Writing a file with radare2 `w`?

Question

Radare2 supports a w which writes a string.

w foobar             write string 'foobar'

However, it doesn't seem to work for me,

$ touch foo

$ radare2 ./foo
w foobar

The file foo remains empty. Am I supposed to flush or save?

Megabeets · Accepted Answer

TLDR

radare2 opens a file in read-only mode by default. (see the manual r2 -h for further information.)
to allow writing to a file, start radare2 in write mode (r2 -w file).
when in a read-only session already, oo+ will re-open the file in write mode.
the cache mode (e io.cache=true) mimics writing access but changes in radare2 are not actually written to disk.

Write Mode - Example

Open the file foo in write mode:
$ r2 -w foo

Print 32 bytes (with px 32):
[0x00000000]> px 32
- offset -   0 1  2 3  4 5  6 7  8 9  A B  C D  E F  0123456789ABCDEF
0x00000000  ffff ffff ffff ffff ffff ffff ffff ffff  ................
0x00000010  ffff ffff ffff ffff ffff ffff ffff ffff  ................

Note: The first 32 bytes (and the rest of the file) are empty.

Let's write "Hello World!" into the file:
[0x00000000]> w Hello World!

And print the 32 bytes again:
[0x00000000]> px 32
- offset -   0 1  2 3  4 5  6 7  8 9  A B  C D  E F  0123456789ABCDEF
0x00000000  4865 6c6c 6f20 576f 726c 6421 ffff ffff  Hello World!....
0x00000010  ffff ffff ffff ffff ffff ffff ffff ffff  ................

Great! Now we can see that "Hello World!" is written to the file.

Let's quit radare2 and cat the file to see the content:
 [0x00000000]> q

$ cat foo
Hello World!

Cache Mode - Example

Open the file foo in write mode:
$ r2 foo

Print the first 32 bytes@
[0x00000000]> px 32
- offset -   0 1  2 3  4 5  6 7  8 9  A B  C D  E F  0123456789ABCDEF
0x00000000  4865 6c6c 6f20 576f 726c 6421 ffff ffff  Hello World!....
0x00000010  ffff ffff ffff ffff ffff ffff ffff ffff  ................

Activate 'cache mode`:
[0x00000000]> e io.cache=true

Make some changes in radare2:
[0x00000000]> w Goodbyte World! :)

Proof that radare2 mimicked writing as if in write mode:
[0x00000000]> px 32
- offset -   0 1  2 3  4 5  6 7  8 9  A B  C D  E F  0123456789ABCDEF
0x00000000  476f 6f64 6279 7465 2057 6f72 6c64 2120  Goodbyte World!
0x00000010  3a29 ffff ffff ffff ffff ffff ffff ffff  :)..............

Quit radare2:
[0x00000000]> q

Examine actual file contents:
$ cat foo
Hello World!

Note: The file didn't change! That's the effect of cache mode.

Paweł Łukasik · Answer

Just to update @Megabeets answer.

When you start with an empty file (created by touch), the w command will not work by default even if the r2 will be started in write mode. The missing information is, in that case, the mapping

[0x00000000]> om
[0x00000000]

Returns an empty result. Apart from listing, the om command can also be used to create the mapping:

Usage: om[-] [arg]   # map opened files
| om                                       list all defined IO maps
...
| om fd vaddr [size] [paddr] [rwx] [name]  create new io map
...

In order to do the mapping, one needs to specify (for example) the following command

[0x00000000]> om 3 0x0 12
[0x00000000]> om
 1 fd: 3 +0x00000000 0x00000000 - 0x0000000b rwx

That will create, for the file description 3, a 12 bytes in size mapping starting from an address 0x0.

After that w will work:

[0x00000000]> w Hello world!
[0x00000000]> px 32
- offset -   0 1  2 3  4 5  6 7  8 9  A B  C D  E F  0123456789ABCDEF
0x00000000  4865 6c6c 6f20 776f 726c 6421 ffff ffff  Hello world!....
0x00000010  ffff ffff ffff ffff ffff ffff ffff ffff  ................
[0x00000000]>

Semnodime · Answer

Create an input/output mapping to allow writing to a non-mapped file
To allow writing up to 64 byte starting at offset 0x00000000, map changes in radare2 to file descriptor 3 (the file opened in radare2).
[0x00000000]> om 3 0x0 64
[0x00000000]> om
1 fd: 3 +0x00000000 0x00000000 - 0x0000000b rwx

Explanation
An empty file (as created by touch) has no input/output mappings (even when opened with in write mode).
To confirm no region has been mapped, you can list all defined IO maps with the radare2-command om.)
To create an i/o-mapping use om with parameters:

om fd vaddr [size] [paddr] [rwx] [name]  create new io map

Example
radare2 -w test_file
 -- What has been executed cannot be unexecuted
[0x00000000]> w This won't be written anywhere, because no mapping exists.
[0x00000000]> om
[0x00000000]> om 3 0 64
[0x00000000]> om
 1 fd: 3 +0x00000000 0x00000000 - 0x0000003f rwx 
[0x00000000]> w Hello World!
[0x00000000]> V

Writing a file with radare2 `w`?

3 Answers

TLDR

Write Mode - Example

Cache Mode - Example

Create an input/output mapping to allow writing to a non-mapped file

Explanation

Example

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