Emptying a text file in bash

[wke...@gmail.com]

Hi,
How can I empty a text file in a bash script or from command line ?

To avoid misunderstanding, what I mean is that if I have a file name myFile.txt,
which contains 3 lines like

aaa
bbb
ccc

Then after running this script/command, then running
cat myFile.txt will show nothing, and ls -al myFile.txt will show size of 0.

I of course know about the possibility of "rm myFile.txt; touch myFile.txt", but is there a way to do it in a single command ?

Regards,
Kevin

[paul]

One way is

> myFile.txt

(exactly as written) or

truncate -s 0 myFile.txt

[Ben Bacarisse]

wke...@gmail.com writes:

> How can I empty a text file in a bash script or from command line ?

<snip>

printf "" >myFile.txt

From the command line I use cat and type Ctl-D (just because of muscle
mempry, I suspect) though you could use cat in a self-contained way like
this:

cat >myFile.txt </dev/null

--
Ben.

[Chris Elvidge]

Try: "> myFile.txt" (without the " obviously)


--

Chris Elvidge, England

[Martin Vaeth]

Chris Elvidge <ch...@mshome.net> wrote:
>>
> Try: "> myFile.txt" (without the " obviously)

This wouldn't work in zsh unless e.g. NULLCMD=: is set.
More compatible is

: >|myFile.txt

(the "|" overrides a possible noclobber option of some shells)

[Christian Weisgerber]

On 2016-11-07, wke...@gmail.com <wke...@gmail.com> wrote:

> I of course know about the possibility of "rm myFile.txt; touch myFile.txt", but is there a way to do it in a single command ?

: >myFile.txt

--
Christian "naddy" Weisgerber na...@mips.inka.de

[Ivan Shmakov]

>>>>> paul <pa...@donotuse.me.invalid> writes:
>>>>> On Mon, 7 Nov 2016 02:52:11 -0800 (PST), wke...@gmail.com wrote:

[...]

>> Then after running this script/command, then running cat myFile.txt
>> will show nothing, and ls -al myFile.txt will show size of 0.

>> I of course know about the possibility of "rm myFile.txt; touch
>> myFile.txt", but is there a way to do it in a single command ?

> One way is

> > myFile.txt

Or, as already suggested, ">|" can be used in place of ">" so
that the command does not fail when "set -o noclobber"
("set -C") is in effect.

But please note that there's a semantic difference between
"rm && touch" and ">". Namely, "rm && touch" /replaces/ the
file with a /new/, empty one. Should the file happen to have
other names ("hardlinks"), they will still refer to the original
data; also, the original data will still be accessible by the
processes, if any, having the file open.

On the contrary, ">" (">|") replaces the file contents in place.
Any and all processes accessing that file will find it empty.

Generally, unless the intent is to, for some reason, instantly
remove the data stored in a file, it makes sense to replace one
instead. A better way to do so is to create a temporary file
and use "mv" to replace the original /atomically/; say:

## Usage: replace_with_empty FILENAME
replace_with_empty () {
## NB: mktemp(1) is non-POSIX.
rwe_f_=$(mktemp -- "$1".XXXXXXXX) \
&& mv -f -- "$rwe_f_" "$1"
}

Although that's not without a flaw, as the new file created will
be accessible only by the user. With GNU Coreutils, it's
possible to copy the access mode by adding chmod(1), and perhaps
chown(1), calls to the code above; like:

&& (chmod --reference="$1" -- "$rwe_f_" ;
chown --reference="$1" -- "$rwe_f_" ; :) \

> (exactly as written) or

> truncate -s 0 myFile.txt

Note, however, that like mktemp(1), truncate(1) is non-POSIX.
But with the GNU Coreutils version, it's possible to also use
"-c" (--no-create), so to do nothing if the file does not exist.

--
FSF associate member #7257 np. Memories and Stories -- Night Breeze