/usr/share/icons/trisquel/scalable/places/trisquel.svg belongs to the "root"
user (the so-called superuser) and other users can only *r*ead that file
(what is obviously necessary to display the icon). Using 'ls -l' in a
terminal, that is what "r--", at the end of "-rw-r--r--" tells (you can also
right click on the file in the file manager to display the "Properties",
including the "Permissions"):
$ ls -l /usr/share/icons/trisquel/scalable/places/trisquel.svg
-rw-r--r-- 1 root root 60536 juil. 18 2014
/usr/share/icons/trisquel/scalable/places/trisquel.svg
To know everything about file permissions:
$ info 'File permissions'
Modifying /usr/share/icons/trisquel/scalable/places/trisquel.svg therefore
requires administrative permissions. With those permissions, that 'sudo'
grants, you can directly edit the file. Or you can, with no special
permission, copy the file (what only requires to read the file, not to write
it) in your home directory, modify the copy, again with no special permission
(because the copy belongs to you, not to root) and then move the modified
file to /usr/share/icons/trisquel/scalable/places/, typically using that
command (executed in a terminal opened in the folder where the modified
trisquel.svg file is):
$ sudo mv trisquel.svg /usr/share/icons/trisquel/scalable/places/
Here you need administrative permissions because nobody but "root" is allowed
to overwrite /usr/share/icons/trisquel/scalable/places/trisquel.svg. You do
not even have the permission to write a new file in
/usr/share/icons/trisquel/scalable/places/:
$ ls -l /usr/share/icons/trisquel/scalable/
total 28
drwxr-xr-x 2 root root 12288 mai 14 09:36 actions
drwxr-xr-x 2 root root 4096 mai 14 09:36 apps
drwxr-xr-x 2 root root 4096 mai 14 09:36 devices
drwxr-xr-x 2 root root 4096 mai 14 09:36 places
drwxr-xr-x 2 root root 4096 nov. 12 2012 status
(the line about "places" says this directory belongs to "root" and, in the
first columns, "w" being only in the first triplet "rwx" means that only the
owner can *w*rite in it)
