linux command-line files tmp – How to Access Temporary File After Creation

command linefileslinuxtmp

I've script which is connecting to remote host via SSH, creates temporary file and executing the following command:

Calling system(mysql –database=information_schema –host=localhost < /tmp/drush_1JAjtt)

Each time it's creating different file (pattern: drush_xxxxxx).

I've tried couple of times manually running on the remote:

tail -f /tmp/drush_*

but my connection is too slow and most of the time I've end up with the error:

tail: cannot open `/tmp/drush_*' for reading: No such file or directory

Is there any trick accessing such file straight after it's being created to show it's content?

Best Answer

I've ran into this problem when examining behavior of a particular application I didn't trust. The application would create and later delete its temp files. kenorb's solution is good, however using cat can lead to race condition( that is a file may be removed while cat is working, so partial data only is retrieved).

A lower chance of race condition can be done with creating a hard-link to the file itself. greping and creating hard link can be combined via awk. Thus what I've came up with is the following;

inotifywait -e create -m --format "%w/%f" /tmp/suspicious_dir/ 2>&1 | 
awk 'NR>2{n=split($0,a,"/");system("ln "$0" /tmp/hardlink_to_"a[n]);}'

Since we're using -e create flag , we're only interested in temporary files that are going to be immediately created, and the output format gives us the full path to created temp file. Ignoring first two lines of output via NR>2. For every reported file, there will be created a hard link in form /tmp/hardlink_to_<original filename>

Related Solutions

Single process accessible temporary file

Well, it depends on what you mean by "file" and how exactly the library accesses it. Here are a few approaches (both kluges) that come to mind:

Use a temporary file, in the normal mkstemp meaning. mkstemp returns a file descriptor number to an unlinked file. You can then create $FOO/mypasswd as a symlink to /proc/self/fd/returned_number. There is a small window where the file exists (between open and unlink), and something else running as the same user could open it, and hold the file descriptor to later read the password. You can be sure it'll not be written to disk if your temporary directory is on a tmpfs.
If it doesn't need to seek the password file, use pipe/pipe2 to create your file descriptors, then proceed as above with the symlink. You'll need to fork a child process to handle feeding the password to the pipe.
You can probably abuse FUSE to do this.
You could use LD_PRELOAD (or other dynamic linker tricks) to "fix" the library by intercepting the open/read/close, and feed it the password from somewhere sane.

None of these are really secure from other processes. Other processes running as the same user could, for example, ptrace your process and read the password directly out of memory.

shell-script – How to Create a Temporary File in Shell Script

tmpfile=$(mktemp /tmp/abc-script.XXXXXX)
: ...
rm "$tmpfile"

You can make sure that a file is deleted when the scripts exits (including kills and crashes) by opening a file descriptor to the file and deleting it. The file keeps available (for the script; not really for other processes but /proc/$PID/fd/$FD is a work-around) as long as the file descriptor is open. When it gets closed (which the kernel does automatically when the process exits) the filesystem deletes the file.

tmpfile=$(mktemp /tmp/abc-script.XXXXXX)
exec 3>"$tmpfile"
rm "$tmpfile"
: ...
echo foo >&3

Best Answer

Related Solutions

Single process accessible temporary file

shell-script – How to Create a Temporary File in Shell Script

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