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How can I create a file (e.g. using the touch command or another similar way), but with a specific size (e.g. 14 MB)?

The use case is to test FTP with various file sizes. If we can create a file with a specific size, it will be useful to verify FTP transfer etc.

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Related SO question: stackoverflow.com/questions/257844/… –  Oliver Salzburg Jun 18 '13 at 13:49

7 Answers 7

up vote 121 down vote accepted

Use truncate:

truncate -s 14M filename

From man truncate:

   Shrink or extend the size of each FILE to the specified size


 -s, --size=SIZE
          set or adjust the file size by SIZE

Note: truncate may not be available on your system, e.g. on Mac OS X it's not installed by default (but you can easily install it, using macports for example). In these cases you may need to use dd or head instead.

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wow. very nice, I didn't know this. –  naxa Jun 18 '13 at 11:28
man says it all: truncate - shrink or extend the size of a file to the specified size –  ssedano Jun 18 '13 at 12:48
Very nice indeed. The touch is not needed though, you can simply run truncate -s 14M filename it will create filename directly. –  terdon Jun 18 '13 at 13:00
@fasil if this answers your question, please mark it as accepted. –  terdon Jun 18 '13 at 13:43
This seems to create a sparse file, whereas the below answer (that uses "dd") doesn't. I don't know if the difference is important to you, but I figured I'd point it out –  offby1 Jun 18 '13 at 17:33

EDIT: The simplest way is probably the truncate of Ashutosh Vishwa Bandhu's answer, but as pointed out by @offby1 that creates sparse files which may not be what you want. The solutions below create normal, full files.

The following commands create a 14MB file called foo:

  1. fallocate (thanks to @Breakthrough who suggested it in the comments and vote up Ahmed Masud's answer below which also mentions it.)

    fallocate -l 14000000 foo

    This command is particularly impressive since it is as fast as truncate (instantaneous) irrespective of the desired file size (unlike the other solutions which will be slow for large files) and yet creates normal files, not sparse ones :

    $ truncate -s 14MB foo1.txt 
    $ fallocate -l 14000000 foo2.txt 
    $ ls -ls foo?.txt 
        0 -rw-r--r-- 1 terdon terdon 14000000 Jun 21 03:54 foo1.txt
    13672 -rw-r--r-- 1 terdon terdon 14000000 Jun 21 03:55 foo2.txt
  2. Create a file filled with random data

    dd if=/dev/urandom of=foo bs=14MB count=1


    head -c 14MB /dev/urandom > foo
  3. Create a file filled with \0s:

    dd if=/dev/zero of=foo.txt bs=14MB count=1


    head -c 14MB /dev/zero > foo
  4. Create a file filled with the first 14MB of data of another file:

    head -c 14MB bar.txt > foo
  5. Create a file filled with the last 14MB of data of another file:

    tail -c 14MB bar.txt > foo

In all of the above examples, the file will be 14*1000*1000 if you want 14*1024*1024, replace MB with M. For example:

dd if=/dev/urandom of=foo bs=14M count=1
head -c 14M /dev/zero > foo

fallocate only deals in bytes, so you'd have to do (14*1024*1024=14680064)

fallocate -l 14680064 foo
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Thanks... This will also meet my purpose of creating a file of desired size... –  fasil Jun 18 '13 at 13:47
i'd recommend against the use of dd if there is a simpler, less potentially-destructive approach. –  acolyte Jun 18 '13 at 15:50
@acolyte so would I. That's why each dd solution has a non-dd alternative. –  terdon Jun 18 '13 at 17:54
The wiki entry says than even dd (with zeros) will create a sparse file, doesn't it? en.wikipedia.org/wiki/Sparse_file –  Michael Jun 19 '13 at 22:01
@Michael I checked, just to be sure, and none of my commands produces sparse files. The dd command on the wiki page is completely different and does not use /dev/zero. As for whether M or MB is more logical, take it up with the developers. The man pages of head, tail and dd all clearly state that "MB =1000*1000, M =1024*1024". This may be different on other *nixes. –  terdon Jun 20 '13 at 1:16
dd bs=1MB count=14 if=/dev/zero of=<yourfilename>

WARNING dd will silently overwrite anything you have the rights to write to, if you specify it either willingly or by accident. Make sure you understand dd, eg. with reading dd --help, and you don't screw up your dd command lines or else you can easily create irreversible data loss.

For start, if= is the input file and of= is the output file. Always double check that you got it right, before running the command. :)

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Use magic dd ;) man page

$ dd if=/dev/zero of=output.dat  bs=1024  count=14336

cause 14MB*1024 = 14336KB, or:

$ dd if=/dev/zero of=output.dat  bs=1MB  count=14

And you'll get 14mb output.dat file, filled with zeroes :D

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This should be the fastest way to do it:

dd if=/dev/zero of=outfile bs=1 seek=$SEEK count=1 

Otherwise just write a quick and dirty C or perl program that does a seek to the exact position and writes a single byte. This is a LOT faster than actually dumping data onto the blocks.

If you want to avoid sparse files then on Linux (and possibly other systems) you can use fallocate -l <length> from util-linux.

If that's unavailable, and you have a POSIX compliant system then you can write a quick and dirty program using posix_fallocate() library call. posix_fallocate guarantees allocation of the range specified so you will not get a disk-full on subsequent use of the range if it returns successfully

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While dd is the portable way to do it, the commands mkfile and xfs_mkfile are written specifically for this purpose and are faster:

mkfile size[b|k|m|g] filename
xfs_mkfile size[b|k|m|g] filename

The catch is that it is that neither command is universally available, so they are not good for writing portable scripts. However, if I'm doing it interactively via the command line, those are the commands I use.

By default the create non-holey zero-filled files. They both take the -n option to create holey files instead.

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There is another option with Perl:

perl -e 'print "a" x 1024' > file.txt

This will create 1KB file. Replace 1024 with any number you like.

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