Shell – How to make a temporary file in RAM

filesfilesystemsscriptingshell-scripttmp

I have a script that will pipe its output to |tee scriptnameYYMMDD.txt. After each cycle of the for loop in which the output is generated, I'll be reversing the file contents with tac scriptnameYYYYMMDD.txt > /var/www/html/logs/scriptname.txt so that the log output is visible in a browser window with the newest lines at the top.

I'll have several scripts doing this in parallel. I'm trying to minimize the disk activity, so output from |tee scriptnameYYYYMMDD.txt to a RAMdisk would be best. mktemp creates a file in the /tmp folder, but that doesn't appear to be off-disk.

Best Answer

You can mount a tmpfs partititon and write the file there:

mount -t tmpfs -o size=500m tmpfs /mountpoint

This partition now is limited to 500 MB. If your temporary file grows larger than 500 MB an error will occur: no space left on device. But, it doesn't matter when you specify a larger amount of space than your systems RAM has. tmpfs uses swap space too, so you cannot force a system crash, as opposed to ramfs.

You can now write your file into /mountpoint:

command | tee /mountpoint/scriptnameYYYYMMDD.txt

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.

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

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>

Best Answer

Related Solutions

Single process accessible temporary file

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

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