Shells.md

User Shells

Almost every linux shell has ways to modify the user experience. There are multiple ways of changing your shell environment.

Here are some topics listed:

• modifying the shell user limits
• virtual terminals

Limits

It is possible to address resource limits in a user shell, which make sure that programs do not run out of control. This can be useful for a code that might have an infinite recursion. This is usually controlled by the stack size limit.

Check what resource limits you have:

$ ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 3091355
max locked memory       (kbytes, -l) unlimited
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) 10000
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

# in an slurm job it might be necessary to use
ulimit -s unlimited
# to allow a larger stack size for e.g. FORTRAN programs.

Stack contains all local variables & data, intermediate storage for registers, and function parameters. This setting is used to block infinite recursion.

flags	description
-H	Specifies that the hard limit for the given resource is set. If you have root user authority, you can increase the hard limit. Anyone can decrease it
-S	Specifies that the soft limit for the given resource is set. A soft limit can be increased up to the value of the hard limit. If neither the -H nor -S flags are specified, the limit applies to both
-a	Lists all of the current resource limits
-b	The maximum socket buffer size
-c	The maximum size of core files created
-d	The maximum size of a process's data segment
-e	The maximum scheduling priority ("nice")
-f	The maximum size of files written by the shell and its children
-i	The maximum number of pending signals
-l	The maximum size that may be locked into memory
-m	The maximum resident set size (many systems do not honor this limit)
-n	The maximum number of open file descriptors (most systems do not allow this value to be set)
-p	The pipe size in 512-byte blocks (this may not be set)
-q	The maximum number of bytes in POSIX message queues
-r	The maximum real-time scheduling priority
-s	The maximum stack size
-t	The maximum amount of cpu time in seconds
-u	The maximum number of processes available to a single user
-v	The maximum amount of virtual memory available to the shell and, on some systems, to its children
-x	The maximum number of file locks
-T	The maximum number of threads