What is Linux¶

mkdir: invalid option -- 'h'
Try 'mkdir --help' for more information.
[root@rhel ~]#

mkdir -h
mkdir --help

[root@rhel ~]# mkdir --help
Usage: mkdir [OPTION]... DIRECTORY...
Create the DIRECTORY(ies), if they do not already exist.

Mandatory arguments to long options are mandatory for short options too.
  -m, --mode=MODE   set file mode (as in chmod), not a=rwx - umask
  -p, --parents     no error if existing, make parent directories as needed
  -v, --verbose     print a message for each created directory
  -Z                   set SELinux security context of each created directory
                         to the default type
      --context[=CTX]  like -Z, or if CTX is specified then set the SELinux
                         or SMACK security context to CTX
      --help     display this help and exit
      --version  output version information and exit

GNU coreutils online help: <https://www.gnu.org/software/coreutils/>
Full documentation <https://www.gnu.org/software/coreutils/mkdir>
or available locally via: info '(coreutils) mkdir invocation'
[root@rhel ~]#

-h
--help
man 
info 
apropos
whatis 
dpkg -S

ls - List Directory Contents 
cd
pwd
mkdir
cp
mv
rm
touch
cat nano

uname - 
df
free
hostname

[root@oracle ~]# ipcs -l

------ Messages Limits --------
max queues system wide = 32000
max size of message (bytes) = 8192
default max size of queue (bytes) = 16384

------ Shared Memory Limits --------
max number of segments = 4096
max seg size (kbytes) = 18014398509465599
max total shared memory (kbytes) = 18446744073709551612
min seg size (bytes) = 1

------ Semaphore Limits --------
max number of arrays = 32000
max semaphores per array = 32000
max semaphores system wide = 1024000000
max ops per semop call = 500
semaphore max value = 32767

[root@oracle ~]# cat /proc/sys/kernel/sem
32000   1024000000      500     32000
[root@oracle ~]# ipcs --help

Usage:
 ipcs [resource-option...] [output-option]
 ipcs -m|-q|-s -i <id>

Show information on IPC facilities.

Options:
 -i, --id <id>  print details on resource identified by <id>
 -h, --help     display this help
 -V, --version  display version

Resource options:
 -m, --shmems      shared memory segments
 -q, --queues      message queues
 -s, --semaphores  semaphores
 -a, --all         all (default)

Output options:
 -t, --time        show attach, detach and change times
 -p, --pid         show PIDs of creator and last operator
 -c, --creator     show creator and owner
 -l, --limits      show resource limits
 -u, --summary     show status summary
     --human       show sizes in human-readable format
 -b, --bytes       show sizes in bytes

For more details see ipcs(1).
[root@oracle ~]# ipcs man

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x9700f349 98304      root       600        3294       2
0x8c00f34a 98305      root       600        1473       2
0x2e00f34b 98306      root       600        3294       2
0x2f00f34c 98307      root       600        1473       2
0x6500f34d 98308      root       600        3294       2
0xb700f355 98309      root       600        1473       2
0x8b00f356 98310      root       600        3294       2
0x5d00f357 98311      root       600        1473       2
0x6200f345 65596      root       600        3294       2
0x1000f346 65597      root       600        1473       2
0x3700f347 65598      root       600        3294       2
0x4e00f348 65599      root       600        1473       2

------ Semaphore Arrays --------
key        semid      owner      perms      nsems
0x00000000 19         lpfsg      600        32

[root@oracle ~]# see ipcs
bash: see: command not found...
man ipcs
^C
[root@oracle ~]# man ipcs
IPCS(1)                                                                        User Commands                                                                        IPCS(1)

NAME
       ipcs - show information on IPC facilities

SYNOPSIS
       ipcs [options]

DESCRIPTION
       ipcs shows information on System V inter-process communication facilities. By default it shows information about all three resources: shared memory segments,
       message queues, and semaphore arrays.

OPTIONS
       -i, --id id
           Show full details on just the one resource element identified by id. This option needs to be combined with one of the three resource options: -m, -q or -s.

       -h, --help
           Display help text and exit.

       -V, --version
           Display version information and exit.

   Resource options
       -m, --shmems
           Write information about active shared memory segments.

       -q, --queues
           Write information about active message queues.

       -s, --semaphores
           Write information about active semaphore sets.

       -a, --all
           Write information about all three resources (default).

   Output formats
       Of these options only one takes effect: the last one specified.

       -c, --creator
           Show creator and owner.

       -l, --limits
           Show resource limits.

       -p, --pid
           Show PIDs of creator and last operator.

       -t, --time
           Write time information. The time of the last control operation that changed the access permissions for all facilities, the time of the last msgsnd(2) and
           msgrcv(2) operations on message queues, the time of the last shmat(2) and shmdt(2) operations on shared memory, and the time of the last semop(2) operation on
           semaphores.

       -u, --summary
           Show status summary.

   Representation
       These affect only the -l (--limits) option.

       -b, --bytes
 Manual page ipcs(1) line 1/87 55% (press h for help or q to quit)

cat /proc/sys/kernel/sem
250     32000   32      128

ipcs -ls