Bash built-in.
POSIX 2013 CLI utility: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/ulimit.html and deprecated C API function.
Get and set resource limits for a process and its children.
Linux and POSIX offer several per process limits.
POSIX ones are documented with the getrlimit interface at http://pubs.opengroup.org/onlinepubs/9699919799/functions/getrlimit.html
Linux implements both getrlimit() and ulimit() with sys_getrlimit. ulimit() was deprecated in POSIX 7, but not the CLI utility.
On Ubuntu, implemented as a sh and bash built-in:
help ulimit
sudo ulimit fails because ulimit is a built-in and sudo takes executables as argument. Alternatives: http://stackoverflow.com/questions/17483723/command-not-found-when-using-sudo-ulimit
ulimit is a front-end for http://pubs.opengroup.org/onlinepubs/9699919799/functions/ulimit.html, which in Linux is a front-end for http://pubs.opengroup.org/onlinepubs/9699919799/functions/setrlimit.html
Limits are enforced per process, so if a process is able to fork it can blow the limits by using the children. Limiting the children is non-trivial, so ulimit should only be used to prevent system crashes on trusted programs (e.g. one that may take too much RAM with certain parameters).
If programs try to break those limits, the system call fails, and often the program crashes.
http://serverfault.com/questions/610130/how-to-set-ulimit-value-permanently
libpam-modules:amd64 package on Ubuntu 14.04.
POSIX only specifies this option: the maximum file size that can be written in 512 byte blocks.
Get value of the (soft) limit:
ulimit -f
On most distros is unlimited by default.
Set value:
ulimit -Sf 0
# Shell crashes because `>` happens in current process.
printf a > a
ulimit -Sf 1
printf a > a
# TODO why works? Was the limit not 512?
python -c 'print "a" * 1023' > a
# Fails.
python -c 'print "a" * 1024' > a
sudo -c ulimit -Sf unlimited
GNU adds many more options in its sh and bash implementations.
In both cases ulimit is implemented as a built-in.
List all limits (including the unit and the option name):
ulimit -a
Linux has the concept of hard and soft limits.
Soft limits can be increased by any user, up to the hard limit.
The hard limit can only be increased by sudo, but decreased without.
-S specifies soft limits, -H hard, and the absence of both sets both at once.
Print soft limit:
ulimit -Sc
Same:
ulimit -c
Print hard limit:
ulimit -Hc
Bash also provides the soft and hard special limit values which refer to the current soft and hard values.
Virtual memory in bytes:
ulimit -Sv 500000
This is one of the most useful limits, when you are running a potentially memory heavy algorithm and don't want it to crash your computer.
Linux deals well with CPU limits since CPU usage rotates according to nice, but as for memory it just takes up the entire RAM / swap and halts everything.
http://unix.stackexchange.com/questions/44985/limit-memory-usage-for-a-single-linux-process
The timeout tool aims to limit CPU and RAM for the children as well as parent: https://github.com/pshved/timeout
- http://stackoverflow.com/questions/386945/limiting-certain-processes-to-cpu-linux
- http://unix.stackexchange.com/questions/151883/limiting-processes-to-not-exceed-more-than-10-of-cpu-usage
TODO confirm this section.
Module that sets per user resource quotas.
http://www.cyberciti.biz/tips/linux-limiting-user-process.html
Allows for several useful limits, e.g. nproc for the number or processes.
man limits.conf
man pam_limits