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Subject: |
sed happily modifies read-only files. |
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Date: |
Fri, 2 Jun 2017 17:57:37 +0200 |
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User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.1.1 |
sed happily modifies read-only files.
*Modifying* *write-protected* files sound me contradicting..
$ ls -l aa
-r--r--r-- 1 un un 4 jún 2 14:56 aa
$ cat aa
CCC
$ sed -ri 's/C/zi/g' aa
$ ls -l aa
-r--r--r-- 1 un un 7 jún 2 17:54 aa
$ cat aa
zizizi
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--- Begin Message ---
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Subject: |
Re: bug#27200: sed happily modifies read-only files. |
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Date: |
Fri, 2 Jun 2017 21:52:01 +0000 |
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Mutt/1.5.23 (2014-03-12) |
tag 27200 notabug
thanks
Hello,
On Fri, Jun 02, 2017 at 05:57:37PM +0200, Péter wrote:
sed happily modifies read-only files.
*Modifying* *write-protected* files sound me contradicting..
This is not a bug - just a sligthly wrong expectation of what
'write-protected' means.
When a FILE is 'write-protected' (technically: does not have
the write bit enabled in its access mode), it means you can not modify
fhe file's content.
Example:
$ echo hello > 1.txt
$ chmod a-w 1.txt
$ echo world > 1.txt
-bash: 1.txt: Permission denied
However, the directory where the file is stored is still writable
(which is why creating files works fine). Deleting
a file and renaming a file also works - it requires write-permission
for the directory, *not* for the file:
$ echo a > 1.txt
-bash: 1.txt: Permission denied
$ mv -f 1.txt old1.txt
$ ls -l
total 4
-r--r--r-- 1 gordon gordon 6 Jun 2 17:36 old1.txt
$ rm -f old1.txt
$ ls -l
total 0
To summarize: read-only access mode on a file protects *only*
the file's content. It does not protect the file from being deleted
or renamed.
Knowing the above, it's easier to see how 'sed -i' works:
it creates a temporary new file, then renames it to replace
the input file.
# Create a write-protected file:
$ echo CCC > 1.txt
$ chmod a-w 1.txt
$ echo a > 1.txt
-bash: 1.txt: Permission denied
# Replace it using a temporary file:
$ sed 's/C/zi/g' 1.txt > 1.tmp
$ mv -f 1.tmp 1.txt
# Or use 'sed -i', which is equivalent to using a temp file:
$ sed -i 's/C/zi/g' 1.txt
$ cat 1.txt
zizizi
In the example above I've used 'rm -f' and 'mv -f'
because these programs detect that a file does not have
the write-bit enabled, and as a courtesy ask the user
if he/she is sure about overwriting it - but there is
nothing technically that 'protects' the file from being deleted/renamed.
As such I'm closing this bug-report, but discussion can continue by
replying to this thread.
regards,
- assaf
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