Databases
StackOverflow2013
Postsdbo
PODifference between "test -a file" and "test file -ef file"
 

Note that there are some explanatory texts on larger screens.

plurals
  1. PODifference between "test -a file" and "test file -ef file"
    primarykey
    data
    text
    <p>QNX (Neutrino 6.5.0) uses an open source implementation of ksh as its shell. A lot of the provided scripts, including the system startup scripts, use constructs such as</p> <pre><code>if ! test /dev/slog -ef /dev/slog; then # do something fi </code></pre> <p>to check whether a resource manager exists or not in the filesystem. I've searched and could only find very dray explanations that <code>-ef</code> checks to see whether the two parameters are in fact the same file. Since the filename specified is the same it seems to just reduce to checking that the file exists.</p> <p>I have checked the behaviour of <code>test -a</code> and <code>test -e</code> (both seem to check for file existance of any type of file according to the various docs I've read) and they seem to also work.</p> <p>Is there any difference in the checks performed between <code>-ef</code> and <code>-a</code>/<code>-e</code>? Is using <code>-ef</code> some kind of attempt to protect against a race condition in the existence of the file?</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. UStinman
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. PODifference between "test -a file" and "test file -ef file"
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. PODifference between "test -a file" and "test file -ef file"
      1. UStinman
      1. VTBountyStart
    3. VO
      singulars
      1. PODifference between "test -a file" and "test file -ef file"
      1. This table or related slice is empty.
      1. VTUpMod
    1. COIt seems like it could, indeed, be checking that the file exists for two different stat() calls. Does the /dev entry get removed by the driver/manager (e.g., after being shutdown) only if someone checks for its existence?
      singulars
      1. PODifference between "test -a file" and "test file -ef file"
      1. USjhfrontz
    2. CO@jhfrontz: I'm not entirely sure what you mean but (as an example) the /dev/slog pseudo device only gets removed when the driver is killed which would not normally happen in the middle of the call to `test` since it is part of the initial system startup script. In the normal case of /dev/slog a process can write to the device to log to a file/console/memory, and while it may end up with a different filename, the device exposed in /dev should be the same as before the write to /dev/slog.
      singulars
      1. PODifference between "test -a file" and "test file -ef file"
      1. UStinman
    3. COI'm speculating that stat'ing the device has some side effect (e.g., prompting the driver/manager to do some sort of housekeeping) such that as the first stat() call (from `test`) is completed, the file/device is removed/replaced by the driver -- thus causing the second stat() to get different information, in turn causing the `test` to fail. It's been a while since I've used QNX, but [the slogger man page](http://www.qnx.com/developers/docs/6.3.0SP3/neutrino/utilities/s/slogger.html) suggests there are side effects from interacting with /dev/slog (e.g., unlinking it purges the log).
      singulars
      1. PODifference between "test -a file" and "test file -ef file"
      1. USjhfrontz
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload