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?
Best Answer
The mktemp(1) man page explains it fairly well:
Traditionally, many shell scripts take the name of the program withthe pid as a suffix and use that as a temporary file name. This kindof naming scheme is predictable and the race condition it creates iseasy for an attacker to win. A safer, though still inferior, approachis to make a temporary directory using the same naming scheme. Whilethis does allow one to guarantee that a temporary file will not besubverted, it still allows a simple denial of service attack. Forthese 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…
Yes, use mktemp.
It will create a temporary file inside a folder that is designed for storing temporary files, and it will guarantee you a unique name. It outputs the name of that file:
> mktemp/tmp/tmp.xx4mM3ePQY>You might want to look at mktemp
The mktemp utility takes the given filename template and overwrites aportion of it to create a unique filename. The template may be anyfilename with some number of 'Xs' appended to it, for example/tmp/tfile.XXXXXXXXXX. The trailing 'Xs' are replaced with a combination of the current process number and random letters.
For more details: man mktemp
Is there any advantage in creating a temporary file in a more careful way
The temporary files are usually created in the temporary directory (such as /tmp) where all other users and processes has read and write access (any other script can create the new files there). Therefore the script should be careful about creating the files such as using with the right permissions (e.g. read only for the owner, see: help umask) and filename should be be not easily guessed (ideally random). Otherwise if the filenames aren't unique, it can create conflict with the same script ran multiple times (e.g. race condition) or some attacker could either hijack some sensitive information (e.g. when permissions are too open and filename is easy to guess) or create/replacing the file with their own version of the code (like replacing the commands or SQL queries depending on what is being stored).
You could use the following approach to create the temporary directory:
TMPDIR=".${0##*/}-$$" && mkdir -v "$TMPDIR"or temporary file:
TMPFILE=".${0##*/}-$$" && touch "$TMPFILE"However it is still predictable and not considered safe.
As per man mktemp, we can read:
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.
So to be safe, it is recommended to use mktemp command to create unique temporary file or directory (-d).
mktemp is probably the most versatile, especially if you plan to work with the file for a while.
You can also use a process substitution operator <() if you only need the file temporarily as input to another command, e.g.:
$ diff <(echo hello world) <(echo foo bar)To somewhat expand on previous answers here, you want to run mktemp and make sure you also clean up afterwards. The usual way to do that is with trap, which lets you set up a hook that can be run when your script is interrupted.
Bash also provides the EXIT pseudo-signal so that you can set up a trap to be run when your script exits successfully, and ERR which triggers if your script produces an error. (See also What does set -e mean in a bash script? for some unobvious consequences.)
t=$(mktemp -d -p temporary.XXXXXXXXXXXX) || exittrap 'rm -rf "$t"; exit' ERR EXIT # HUP INT TERM: # use "$t" to your heart's content ...You might want to set up additional signals besides ERR and EXIT; obviously, kill -9 cannot be trapped (which is why it should not be used, except in emergencies). HUP (signal 1) and INT (signal 2) are generated when your script's session is hung up, or the user presses ctrl-C, respectively. TERM (signal 15) is the default signal sent by kill, and requests the script to be terminated.
mktemp -p replaces mktemp -t which is regarded as obsolete. The -d option says to create a directory; if you only need a single temporary file, obviously, that's not necessary.
The mktemp docs have some nice examples.
If you require a certain suffix (file extension) for your temporary file. You can do the following
$ myfile=$(mktemp --suffix ".txt")$ echo "$myfile"/tmp/tmp.9T9soL2QNp.txtIf you don't want the file to be created, but just want a name, you can additionally use the -u/--dry-run flag.
$ myfile=$(mktemp -u --suffix ".txt")$ echo "$myfile"/tmp/tmp.Y8cMDJ1DDr.txtBUT NOTE, when using -u/--dry-run
Using the output of this command to create a new file is inherently unsafe, as there is a window of time between generating the name and using it where another process can create an object by the same name.