Show Process Limits Using /proc Filesystem

**Show process limits with /proc filesystem**

I think I mentioned the special /proc filesystem before, it's available in Linux distros and helps you obtain system and process information via normal files created in a special structure. Today I'd like to show you another cool trick /proc has.

Show Process Info Using /proc

Just to remind you, here's what I mean: on my Red Hat PC I have this sshd daemon process running:

root@redhat:/ # ps -aef | grep [o]penssh
 root      5130     1  0 Oct03 ?        00:00:00 /usr/sbin/sshd -D [email protected],[email protected],aes256-ctr,aes256-cbc,[email protected],aes128-ctr,aes128-cbc [email protected],[email protected],[email protected],[email protected],hmac-sha2-256,hmac-sha1,[email protected],hmac-sha2-512 -oGSSAPIKexAlgorithms=gss-gex-sha1-,gss-group14-sha1- [email protected],ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1 -oHostKeyAlgorithms=rsa-sha2-256,ecdsa-sha2-nistp256,[email protected],ecdsa-sha2-nistp384,[email protected],rsa-sha2-512,ecdsa-sha2-nistp521,[email protected],ssh-ed25519,[email protected],ssh-rsa,[email protected] -oPubkeyAcceptedKeyTypes=rsa-sha2-256,ecdsa-sha2-nistp256,[email protected],ecdsa-sha2-nistp384,[email protected],rsa-sha2-512,ecdsa-sha2-nistp521,[email protected],ssh-ed25519,[email protected],ssh-rsa,[email protected]

So the sshd process ID (PID) is 5130. That means I can use /proc filesystem to learn quite a bit about the process:

root@redhat:/ # cd /proc/5130
root@redhat:/proc/5130 # ls
 attr        cmdline          environ  io         mem         ns             pagemap      sched      smaps_rollup  syscall        wchan
 autogroup   comm             exe      limits     mountinfo   numa_maps      patch_state  schedstat  stack         task
 auxv        coredump_filter  fd       loginuid   mounts      oom_adj        personality  sessionid  stat          timers
 cgroup      cpuset           fdinfo   map_files  mountstats  oom_score      projid_map   setgroups  statm         timerslack_ns
 clear_refs  cwd              gid_map  maps       net         oom_score_adj  root         smaps      status        uid_map

Each file or directory in this /proc/5130 location shows some information specific to this PID 5130.

For instance, if we list files in the fd directory there, we'll see all the files and sockets open by sshd at the moment:

root@redhat:/proc/5130 # ls -al fd/*
 lr-x------. 1 root root 64 Oct  3 14:10 fd/0 -> /dev/null
 lrwx------. 1 root root 64 Oct  3 14:10 fd/1 -> 'socket:[39555]'
 lrwx------. 1 root root 64 Oct  3 14:10 fd/2 -> 'socket:[39555]'
 lr-x------. 1 root root 64 Oct  3 14:10 fd/3 -> /dev/urandom
 lr-x------. 1 root root 64 Oct  3 14:10 fd/4 -> /var/lib/sss/mc/passwd
 lrwx------. 1 root root 64 Oct  3 14:10 fd/5 -> 'socket:[45446]'
 lrwx------. 1 root root 64 Oct  3 14:10 fd/6 -> 'socket:[45450]'
 lr-x------. 1 root root 64 Oct  3 14:10 fd/7 -> /var/lib/sss/mc/group
 lrwx------. 1 root root 64 Oct  3 14:10 fd/8 -> 'socket:[45452]'

TODO: I'll be sure to write a separate post on the /proc filesystem with more thorough walkthrough.