Empty the contents of a file
up vote
200
down vote
favorite
I am aware of three methods to delete all entries from a file.
They are
>filenametouch filenamefilename < /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?
shell-script files
asked Aug 30 '13 at 5:13
debal
1,59451118
add a comment |
up vote
200
down vote
favorite
I am aware of three methods to delete all entries from a file.
They are
>filenametouch filenamefilename < /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?
shell-script files
asked Aug 30 '13 at 5:13
debal
1,59451118
22
What abouttruncate -s 0 filename?
– Martin Thoma
Jun 29 '14 at 16:18
Very similar to Difference between cat and '>' to zero out a file where you'll find more information.
– Stéphane Chazelas
Oct 26 '15 at 15:27
The first will work only if called from bash command line, but won't work if executed in a .sh file
– Marco Marsala
Nov 24 '15 at 11:04
6
touch does not delete contents, but does change access time on the file. It does create an empty file if none existed.
– hbogert
Sep 28 '16 at 10:12
add a comment |
up vote
200
down vote
favorite
up vote
200
down vote
favorite
I am aware of three methods to delete all entries from a file.
They are
>filenametouch filenamefilename < /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?
shell-script files
asked Aug 30 '13 at 5:13
debal
1,59451118
I am aware of three methods to delete all entries from a file.
They are
>filenametouch filenamefilename < /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?
shell-script files
shell-script files
asked Aug 30 '13 at 5:13
debal
1,59451118
asked Aug 30 '13 at 5:13
debal
1,59451118
asked Aug 30 '13 at 5:13
debal
1,59451118
asked Aug 30 '13 at 5:13
debal
1,59451118
asked Aug 30 '13 at 5:13
debal
1,59451118
1,59451118
22
What abouttruncate -s 0 filename?
– Martin Thoma
Jun 29 '14 at 16:18
Very similar to Difference between cat and '>' to zero out a file where you'll find more information.
– Stéphane Chazelas
Oct 26 '15 at 15:27
The first will work only if called from bash command line, but won't work if executed in a .sh file
– Marco Marsala
Nov 24 '15 at 11:04
6
touch does not delete contents, but does change access time on the file. It does create an empty file if none existed.
– hbogert
Sep 28 '16 at 10:12
add a comment |
22
What abouttruncate -s 0 filename?
– Martin Thoma
Jun 29 '14 at 16:18
Very similar to Difference between cat and '>' to zero out a file where you'll find more information.
– Stéphane Chazelas
Oct 26 '15 at 15:27
The first will work only if called from bash command line, but won't work if executed in a .sh file
– Marco Marsala
Nov 24 '15 at 11:04
6
touch does not delete contents, but does change access time on the file. It does create an empty file if none existed.
– hbogert
Sep 28 '16 at 10:12
22
22
What about
truncate -s 0 filename?– Martin Thoma
Jun 29 '14 at 16:18
What about
truncate -s 0 filename?– Martin Thoma
Jun 29 '14 at 16:18
Very similar to Difference between cat and '>' to zero out a file where you'll find more information.
– Stéphane Chazelas
Oct 26 '15 at 15:27
Very similar to Difference between cat and '>' to zero out a file where you'll find more information.
– Stéphane Chazelas
Oct 26 '15 at 15:27
The first will work only if called from bash command line, but won't work if executed in a .sh file
– Marco Marsala
Nov 24 '15 at 11:04
The first will work only if called from bash command line, but won't work if executed in a .sh file
– Marco Marsala
Nov 24 '15 at 11:04
6
6
touch does not delete contents, but does change access time on the file. It does create an empty file if none existed.
– hbogert
Sep 28 '16 at 10:12
touch does not delete contents, but does change access time on the file. It does create an empty file if none existed.
– hbogert
Sep 28 '16 at 10:12
add a comment |
3 Answers
3
active
oldest
votes
up vote
272
down vote
accepted
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: http://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 the run it.
answered Aug 30 '13 at 5:19
ash
4,57011311
7
So why do you say thattruncateis the most efficient?
– Stéphane Chazelas
Aug 30 '13 at 6:24
6
The truncate operation uses the ftruncate() or truncate() system call which does not bother to open the file. It also avoids the close() system call that cp and> filenamemethods need to call.
– ash
Aug 30 '13 at 6:26
3
Actually, it (at least the GNU one) does an open+ftruncate+close (in addition to the many system calls it does to load and initialise itself), as anyway, it would have to create the file if it didn't exist andtruncate(2)doesn't do that.
– Stéphane Chazelas
Aug 30 '13 at 8:01
If we usetouch filename, will the inode remain same (provided there was a file before)?
– pMan
Aug 30 '13 at 8:30
1
@pMan yes, you can try it and check withls -i
– terdon♦
Aug 30 '13 at 13:02
|
show 11 more comments
up vote
36
down vote
Other option could be:
echo -n > filename
From the man page of echo:
-n Do not print the trailing newline character.
answered Apr 23 '16 at 20:35
Arturo Herrero
95121117
How can I set the size? Say if I want 30000 null characters?
– User
Nov 16 '16 at 23:52
add a comment |
up vote
0
down vote
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.
answered Dec 3 at 9:59
Mirko Steiner
292
man bashdescribes the:shell builtin as having no effect.
– Haxiel
Dec 3 at 10:06
Yes, and you redirect this with>in to the file, which creates the file if it does not exists, and if it exists you truncate it to zero. Better said: you use the:to do nothing, and use>to redirect nothing to a file, and truncate it.
– Mirko Steiner
Dec 3 at 10:10
1
Why would you do that?> fileis enough to truncate a file. You don't need any command, just the redirection operator.
– terdon♦
Dec 3 at 11:12
sometimes,> filenamewon't work. for example, in zsh. but: > filenameworks still.
– CS Pei
yesterday
add a comment |
protected by Anthon Jun 20 '17 at 10:06
Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
272
down vote
accepted
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: http://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 the run it.
answered Aug 30 '13 at 5:19
ash
4,57011311
7
So why do you say thattruncateis the most efficient?
– Stéphane Chazelas
Aug 30 '13 at 6:24
6
The truncate operation uses the ftruncate() or truncate() system call which does not bother to open the file. It also avoids the close() system call that cp and> filenamemethods need to call.
– ash
Aug 30 '13 at 6:26
3
Actually, it (at least the GNU one) does an open+ftruncate+close (in addition to the many system calls it does to load and initialise itself), as anyway, it would have to create the file if it didn't exist andtruncate(2)doesn't do that.
– Stéphane Chazelas
Aug 30 '13 at 8:01
If we usetouch filename, will the inode remain same (provided there was a file before)?
– pMan
Aug 30 '13 at 8:30
1
@pMan yes, you can try it and check withls -i
– terdon♦
Aug 30 '13 at 13:02
|
show 11 more comments
up vote
272
down vote
accepted
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: http://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 the run it.
answered Aug 30 '13 at 5:19
ash
4,57011311
7
So why do you say thattruncateis the most efficient?
– Stéphane Chazelas
Aug 30 '13 at 6:24
6
The truncate operation uses the ftruncate() or truncate() system call which does not bother to open the file. It also avoids the close() system call that cp and> filenamemethods need to call.
– ash
Aug 30 '13 at 6:26
3
Actually, it (at least the GNU one) does an open+ftruncate+close (in addition to the many system calls it does to load and initialise itself), as anyway, it would have to create the file if it didn't exist andtruncate(2)doesn't do that.
– Stéphane Chazelas
Aug 30 '13 at 8:01
If we usetouch filename, will the inode remain same (provided there was a file before)?
– pMan
Aug 30 '13 at 8:30
1
@pMan yes, you can try it and check withls -i
– terdon♦
Aug 30 '13 at 13:02
|
show 11 more comments
up vote
272
down vote
accepted
up vote
272
down vote
accepted
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: http://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 the run it.
answered Aug 30 '13 at 5:19
ash
4,57011311
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: http://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 the run it.
answered Aug 30 '13 at 5:19
ash
4,57011311
answered Aug 30 '13 at 5:19
ash
4,57011311
answered Aug 30 '13 at 5:19
ash
4,57011311
answered Aug 30 '13 at 5:19
ash
4,57011311
4,57011311
7
So why do you say thattruncateis the most efficient?
– Stéphane Chazelas
Aug 30 '13 at 6:24
6
The truncate operation uses the ftruncate() or truncate() system call which does not bother to open the file. It also avoids the close() system call that cp and> filenamemethods need to call.
– ash
Aug 30 '13 at 6:26
3
Actually, it (at least the GNU one) does an open+ftruncate+close (in addition to the many system calls it does to load and initialise itself), as anyway, it would have to create the file if it didn't exist andtruncate(2)doesn't do that.
– Stéphane Chazelas
Aug 30 '13 at 8:01
If we usetouch filename, will the inode remain same (provided there was a file before)?
– pMan
Aug 30 '13 at 8:30
1
@pMan yes, you can try it and check withls -i
– terdon♦
Aug 30 '13 at 13:02
|
show 11 more comments
7
So why do you say thattruncateis the most efficient?
– Stéphane Chazelas
Aug 30 '13 at 6:24
6
The truncate operation uses the ftruncate() or truncate() system call which does not bother to open the file. It also avoids the close() system call that cp and> filenamemethods need to call.
– ash
Aug 30 '13 at 6:26
3
Actually, it (at least the GNU one) does an open+ftruncate+close (in addition to the many system calls it does to load and initialise itself), as anyway, it would have to create the file if it didn't exist andtruncate(2)doesn't do that.
– Stéphane Chazelas
Aug 30 '13 at 8:01
If we usetouch filename, will the inode remain same (provided there was a file before)?
– pMan
Aug 30 '13 at 8:30
1
@pMan yes, you can try it and check withls -i
– terdon♦
Aug 30 '13 at 13:02
7
7
So why do you say that
truncate is the most efficient?– Stéphane Chazelas
Aug 30 '13 at 6:24
So why do you say that
truncate is the most efficient?– Stéphane Chazelas
Aug 30 '13 at 6:24
6
6
The truncate operation uses the ftruncate() or truncate() system call which does not bother to open the file. It also avoids the close() system call that cp and
> filename methods need to call.– ash
Aug 30 '13 at 6:26
The truncate operation uses the ftruncate() or truncate() system call which does not bother to open the file. It also avoids the close() system call that cp and
> filename methods need to call.– ash
Aug 30 '13 at 6:26
3
3
Actually, it (at least the GNU one) does an open+ftruncate+close (in addition to the many system calls it does to load and initialise itself), as anyway, it would have to create the file if it didn't exist and
truncate(2) doesn't do that.– Stéphane Chazelas
Aug 30 '13 at 8:01
Actually, it (at least the GNU one) does an open+ftruncate+close (in addition to the many system calls it does to load and initialise itself), as anyway, it would have to create the file if it didn't exist and
truncate(2) doesn't do that.– Stéphane Chazelas
Aug 30 '13 at 8:01
If we use
touch filename, will the inode remain same (provided there was a file before)?– pMan
Aug 30 '13 at 8:30
If we use
touch filename, will the inode remain same (provided there was a file before)?– pMan
Aug 30 '13 at 8:30
1
1
@pMan yes, you can try it and check with
ls -i– terdon♦
Aug 30 '13 at 13:02
@pMan yes, you can try it and check with
ls -i– terdon♦
Aug 30 '13 at 13:02
|
show 11 more comments
up vote
36
down vote
Other option could be:
echo -n > filename
From the man page of echo:
-n Do not print the trailing newline character.
answered Apr 23 '16 at 20:35
Arturo Herrero
95121117
How can I set the size? Say if I want 30000 null characters?
– User
Nov 16 '16 at 23:52
add a comment |
up vote
36
down vote
Other option could be:
echo -n > filename
From the man page of echo:
-n Do not print the trailing newline character.
answered Apr 23 '16 at 20:35
Arturo Herrero
95121117
How can I set the size? Say if I want 30000 null characters?
– User
Nov 16 '16 at 23:52
add a comment |
up vote
36
down vote
up vote
36
down vote
Other option could be:
echo -n > filename
From the man page of echo:
-n Do not print the trailing newline character.
answered Apr 23 '16 at 20:35
Arturo Herrero
95121117
Other option could be:
echo -n > filename
From the man page of echo:
-n Do not print the trailing newline character.
answered Apr 23 '16 at 20:35
Arturo Herrero
95121117
answered Apr 23 '16 at 20:35
Arturo Herrero
95121117
answered Apr 23 '16 at 20:35
Arturo Herrero
95121117
answered Apr 23 '16 at 20:35
Arturo Herrero
95121117
95121117
How can I set the size? Say if I want 30000 null characters?
– User
Nov 16 '16 at 23:52
add a comment |
How can I set the size? Say if I want 30000 null characters?
– User
Nov 16 '16 at 23:52
How can I set the size? Say if I want 30000 null characters?
– User
Nov 16 '16 at 23:52
How can I set the size? Say if I want 30000 null characters?
– User
Nov 16 '16 at 23:52
add a comment |
up vote
0
down vote
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.
answered Dec 3 at 9:59
Mirko Steiner
292
man bashdescribes the:shell builtin as having no effect.
– Haxiel
Dec 3 at 10:06
Yes, and you redirect this with>in to the file, which creates the file if it does not exists, and if it exists you truncate it to zero. Better said: you use the:to do nothing, and use>to redirect nothing to a file, and truncate it.
– Mirko Steiner
Dec 3 at 10:10
1
Why would you do that?> fileis enough to truncate a file. You don't need any command, just the redirection operator.
– terdon♦
Dec 3 at 11:12
sometimes,> filenamewon't work. for example, in zsh. but: > filenameworks still.
– CS Pei
yesterday
add a comment |
up vote
0
down vote
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.
answered Dec 3 at 9:59
Mirko Steiner
292
man bashdescribes the:shell builtin as having no effect.
– Haxiel
Dec 3 at 10:06
Yes, and you redirect this with>in to the file, which creates the file if it does not exists, and if it exists you truncate it to zero. Better said: you use the:to do nothing, and use>to redirect nothing to a file, and truncate it.
– Mirko Steiner
Dec 3 at 10:10
1
Why would you do that?> fileis enough to truncate a file. You don't need any command, just the redirection operator.
– terdon♦
Dec 3 at 11:12
sometimes,> filenamewon't work. for example, in zsh. but: > filenameworks still.
– CS Pei
yesterday
add a comment |
up vote
0
down vote
up vote
0
down vote
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.
answered Dec 3 at 9:59
Mirko Steiner
292
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.
answered Dec 3 at 9:59
Mirko Steiner
292
edited Dec 3 at 10:12
answered Dec 3 at 9:59
Mirko Steiner
292
answered Dec 3 at 9:59
Mirko Steiner
292
answered Dec 3 at 9:59
Mirko Steiner
292
292
man bashdescribes the:shell builtin as having no effect.
– Haxiel
Dec 3 at 10:06
Yes, and you redirect this with>in to the file, which creates the file if it does not exists, and if it exists you truncate it to zero. Better said: you use the:to do nothing, and use>to redirect nothing to a file, and truncate it.
– Mirko Steiner
Dec 3 at 10:10
1
Why would you do that?> fileis enough to truncate a file. You don't need any command, just the redirection operator.
– terdon♦
Dec 3 at 11:12
sometimes,> filenamewon't work. for example, in zsh. but: > filenameworks still.
– CS Pei
yesterday
add a comment |
man bashdescribes the:shell builtin as having no effect.
– Haxiel
Dec 3 at 10:06
Yes, and you redirect this with>in to the file, which creates the file if it does not exists, and if it exists you truncate it to zero. Better said: you use the:to do nothing, and use>to redirect nothing to a file, and truncate it.
– Mirko Steiner
Dec 3 at 10:10
1
Why would you do that?> fileis enough to truncate a file. You don't need any command, just the redirection operator.
– terdon♦
Dec 3 at 11:12
sometimes,> filenamewon't work. for example, in zsh. but: > filenameworks still.
– CS Pei
yesterday
man bash describes the : shell builtin as having no effect.– Haxiel
Dec 3 at 10:06
man bash describes the : shell builtin as having no effect.– Haxiel
Dec 3 at 10:06
Yes, and you redirect this with
> in to the file, which creates the file if it does not exists, and if it exists you truncate it to zero. Better said: you use the : to do nothing, and use > to redirect nothing to a file, and truncate it.– Mirko Steiner
Dec 3 at 10:10
Yes, and you redirect this with
> in to the file, which creates the file if it does not exists, and if it exists you truncate it to zero. Better said: you use the : to do nothing, and use > to redirect nothing to a file, and truncate it.– Mirko Steiner
Dec 3 at 10:10
1
1
Why would you do that?
> file is enough to truncate a file. You don't need any command, just the redirection operator.– terdon♦
Dec 3 at 11:12
Why would you do that?
> file is enough to truncate a file. You don't need any command, just the redirection operator.– terdon♦
Dec 3 at 11:12
sometimes,
> filename won't work. for example, in zsh. but : > filename works still.– CS Pei
yesterday
sometimes,
> filename won't work. for example, in zsh. but : > filename works still.– CS Pei
yesterday
add a comment |
protected by Anthon Jun 20 '17 at 10:06
Thank you for your interest in this question.
Because it has attracted low-quality or spam answers that had to be removed, posting an answer now requires 10 reputation on this site (the association bonus does not count).
Would you like to answer one of these unanswered questions instead?
22
What about
truncate -s 0 filename?– Martin Thoma
Jun 29 '14 at 16:18
Very similar to Difference between cat and '>' to zero out a file where you'll find more information.
– Stéphane Chazelas
Oct 26 '15 at 15:27
The first will work only if called from bash command line, but won't work if executed in a .sh file
– Marco Marsala
Nov 24 '15 at 11:04
6
touch does not delete contents, but does change access time on the file. It does create an empty file if none existed.
– hbogert
Sep 28 '16 at 10:12