make your overall script simpler, conceptually at least, and probably more robust.

Maybe try something like this (untested):

#!/usr/bin/env bash

while IFS= read -r str; do
    case $str in
        Return ) exit 0 ;;
        BackSpace ) truncate -s-1 timer.txt ;;
        ? ) printf "%s" "$str" | tee -a timer.txt ;;
    esac
    pkill -RTMIN+3 dwmblocks
done < <(
    xev -root |
    awk -F'[ )]+' '/^KeyPress/{a[NR+2]} NR in a{print $8; fflush()}'
)

I moved the write to timer.txt inside the loop to make sure tees not trying to write to it while you're truncating it - that may not be necessary.

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