The recursive function
Functions in Bash also support recursion (the function can call itself). For example, F() { echo $1; F hello; sleep 1; }.
Fork bomb
A recursive function is a function that calls itself: recursive functions must have an exit condition, or they will spawn until the system exhausts a resource and crashes.
This function: :(){ :|:& };: spawns processes forever and ends up in a denial-of-service attack.
The & character is postfixed with the function call to bring the subprocess into the background. This dangerous code forks processes forever and is called a fork bomb.
You may find it difficult to interpret the preceding code. Refer to the Wikipedia page h t t p ://e n . w i k i p e d i a . o r g /w i k i /F o r k _ b o m b for more details and interpretation of the fork bomb.
Prevent this attack by restricting the maximum number of processes that can be spawned by defining the nproc value in /etc/security/limits.conf.
This line will limit all users to 100 processes:
hard nproc 100
Exporting functions
Functions can be exported, just like environment variables, using the export command. Exporting extends the scope of the function to subprocesses:
export -f fname
$> function getIP() { /sbin/ifconfig $1 | grep 'inet '; }
$> echo "getIP eth0" >test.sh
$> sh test.sh
sh: getIP: No such file or directory
$> export -f getIP
$> sh test.sh
inet addr: 192.168.1.2 Bcast: 192.168.255.255 Mask:255.255.0.0