Libguestfs - Dec 2016 - IRC question: appending newline to end of file

16:43 < martingo_> hi all
16:43 < martingo_> I am using write_append to write at the end of one file
16:43 < martingo_> but newline he is not taking
16:43 < martingo_> guestfish add ./overcloud-full.qcow2 : run : mount
/dev/sda / : write_append /etc/fstab "nodev /mnt/huge_qemu_1G
                   hugetlbfs rw,pagesize=1G  0 0\n"
16:43 < martingo_> any ideas
16:48 < martingo_> any help?

An example of what can go wrong is shown here:

$ guestfish -N fs -m /dev/sda1 \
    write /foo "hello" : write-append /foo "goodbye\n" :
hexdump /foo
00000000  68 65 6c 6c 6f 67 6f 6f  64 62 79 65 5c 6e        |hellogoodbye\n|
0000000e

Notice that the literal characters '\' and 'n' have been added
to the file.

It helps to look at the API calls being made, by adding the
guestfish -x option:

$ guestfish -x -N fs -m /dev/sda1 \
    write /foo "hello" : write-append /foo "goodbye\n" :
hexdump /foo
[...]
libguestfs: trace: write_append "/foo" "goodbye\n"

(Notice that the trace output escapes non-printable characters using
\x.., so again that's the literal two characters '\' and
'n').

This isn't really surprising because we're asking the shell to send
those two characters to guestfish, as you can see by doing:

$ echo "\n" | hexdump -C
00000000  5c 6e 0a                                          |\n.|
00000003

The guestfish *command line* will turn \n into a newline character as
described here:

http://libguestfs.org/guestfish.1.html#escape-sequences-in-double-quoted-arguments

but this doesn't apply to the shell command line because it uses a
different code path.

Anyway to fix this you can either do:

$ guestfish -N fs -m /dev/sda1 \
write /foo "hello" : write-append /foo "goodbye
" : hexdump /foo

You have to actually press the newline key: ``"goodbye<CR>''.

Or slightly less cumbersome is to use a script which invokes the
guestfish command line code path:

$ cat test.sh
guestfish -N fs -m /dev/sda1 <<'EOF'
  write /foo "hello"
  write-append /foo "goodbye\n"
  hexdump /foo
EOF

$ bash test.sh 
00000000  68 65 6c 6c 6f 67 6f 6f  64 62 79 65 0a           |hellogoodbye.|
0000000d

HTH,

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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