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Whilst I was aware that "ulimit" was responsible for setting the maximum number of files a user could have open in Linux, I rarely need to alter this value. Today when I needed to increase it for a relatively niche purpose, and I was shocked to realise that I couldn't remember how to do this.
I knew that I could check all my ulimit values using "ulimit -a" (output on default install of ubuntu below) and that I could check how many files specifically using "ulimit -n".
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) 53248 max locked memory (kbytes, -l) 32 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) 53248 virtual memory (kbytes, -v) unlimited file locks (-x) unlimited
So, how do I change this? Well, I tried, as root, "ulimit -n 4096" but this only affected the root user seemingly. It appears the correct way is to edit /etc/security/limits.conf and append/edit the following values (where $username is the username of the user you wish to edit the value for):
$username soft nofile 4096 $username hard nofile 4096
Subsequent logins should then have a max number of files open of 4096. I'm writing this down as I know that I have changed it previously, and appeared to have totally forgotten how to do it this time around.