Test -e not working as expected (by me)


 
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# 1  
Old 03-11-2015
Test -e not working as expected (by me)

I ran into the following and still do not understand entirely the rationale behind this. If someone could explain why things are as they are I'd be thankful.

The following was tested on AIX 7.1 with ksh88, but i suspect that to be ubiquitous. In an installation routine i had to create a set of symbolic links. Because they might already exist i used test -e filename to test it:

Code:
if [ ! -e /some/link ] ; then
     ln -s /path/to/file /some/link
fi

This didn't work as expected because test returned 0 only if the link AND its target existed. If only the link exists but not the file referenced by it test will return 1.

The POSIX Documentation about test says:

Code:
-e  pathname
    True if pathname resolves to a file that exists. False if pathname cannot be resolved.

A "file that exists" is IMHO covered by a link, even if this link points to a file which doesn't. I can stat() this file and do many other operations on it which can be done with a file.

I do understand that i can use "-L", which tests for a link but i would like to understand the rationale behind the behavior. Or is the POSIX documentation inconsistent here?

bakunin
# 2  
Old 03-11-2015
Single UNIX Specification v3

Quote:
3.163 File

An object that can be written to, or read from, or both. A file has certain attributes, including access permissions and type. File types include regular file, character special file, block special file, FIFO special file, symbolic link, socket, and directory. Other types of files may be supported by the implementation.



---------- Post updated at 01:30 PM ---------- Previous update was at 01:18 PM ----------

It seems to be POSIX-compliant behaviour. Look at this excerpt:

Quote:
4.11 Pathname Resolution

Pathname resolution is performed for a process to resolve a pathname to a particular file in a file hierarchy. There may be multiple pathnames that resolve to the same file.
Each filename in the pathname is located in the directory specified by its predecessor (for example, in the pathname fragment a/b, file b is located in directory a). Pathname resolution shall fail if this cannot be accomplished. If the pathname begins with a slash, the predecessor of the first filename in the pathname shall be taken to be the root directory of the process (such pathnames are referred to as "absolute pathnames"). If the pathname does not begin with a slash, the predecessor of the first filename of the pathname shall be taken to be the current working directory of the process (such pathnames are referred to as "relative pathnames").
The interpretation of a pathname component is dependent on the value of {NAME_MAX} and _POSIX_NO_TRUNC associated with the path prefix of that component. If any pathname component is longer than {NAME_MAX}, the implementation shall consider this an error.
A pathname that contains at least one non-slash character and that ends with one or more trailing slashes shall be resolved as if a single dot character ( '.' ) were appended to the pathname.
If a symbolic link is encountered during pathname resolution, the behavior shall depend on whether the pathname component is at the end of the pathname and on the function being performed. If all of the following are true, then pathname resolution is complete:
  • This is the last pathname component of the pathname.
  • The pathname has no trailing slash.
  • The function is required to act on the symbolic link itself, or certain arguments direct that the function act on the symbolic link itself.
In all other cases, the system shall prefix the remaining pathname, if any, with the contents of the symbolic link. If the combined length exceeds {PATH_MAX}, and the implementation considers this to be an error, errnoshall be set to [ENAMETOOLONG] and an error indication shall be returned. Otherwise, the resolved pathname shall be the resolution of the pathname just created. If the resulting pathname does not begin with a slash, the predecessor of the first filename of the pathname is taken to be the directory containing the symbolic link.
If the system detects a loop in the pathname resolution process, it shall set errno to [ELOOP] and return an error indication. The same may happen if during the resolution process more symbolic links were followed than the implementation allows. This implementation-defined limit shall not be smaller than {SYMLOOP_MAX}.
The special filename dot shall refer to the directory specified by its predecessor. The special filename dot-dot shall refer to the parent directory of its predecessor directory. As a special case, in the root directory, dot-dot may refer to the root directory itself.
A pathname consisting of a single slash shall resolve to the root directory of the process. A null pathname shall not be successfully resolved. A pathname that begins with two successive slashes may be interpreted in an implementation-defined manner, although more than two leading slashes shall be treated as a single slash.



and -e tests, if a pathname resolves to a file that exists, not that pathname exists.



Man pages, as e.g. in AIX:
Quote:
-e FileName
Returns a True exit value if the specified FileName exists.



or in Linux:
Quote:
-e FILE FILE exists
are lying ;-)
# 3  
Old 03-11-2015
Consider testing for a link instead:-
Code:
$ ls -l a b
ls: cannot access a: No such file or directory
lrwxrwxrwx 1 RBATTE1 TS 1 Mar 11 13:32 b -> a

$ if [ -L b ]
> then
>   echo hit
> else
>   echo miss
> fi
hit

# 4  
Old 03-11-2015
Quote:
Originally Posted by rbatte1
Consider testing for a link instead:-
Code:
$ if [ -L b ]

See my first post above, i know that. The question is: why does "-e" work like it does and: is the POSIX documentation buggy there? According to the definition of "file" it should work like this:

if "file" exists and:
- is a regular file, "-f" will return TRUE
- is a block special file "-b" will return TRUE
- is a directory "-d" will return TRUE
...etc. analogously for "-c", "-h", "L", "-p", ...

and "-e" should return TRUE if any one of the above return TRUE.

bakunin
# 5  
Old 03-11-2015
I suppose it all depends on your definition of what is a file in "resolves to a file". Does it mean a file being with content and just a plain regular file, or does it mean any entity within a directory. How about a directory even? That does seem to be considered as a file.

It should be that everything is a file, but the word resolves perhaps covers the symbolic link issue that's being illustrated.

All very wordy stuff. I bet a lawyer would love it.



Kind regards,
Robin
# 6  
Old 03-12-2015
Robin is correct. It all depends on what resolves means. And, the standards lawyers love stuff like this. And, the text agenda.kgb quoted:
Quote:
If a symbolic link is encountered during pathname resolution, the behavior shall depend on whether the pathname component is at the end of the pathname and on the function being performed. If all of the following are true, then pathname resolution is complete:
  • This is the last pathname component of the pathname.
  • The pathname has no trailing slash.
  • The function is required to act on the symbolic link itself, or certain arguments direct that the function act on the symbolic link itself.
is the key here. If the function you're using is stat(), then pathname resolution is NOT complete when the final component of a path names a symbolic link and the file named by the symlink does not exist. If the function you're using is lstat(), then pathname resolution is complete because the description of the stat() and lstat() functions explicitly states:
Quote:
The lstat() function shall be equivalent to stat(), except when path refers to a symbolic link. In that case lstat() shall return information about the link, while stat() shall return information about the file the link references.
And, in the Commands and Utilities Volume of the POSIX standards, pathname resolution is not complete when a symlink is found at the end of the path unless the utility description explicitly states that a symbolic link is being processed rather than the file the symlink references. The description of test -L pathname is:
Quote:
True if pathname resolves to an existing directory entry for a symbolic link. False if pathname cannot be resolved, or if pathname resolves to an existing directory entry for a file that is not a symbolic link. If the final component of pathname is a symbolic link, that symbolic link is not followed.
which makes it clear that for the -L option, test needs to use the equivalent of lstat(pathname, ...). But, the description of test -e:
Quote:
True if pathname resolves to an existing directory entry. False if pathname cannot be resolved.
does not make any exception for symlinks. So, it needs to use the equivalent of stat(pathname, ...).

Speaking as a standards lawyer, it appears to me that the ksh88 AIX 7.1 test -e and test -L utility is behaving properly.
These 3 Users Gave Thanks to Don Cragun For This Post:
# 7  
Old 03-12-2015
I did some tracings.
Indeed test -L does an lstat() i.e. does not try to follow a symlink.
test -e does either a stat() or an access() i.e. follows a symlink and a symlink's symlink... This is meant with "resolve".
In order to test for anything, you need test -L "$file" || test -e "$file".
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