Creating temporary files in bash

Question

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

User · Answer

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    gt  mktemp  tmp tmp xx4mM3ePQY  gt

User · Answer

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

User · Answer

You might want to look at mktemp     The  mktemp  utility takes the given filename template and overwrites a          portion of it to create a unique filename   The  template  may  be  any          filename  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

User · Answer

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     -     amp  amp  mkdir -v   TMPDIR    or temporary file   TMPFILE     0     -     amp  amp  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

User · Answer

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  lt    if you only need the file temporarily as input to another command  e g      diff  lt  echo hello world   lt  echo foo bar

[linux] Creating temporary files in bash

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