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13

1

I have a file that I want to pad until it reaches 16 MiB (16777216 bytes). Currently it is 16515072 bytes. The difference is 262144 bytes.

How do I pad it?

This doesn't seem to be working:

cp smallfile.img largerfile.img

dd if=/dev/zero of=largerfile.img bs=1 count=262144

dd

share|improve this question

edited Apr 17 '15 at 11:59

Peter Mortensen

93069

asked Apr 16 '15 at 22:04

tarabytetarabyte

1,11951935

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @terabyte; do you want physical padding or logical padding? In other words; should the file only show a size of 16777216 (and may contain holes) or shall it also occupy that amount of storage on the disk? - BTW, choosing a bs=1 in dd is in my experience very runtime expensive.
  
  – Janis
  Apr 16 '15 at 22:50
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  truncate -s 16M thefile
  
  – frostschutz
  Apr 17 '15 at 9:28
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @frostschutz that'd be a good answer, were you to post it as an answer.
  
  – derobert
  Apr 17 '15 at 9:40
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @derobert, What's with StackExchange site users posting legit, simple answers as comments?
  
  – user1717828
  Apr 20 '15 at 5:29
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @user1717828 not sure, probably a good question for meta.
  
  – derobert
  Apr 20 '15 at 7:27
  

  
  
  
  

add a comment | 

13

1

I have a file that I want to pad until it reaches 16 MiB (16777216 bytes). Currently it is 16515072 bytes. The difference is 262144 bytes.

How do I pad it?

This doesn't seem to be working:

cp smallfile.img largerfile.img

dd if=/dev/zero of=largerfile.img bs=1 count=262144

dd

share|improve this question

edited Apr 17 '15 at 11:59

Peter Mortensen

93069

asked Apr 16 '15 at 22:04

tarabytetarabyte

1,11951935

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  @terabyte; do you want physical padding or logical padding? In other words; should the file only show a size of 16777216 (and may contain holes) or shall it also occupy that amount of storage on the disk? - BTW, choosing a bs=1 in dd is in my experience very runtime expensive.
  
  – Janis
  Apr 16 '15 at 22:50
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  truncate -s 16M thefile
  
  – frostschutz
  Apr 17 '15 at 9:28
  

  
  
  
  


• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  @frostschutz that'd be a good answer, were you to post it as an answer.
  
  – derobert
  Apr 17 '15 at 9:40
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @derobert, What's with StackExchange site users posting legit, simple answers as comments?
  
  – user1717828
  Apr 20 '15 at 5:29
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  @user1717828 not sure, probably a good question for meta.
  
  – derobert
  Apr 20 '15 at 7:27
  

  
  
  
  

add a comment | 

I have a file that I want to pad until it reaches 16 MiB (16777216 bytes). Currently it is 16515072 bytes. The difference is 262144 bytes.

How do I pad it?

This doesn't seem to be working:

cp smallfile.img largerfile.img

dd if=/dev/zero of=largerfile.img bs=1 count=262144

dd

share|improve this question

edited Apr 17 '15 at 11:59

Peter Mortensen

93069

asked Apr 16 '15 at 22:04

tarabytetarabyte

1,11951935

cp smallfile.img largerfile.img

dd if=/dev/zero of=largerfile.img bs=1 count=262144

share|improve this question

edited Apr 17 '15 at 11:59

Peter Mortensen

93069

asked Apr 16 '15 at 22:04

tarabytetarabyte

1,11951935

share|improve this question

edited Apr 17 '15 at 11:59

Peter Mortensen

93069

asked Apr 16 '15 at 22:04

tarabytetarabyte

1,11951935

edited Apr 17 '15 at 11:59

Peter Mortensen

93069

edited Apr 17 '15 at 11:59

Peter Mortensen

93069

asked Apr 16 '15 at 22:04

tarabytetarabyte

1,11951935

asked Apr 16 '15 at 22:04

tarabytetarabyte

1,11951935

5 Answers
5

active

oldest

votes

8

Drop the of=largerfile.txt and append stdout to the file:

dd if=/dev/zero bs=1 count=262144 >> largerfile.txt

share|improve this answer

answered Apr 16 '15 at 22:13

OuturnateOuturnate

984617

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  seek is a right option here.
  
  – 0andriy
  Oct 29 '16 at 17:44
  

  
  
  
  

add a comment | 

10

Besides the answers to get a physical padding you may also leave most of the padding space in the file just empty ("holes"), by seeking to the new end-position of the file and writing a single character:

dd if=/dev/zero of=largerfile.txt bs=1 count=1 seek=16777215

(which has the advantage to be much more performant, specifically with bs=1, and does not occupy large amounts of additional disk space).

That method seems to work even without adding any character, by using if=/dev/null and the final desired file size:

dd if=/dev/null of=largerfile.txt bs=1 count=1 seek=16777216

A performant variant of a physical padding solution that uses larger block-sizes is:

padding=262144 bs=32768 nblocks=$((padding/bs)) rest=$((padding%bs))

{

  dd if=/dev/zero bs=$bs count=$nblocks

  dd if=/dev/zero bs=$rest count=1

} 2>/dev/null >>largerfile.txt

share|improve this answer

edited Apr 16 '15 at 23:51

answered Apr 16 '15 at 22:59

JanisJanis

10.5k21738

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Correct. In this case truncate -s +262144 largerfile.txt would also be fast.
  
  – don_crissti
  Apr 16 '15 at 23:08
  
  
  

  
  
  
  

add a comment | 

4

The best anwser here is Janis's (above) because it lets you forget about the current file size and pad directly to the desired size with no calculation.

It also takes advantage of sparse files, which appending /dev/zero doesn't.

The answer could be tidier though, because 'count' is allowed to be 0 and you still get the padding:

dd if=/dev/null of=largerfile.txt bs=1 count=0 seek=16777216

(Edit: this is correct for GNU dd, but the behaviour of count=0 is platform-specific, see comments)

share|improve this answer

edited Jul 6 '18 at 11:45

answered Apr 17 '15 at 9:21

PeteCPeteC

1663

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You are mistaken: count=0 is unspecified, but typically is the same as when no count parameter was specified. More problems: dd truncates the file to 16777216 bytes, but if you are in hope that this creates a hole at the end, you are mistaken since you would first need to write data after the hole and later truncate it to a size that contains no data.
  
  – schily
  Jul 4 '18 at 14:58
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  count=0 is nothing like specifying no count parameter. Are you saying that dd if=/dev/zero of=somefile is the same as dd if=/dev/zero of=somefile count=0? Try it.
  
  – PeteC
  Jul 6 '18 at 10:32
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Of course! count=0 is the same as if you did not specify a count parameter at all. This is true at least for all implementations that have been derived from the original sources. Try it, it seems that you did never work with the original dd command.
  
  – schily
  Jul 6 '18 at 10:39
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I can't find any documentation that specifies 0 as a unique value for count meaning 'ignore this parameter'. Can you find any? Without such documentation, count=0 means 'write zero blocks' and any deviation from this is a bug... (original sources or no).
  
  – PeteC
  Jul 6 '18 at 11:14
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  This was the POSIX documentation from before May 2015 when the underspecified text was corrected.
  
  – schily
  Jul 6 '18 at 12:43
  

  
  
  
  

 | 
show 2 more comments

1

Do you have to use dd? If you want a file to have a particular (logical) length, just write a zero to the position you want. The bytes between the previous end and the written byte will be displayed as having null bytes. Here's an example using perl.

$ echo Hello > file

$ ls -l file

-rw-r--r-- 1 user group 6 Apr 16 22:59 file

$ perl -le 'open(my $f,"+<","file"); seek($f, 16777216 - 2, 0); print $f ""'

$ ls -ln file

-rw-r--r-- 1 user group 16777216 Apr 16 22:59 file

Why the "- 2" in the line? The script will write a byte, so we subtract 1 to seek to the position before that byte. We take off the other because the seek position is zero-indexed.

share|improve this answer

answered Apr 17 '15 at 6:06

BowlOfRedBowlOfRed

2,705717

add a comment | 

0

yes!
it works for dd if=/dev/zero bs=1 count=262144 >> largerfile.txt

make this transaction empty with that size.
restart database

share

answered 1 min ago

EricEric

1

New contributor

Eric is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

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8

Drop the of=largerfile.txt and append stdout to the file:

dd if=/dev/zero bs=1 count=262144 >> largerfile.txt

share|improve this answer

answered Apr 16 '15 at 22:13

OuturnateOuturnate

984617

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  seek is a right option here.
  
  – 0andriy
  Oct 29 '16 at 17:44
  

  
  
  
  

add a comment | 

8

Drop the of=largerfile.txt and append stdout to the file:

dd if=/dev/zero bs=1 count=262144 >> largerfile.txt

share|improve this answer

answered Apr 16 '15 at 22:13

OuturnateOuturnate

984617

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  seek is a right option here.
  
  – 0andriy
  Oct 29 '16 at 17:44
  

  
  
  
  

add a comment | 

Drop the of=largerfile.txt and append stdout to the file:

dd if=/dev/zero bs=1 count=262144 >> largerfile.txt

share|improve this answer

answered Apr 16 '15 at 22:13

OuturnateOuturnate

984617

dd if=/dev/zero bs=1 count=262144 >> largerfile.txt

share|improve this answer

answered Apr 16 '15 at 22:13

OuturnateOuturnate

984617

answered Apr 16 '15 at 22:13

OuturnateOuturnate

984617

answered Apr 16 '15 at 22:13

OuturnateOuturnate

984617

10

Besides the answers to get a physical padding you may also leave most of the padding space in the file just empty ("holes"), by seeking to the new end-position of the file and writing a single character:

dd if=/dev/zero of=largerfile.txt bs=1 count=1 seek=16777215

(which has the advantage to be much more performant, specifically with bs=1, and does not occupy large amounts of additional disk space).

That method seems to work even without adding any character, by using if=/dev/null and the final desired file size:

dd if=/dev/null of=largerfile.txt bs=1 count=1 seek=16777216

A performant variant of a physical padding solution that uses larger block-sizes is:

padding=262144 bs=32768 nblocks=$((padding/bs)) rest=$((padding%bs))

{

  dd if=/dev/zero bs=$bs count=$nblocks

  dd if=/dev/zero bs=$rest count=1

} 2>/dev/null >>largerfile.txt

share|improve this answer

edited Apr 16 '15 at 23:51

answered Apr 16 '15 at 22:59

JanisJanis

10.5k21738

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Correct. In this case truncate -s +262144 largerfile.txt would also be fast.
  
  – don_crissti
  Apr 16 '15 at 23:08
  
  
  

  
  
  
  

add a comment | 

10

Besides the answers to get a physical padding you may also leave most of the padding space in the file just empty ("holes"), by seeking to the new end-position of the file and writing a single character:

dd if=/dev/zero of=largerfile.txt bs=1 count=1 seek=16777215

(which has the advantage to be much more performant, specifically with bs=1, and does not occupy large amounts of additional disk space).

That method seems to work even without adding any character, by using if=/dev/null and the final desired file size:

dd if=/dev/null of=largerfile.txt bs=1 count=1 seek=16777216

A performant variant of a physical padding solution that uses larger block-sizes is:

padding=262144 bs=32768 nblocks=$((padding/bs)) rest=$((padding%bs))

{

  dd if=/dev/zero bs=$bs count=$nblocks

  dd if=/dev/zero bs=$rest count=1

} 2>/dev/null >>largerfile.txt

share|improve this answer

edited Apr 16 '15 at 23:51

answered Apr 16 '15 at 22:59

JanisJanis

10.5k21738

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  
  Correct. In this case truncate -s +262144 largerfile.txt would also be fast.
  
  – don_crissti
  Apr 16 '15 at 23:08
  
  
  

  
  
  
  

add a comment | 

Besides the answers to get a physical padding you may also leave most of the padding space in the file just empty ("holes"), by seeking to the new end-position of the file and writing a single character:

dd if=/dev/zero of=largerfile.txt bs=1 count=1 seek=16777215

(which has the advantage to be much more performant, specifically with bs=1, and does not occupy large amounts of additional disk space).

That method seems to work even without adding any character, by using if=/dev/null and the final desired file size:

dd if=/dev/null of=largerfile.txt bs=1 count=1 seek=16777216

A performant variant of a physical padding solution that uses larger block-sizes is:

padding=262144 bs=32768 nblocks=$((padding/bs)) rest=$((padding%bs))

{

  dd if=/dev/zero bs=$bs count=$nblocks

  dd if=/dev/zero bs=$rest count=1

} 2>/dev/null >>largerfile.txt

share|improve this answer

edited Apr 16 '15 at 23:51

answered Apr 16 '15 at 22:59

JanisJanis

10.5k21738

dd if=/dev/zero of=largerfile.txt bs=1 count=1 seek=16777215

dd if=/dev/null of=largerfile.txt bs=1 count=1 seek=16777216

padding=262144 bs=32768 nblocks=$((padding/bs)) rest=$((padding%bs))

{

  dd if=/dev/zero bs=$bs count=$nblocks

  dd if=/dev/zero bs=$rest count=1

} 2>/dev/null >>largerfile.txt

share|improve this answer

edited Apr 16 '15 at 23:51

answered Apr 16 '15 at 22:59

JanisJanis

10.5k21738

answered Apr 16 '15 at 22:59

JanisJanis

10.5k21738

answered Apr 16 '15 at 22:59

JanisJanis

10.5k21738

4

The best anwser here is Janis's (above) because it lets you forget about the current file size and pad directly to the desired size with no calculation.

It also takes advantage of sparse files, which appending /dev/zero doesn't.

The answer could be tidier though, because 'count' is allowed to be 0 and you still get the padding:

dd if=/dev/null of=largerfile.txt bs=1 count=0 seek=16777216

(Edit: this is correct for GNU dd, but the behaviour of count=0 is platform-specific, see comments)

share|improve this answer

edited Jul 6 '18 at 11:45

answered Apr 17 '15 at 9:21

PeteCPeteC

1663

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You are mistaken: count=0 is unspecified, but typically is the same as when no count parameter was specified. More problems: dd truncates the file to 16777216 bytes, but if you are in hope that this creates a hole at the end, you are mistaken since you would first need to write data after the hole and later truncate it to a size that contains no data.
  
  – schily
  Jul 4 '18 at 14:58
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  count=0 is nothing like specifying no count parameter. Are you saying that dd if=/dev/zero of=somefile is the same as dd if=/dev/zero of=somefile count=0? Try it.
  
  – PeteC
  Jul 6 '18 at 10:32
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Of course! count=0 is the same as if you did not specify a count parameter at all. This is true at least for all implementations that have been derived from the original sources. Try it, it seems that you did never work with the original dd command.
  
  – schily
  Jul 6 '18 at 10:39
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I can't find any documentation that specifies 0 as a unique value for count meaning 'ignore this parameter'. Can you find any? Without such documentation, count=0 means 'write zero blocks' and any deviation from this is a bug... (original sources or no).
  
  – PeteC
  Jul 6 '18 at 11:14
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  This was the POSIX documentation from before May 2015 when the underspecified text was corrected.
  
  – schily
  Jul 6 '18 at 12:43
  

  
  
  
  

 | 
show 2 more comments

4

The best anwser here is Janis's (above) because it lets you forget about the current file size and pad directly to the desired size with no calculation.

It also takes advantage of sparse files, which appending /dev/zero doesn't.

The answer could be tidier though, because 'count' is allowed to be 0 and you still get the padding:

dd if=/dev/null of=largerfile.txt bs=1 count=0 seek=16777216

(Edit: this is correct for GNU dd, but the behaviour of count=0 is platform-specific, see comments)

share|improve this answer

edited Jul 6 '18 at 11:45

answered Apr 17 '15 at 9:21

PeteCPeteC

1663

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  You are mistaken: count=0 is unspecified, but typically is the same as when no count parameter was specified. More problems: dd truncates the file to 16777216 bytes, but if you are in hope that this creates a hole at the end, you are mistaken since you would first need to write data after the hole and later truncate it to a size that contains no data.
  
  – schily
  Jul 4 '18 at 14:58
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  count=0 is nothing like specifying no count parameter. Are you saying that dd if=/dev/zero of=somefile is the same as dd if=/dev/zero of=somefile count=0? Try it.
  
  – PeteC
  Jul 6 '18 at 10:32
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Of course! count=0 is the same as if you did not specify a count parameter at all. This is true at least for all implementations that have been derived from the original sources. Try it, it seems that you did never work with the original dd command.
  
  – schily
  Jul 6 '18 at 10:39
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I can't find any documentation that specifies 0 as a unique value for count meaning 'ignore this parameter'. Can you find any? Without such documentation, count=0 means 'write zero blocks' and any deviation from this is a bug... (original sources or no).
  
  – PeteC
  Jul 6 '18 at 11:14
  
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  
  This was the POSIX documentation from before May 2015 when the underspecified text was corrected.
  
  – schily
  Jul 6 '18 at 12:43
  

  
  
  
  

 | 
show 2 more comments

The best anwser here is Janis's (above) because it lets you forget about the current file size and pad directly to the desired size with no calculation.

It also takes advantage of sparse files, which appending /dev/zero doesn't.

The answer could be tidier though, because 'count' is allowed to be 0 and you still get the padding:

dd if=/dev/null of=largerfile.txt bs=1 count=0 seek=16777216

(Edit: this is correct for GNU dd, but the behaviour of count=0 is platform-specific, see comments)

share|improve this answer

edited Jul 6 '18 at 11:45

answered Apr 17 '15 at 9:21

PeteCPeteC

1663

dd if=/dev/null of=largerfile.txt bs=1 count=0 seek=16777216

share|improve this answer

edited Jul 6 '18 at 11:45

answered Apr 17 '15 at 9:21

PeteCPeteC

1663

answered Apr 17 '15 at 9:21

PeteCPeteC

1663

answered Apr 17 '15 at 9:21

PeteCPeteC

1663

1

Do you have to use dd? If you want a file to have a particular (logical) length, just write a zero to the position you want. The bytes between the previous end and the written byte will be displayed as having null bytes. Here's an example using perl.

$ echo Hello > file

$ ls -l file

-rw-r--r-- 1 user group 6 Apr 16 22:59 file

$ perl -le 'open(my $f,"+<","file"); seek($f, 16777216 - 2, 0); print $f ""'

$ ls -ln file

-rw-r--r-- 1 user group 16777216 Apr 16 22:59 file

Why the "- 2" in the line? The script will write a byte, so we subtract 1 to seek to the position before that byte. We take off the other because the seek position is zero-indexed.

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answered Apr 17 '15 at 6:06

BowlOfRedBowlOfRed

2,705717

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1

Do you have to use dd? If you want a file to have a particular (logical) length, just write a zero to the position you want. The bytes between the previous end and the written byte will be displayed as having null bytes. Here's an example using perl.

$ echo Hello > file

$ ls -l file

-rw-r--r-- 1 user group 6 Apr 16 22:59 file

$ perl -le 'open(my $f,"+<","file"); seek($f, 16777216 - 2, 0); print $f ""'

$ ls -ln file

-rw-r--r-- 1 user group 16777216 Apr 16 22:59 file

Why the "- 2" in the line? The script will write a byte, so we subtract 1 to seek to the position before that byte. We take off the other because the seek position is zero-indexed.

share|improve this answer

answered Apr 17 '15 at 6:06

BowlOfRedBowlOfRed

2,705717

add a comment | 

Do you have to use dd? If you want a file to have a particular (logical) length, just write a zero to the position you want. The bytes between the previous end and the written byte will be displayed as having null bytes. Here's an example using perl.

$ echo Hello > file

$ ls -l file

-rw-r--r-- 1 user group 6 Apr 16 22:59 file

$ perl -le 'open(my $f,"+<","file"); seek($f, 16777216 - 2, 0); print $f ""'

$ ls -ln file

-rw-r--r-- 1 user group 16777216 Apr 16 22:59 file

Why the "- 2" in the line? The script will write a byte, so we subtract 1 to seek to the position before that byte. We take off the other because the seek position is zero-indexed.

share|improve this answer

answered Apr 17 '15 at 6:06

BowlOfRedBowlOfRed

2,705717

$ echo Hello > file

$ ls -l file

-rw-r--r-- 1 user group 6 Apr 16 22:59 file

$ perl -le 'open(my $f,"+<","file"); seek($f, 16777216 - 2, 0); print $f ""'

$ ls -ln file

-rw-r--r-- 1 user group 16777216 Apr 16 22:59 file

share|improve this answer

answered Apr 17 '15 at 6:06

BowlOfRedBowlOfRed

2,705717

answered Apr 17 '15 at 6:06

BowlOfRedBowlOfRed

2,705717

answered Apr 17 '15 at 6:06

BowlOfRedBowlOfRed

2,705717

0

yes!
it works for dd if=/dev/zero bs=1 count=262144 >> largerfile.txt

make this transaction empty with that size.
restart database

share

answered 1 min ago

EricEric

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Eric is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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add a comment | 

0

yes!
it works for dd if=/dev/zero bs=1 count=262144 >> largerfile.txt

make this transaction empty with that size.
restart database

share

answered 1 min ago

EricEric

1

New contributor

Eric is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

add a comment | 

yes!
it works for dd if=/dev/zero bs=1 count=262144 >> largerfile.txt

make this transaction empty with that size.
restart database

share

answered 1 min ago

EricEric

1

New contributor

Eric is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share

answered 1 min ago

EricEric

1

New contributor

Eric is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered 1 min ago

EricEric

1

New contributor

Eric is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

answered 1 min ago

EricEric

1