Strapakowsky Strapakowsky - 1 year ago 138
Linux Question

Creating temporary files in bash

Are there objectively better ways to create temporary files in bash scripts?

I normally just name them whatever comes to my mind, such as tempfile-123, since it will be deleted when the script is over. Is there any disadvantage in doing this other than overwriting a possible tempfile-123 in current folder? Or is there any advantage in creating a temporary file in a more careful way?

Answer Source

The mktemp(1) man page explains it fairly well:

Traditionally, many shell scripts take the name of the program with the pid as a suffix and use that as a temporary file name. This kind of naming scheme is predictable and the race condition it creates is easy for an attacker to win. A safer, though still inferior, approach is to make a temporary directory using the same naming scheme. While this does allow one to guarantee that a temporary file will not be subverted, it still allows a simple denial of service attack. For these reasons it is suggested that mktemp be used instead.

In a script, I invoke mktemp something like

mydir=$(mktemp -d "${TMPDIR:-/tmp/}$(basename 0).XXXXXXXXXXXX")

which creates a temporary directory I can work in, and in which I can safely name the actual files something readable and useful.

mktemp is not standard, but it does exist on many platforms. The "X"s will generally get converted into some randomness, and more will probably be more random; however, some systems (busybox ash, for one) limit this randomness more significantly than others

By the way, safe creation of temporary files is important for more than just shell scripting. That's why python has tempfile, perl has File::Temp, ruby has Tempfile, etc…

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