Question: I am aware of three methods to delete all entries from a file.

They are

* `>filename`
* `touch filename`1
* `filename < /dev/null`

Of these three I abuse `>filename` the most as that requires the least number of keystrokes.

However, I would like to know which is the most efficient of the three (if there are any more efficient methods) with respect to large log files and small files.

Also, how does the three codes operate and delete the contents?

---

1**Edit**: as discussed in [this answer](https://unix.stackexchange.com/a/88810/377345), this actually *does not* clear the file!

Answer: 
Actually, the second form `touch filename` doesn't delete anything from the file - it only creates an empty file if one did not exist, or updates the last-modified date of an existing file.

And the third `filename < /dev/null` tries to run filename with `/dev/null` as input.

`cp /dev/null filename` works.

As for efficient, the most efficient would be `truncate -s 0 filename` (see [here](https://linux.die.net/man/1/truncate)).

Otherwise, `cp /dev/null filename` or `> filename` are both fine. They both open and then close the file, using the truncate-on-open setting. `cp` also opens `/dev/null`, so that makes it marginally slower.

On the other hand, `truncate` would likely be slower than `> filename` when run from a script since running the truncate command requires the system to open the executable, load it, and then run it.
There is a builtin command ":", which is available in sh,csh,bash and others maybe, which can be easily used with the redirecting output operator `>` truncate a file:

```
#!/usr/bin/env bash
:> filename

```

What I like on this is, that it does not need any external commands like "echo" etc.

One big advantage of truncating files instead of deleate/recreate them is, that running applications which works with this file (e.g. someone makes an `tail -f filename` or a monitoring software, ...) don't have to reopen it. They just can continue using the filedescriptor and gets all the new data.