How to create a file with a given size in Linux

Question

For testing purposes I have to generate a file of a certain size  to test an upload limit      What is a command to create a file of a certain size on Linux

User · Answer

you could do    dsm localhost     perl -e  print   0  x 100   gt  filename ext   Where you replace 100 with the number of bytes you want written

User · Answer

Use fallocate if you don t want to wait for disk   Example   fallocate -l 100G BigFile   Usage   Usage   fallocate  options   lt filename gt   Preallocate space to  or deallocate space from a file   Options   -c  --collapse-range remove a range from the file  -d  --dig-holes      detect zeroes and replace with holes  -i  --insert-range   insert a hole at range  shifting existing data  -l  --length  lt num gt    length for range operations  in bytes  -n  --keep-size      maintain the apparent size of the file  -o  --offset  lt num gt    offset for range operations  in bytes  -p  --punch-hole     replace a range with a hole  implies -n   -z  --zero-range     zero and ensure allocation of a range  -x  --posix          use posix fallocate 3  instead of fallocate 2   -v  --verbose        verbose mode   -h  --help           display this help  -V  --version        display version

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Some of these answers have you using  dev zero for the source of your data   If your testing network upload speeds  this may not be the best idea if your application is doing any compression  a file full of zeros compresses really well   Using this command to generate the file   dd if  dev zero of upload test bs 10000 count 1   I could compress upload test down to about 200 bytes   So you could put yourself in a situation where you think your uploading a 10KB file but it would actually be much less   What I suggest is using  dev urandom instead of  dev zero   I couldn t compress the output of  dev urandom very much at all

User · Answer

Use this command    dd if  INPUT-FILE of  OUTPUT-FILE bs  BLOCK-SIZE count  NUM-BLOCKS   To create a big  empty  file  set  INPUT-FILE  dev zero  Total size of the file will be  BLOCK-SIZE    NUM-BLOCKS  New file created will be  OUTPUT-FILE

User · Answer

You can do it programmatically    include  lt unistd h gt   include  lt sys types h gt   include  lt sys stat h gt   include  lt fcntl h gt   include  lt stdlib h gt   int main         int fd   creat   tmp foo txt   0644       ftruncate fd  SIZE IN BYTES       close fd       return 0      This approach is especially useful to subsequently mmap the file into memory   use the following command to check that the file has the correct size     du -B1 --apparent-size  tmp foo txt   Be careful     du  tmp foo txt   will probably print 0 because it is allocated as Sparse file if supported by your filesystem   see also  man 2 open and man 2 truncate

User · Answer

As shell command    lt   dev zero head -c 1048576  gt   output

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This will generate 4 MB text file with random characters in current directory and its name  4mb txt  You can change parameters to generate different sizes and names   base64  dev urandom   head -c 4000000  gt  4mb txt

User · Answer

Please  modern is easier  and faster  On Linux   pick one   truncate -s 10G foo fallocate -l 5G bar   It needs to be stated that truncate on a file system supporting sparse files will create a sparse file and fallocate will not  A sparse file is one where the allocation units that make up the file are not actually allocated until used  The meta-data for the file will however take up some considerable space but likely no where near the actual size of the file  You should consult resources about sparse files for more information as there are advantages and disadvantages to this type of file  A non-sparse file has its blocks  allocation units  allocated ahead of time which means the space is reserved as far as the file system sees it  Also fallocate nor truncate will not set the contents of the file to a specified value like dd  instead the contents of a file allocated with fallocate or truncate may be any trash value that existed in the allocated units during creation and this behavior may or may not be desired  The dd is the slowest because it actually writes the value or chunk of data to the entire file stream as specified with it s command line options    This behavior could potentially be different - depending on file system used and conformance of that file system to any standard or specification  Therefore it is advised that proper research is done to ensure that the appropriate method is used

User · Answer

Just to follow up Tom s post  you can use dd to create sparse files as well   dd if  dev zero of the file bs 1 count 0 seek 12345   This will create a file with a  hole  in it on most unixes - the data won t actually be written to disk  or take up any space until something other than zero is written into it

User · Answer

For small files  dd if  dev zero of upload test bs file size count 1  Where file size is the size of your test file in bytes  For big files  dd if  dev zero of upload test bs 1M count size in megabytes

User · Answer

There are lots of answers  but none explained nicely what else can be done  Looking into man pages for dd  it is possible to better specify the size of a file   This is going to create  tmp zero big data file bin filled with zeros  that has size of 20 megabytes        dd if  dev zero of  tmp zero big data file bin  bs 1M count 20   This is going to create  tmp zero 1000bytes data file bin filled with zeros  that has size of 1000 bytes        dd if  dev zero of  tmp zero 1000bytes data file bin  bs 1kB count 1   or        dd if  dev zero of  tmp zero 1000bytes data file bin  bs 1000 count 1      In all examples  bs is block size  and count is number of blocks BLOCKS and BYTES may be followed by the following multiplicative suffixes  c  1  w  2  b  512  kB  1000  K  1024  MB  1000 1000  M  1024 1024  xM  M GB  1000 1000 1000  G  1024 1024 1024  and so on for T  P  E  Z  Y

User · Answer

You can do it programmatically    include  lt unistd h gt   include  lt sys types h gt   include  lt sys stat h gt   include  lt fcntl h gt   include  lt stdlib h gt   int main         int fd   creat   tmp foo txt   0644       ftruncate fd  SIZE IN BYTES       close fd       return 0      This approach is especially useful to subsequently mmap the file into memory   use the following command to check that the file has the correct size     du -B1 --apparent-size  tmp foo txt   Be careful     du  tmp foo txt   will probably print 0 because it is allocated as Sparse file if supported by your filesystem   see also  man 2 open and man 2 truncate

User · Answer

dd if  dev zero of my file txt count 12345

User · Answer

On OSX  and Solaris  apparently   the mkfile command is available as well   mkfile 10g big file   This makes a 10 GB file named  big file   Found this approach here

[linux] How to create a file with a given size in Linux?

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