The ulimit command with the -S (soft) and -a (all) flag will display the current users soft limits. In this example, John Doe's soft limits are displayed.

[john.doe@server1 ~]# ulimit -S -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) 31119
max locked memory       (kbytes, -l) 64
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) unlimited
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

The ulimit command with the -H (hard) and -a (all) flag will display the current users hard limits. Notice in this example that John Doe's soft limit for open files is 1024 and the hard limit is 2048.

[john.doe@server1 ~]# ulimit -S -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) 31119
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 2048
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) unlimited
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

Or the prlimit command can be used to list the users soft and hard limits.

[john.doe@server1 ~]# prlimit
RESOURCE   DESCRIPTION                             SOFT      HARD UNITS
AS         address space limit                unlimited unlimited bytes
CORE       max core file size                         0 unlimited bytes
CPU        CPU time                           unlimited unlimited seconds
DATA       max data size                      unlimited unlimited bytes
FSIZE      max file size                      unlimited unlimited bytes
LOCKS      max number of file locks held      unlimited unlimited locks
MEMLOCK    max locked-in-memory address space     65536     65536 bytes
MSGQUEUE   max bytes in POSIX mqueues            819200    819200 bytes
NICE       max nice prio allowed to raise             0         0
NOFILE     max number of open files                1024      2048 files
NPROC      max number of processes                 4096     31665 processes
RSS        max resident set size              unlimited unlimited bytes
RTPRIO     max real-time priority                     0         0
RTTIME     timeout for real-time tasks        unlimited unlimited microsecs
SIGPENDING max number of pending signals          22583     22583 signals
STACK      max stack size                       8388608  10485760 bytes

The ulimit command followed by one of the flags listed above will return the value for only that limit. In this example, the -S and -n flags are used to only return John Doe's soft open files limit.

[john.doe@server1 ~]# ulimit -S -n
1024

A limit can temporarily be updated. In this example, John Doe's open files limit is temporarily updated to 2048. When the user signs out and then signs back into the system, the user's limits will be returned to their prior value.

[john.doe@server1 ~]# ulimit -n 2048

To permanently update a limit, you can set the limit in any of the following files

• /home/username/.bash_profile (on a Red Hat distribution - settings in this file will take precedence over similar settings in the limit.d and limits.conf files)
• /etc/security/limit.d/example.conf (settings in this file will take precedence over similar settings in the limits.conf file)
• /etc/security/limits.conf

If you want to set a limit in $HOME/.bash_profile, you will simply just have the ulimit command.

ulimit -n 2048

If you want to set a limit in a file in the limit.d directory or in the limits.conf file, the syntax is

<user|group|wildcard> <soft|hard> <item> <value>

For example

• If john.doe has more than 1024 files open (the soft limit) a warning will be appended to /var/log/messages.
• If john.doe has 2048 files open (the hard limit) john.doe will not be able to open another file

john.doe  soft  nofile  1024
john.doe  hard  nofile  2048

In this example, the limits are apply to users that are members of the "admins" group.

@admins  soft  nofile  1024
@admins  hard  nofile  2048

Wildcard can be used to apply the limit to all users except for root.

*  soft  nofile  1024
*  hard  nofile  2048

root can be used to apply limits for root.

root  soft  nofile  1024
root  hard  nofile  2048

If you make a change to the .bash_profile file, or a file in the limit.d directory, or the limits.conf file, you'll then need to disconnect and reconnect to the system for the change to take effect.

The lsof (list open files) command can be used, as the name suggests, to list the open files on the system. The lsof command without any options should return something like this. Notice in this example that many of the files have "Permission denied". This is because the lsof command in this example is being run by john.doe. Often, the lsof command is run using sudo or run as root.

[john.doe@server1 ~]$ lsof
COMMAND      PID     USER   FD      TYPE             DEVICE SIZE/OFF       NODE NAME
init           1     root  cwd   unknown                                        /proc/1/cwd (readlink: Permission denied)
init           1     root  rtd   unknown                                        /proc/1/root (readlink: Permission denied)
init           1     root  txt   unknown                                        /proc/1/exe (readlink: Permission denied)
init           1     root NOFD                                                  /proc/1/fd (opendir: Permission denied)
kthreadd       2     root  cwd   unknown                                        /proc/2/cwd (readlink: Permission denied)
kthreadd       2     root  rtd   unknown                                        /proc/2/root (readlink: Permission denied)
kthreadd       2     root  txt   unknown                                        /proc/2/exe (readlink: Permission denied)
kthreadd       2     root NOFD                                                  /proc/2/fd (opendir: Permission denied)
migration      3     root  cwd   unknown                                        /proc/3/cwd (readlink: Permission denied)

The lsof (list open files) command can be piped through the wc (word count) command to determine how many files are open.

[john.doe@server1 ~]$ lsof | wc -l
1234

Often, the lsof command with the -u option is used to display the files opened by a certain user.

[john.doe@server1 ~]$ lsof -u john.doe | head
COMMAND      PID    USER   FD      TYPE             DEVICE  SIZE/OFF       NODE NAME
sshd      116079 john.doe cwd   unknown                                         /proc/116079/cwd (readlink: Permission denied)
sshd      116079 john.doe rtd   unknown                                         /proc/116079/root (readlink: Permission denied)
sshd      116079 john.doe txt   unknown                                         /proc/116079/exe