How to list the records from /var/adm/wtmp file:

gz@aix# last
Another way is to manipulate directly the file with fwtmp command as follows:
# /usr/lib/acct/fwtmp < /var/adm/wtmp | more

How to show the shutdown sessions:

gz@aix# last | grep shutdown

How to show how long the system has been up:

gz@aix# uptime
gz@aix# w -u

How to enter the entry tty002:23:respawn:/usr/sbin/getty /dev/tty in inittab:

gz@aix# chitab tty002:23:respawn:/usr/sbin/getty /dev/tty

How to list the operating system level:

gz@aix# oslevel

How to see the number of license:

gz@aix# lslicense

How to change the fixed user license to 30:

gz@aix# chlicense -u30

How to enable floating user license:

gz@aix# chlicense -f on

How to display complete summary of report (how to show all errors and events):

gz@aix# errpt

How to list complete detailed report (how to show all errors and events in detailed view):

gz@aix# errpt -a

How to list all hardware related errors:

gz@aix# errpt -d H

How to list all software related errors:

gz@aix# errpt -d S

How to list detailed error report of error id 88965309:

gz@aix# errpt -a -j 88965309

How to list all events for which reporting is currently disabled:

gz@aix# errpt -t -F Report=0

How to disable the reporting of the ERRLOG_OFF event (error ID 192AC071):

gz@aix# errupdate <Enter>
=192AC071: <Enter>
Report=False <Enter>
<Ctrl-D>
<Ctrl-D>

Example:

gz@aix# errpt -t -F Report=0  
Id       Label               Type CL Description
1850F542 MINIDUMP_INFO       INFO O  NVRAM WAS RE-INITIALIZED
45E4E066 DUPCHECK_OFF        TEMP O  Duplicate checking turned off
86DC0701 DUPCHECK_ON         TEMP O  Duplicate checking turned on
B0787F02 NVRAM_ERRDATA       UNKN U  CORRUPT DATA DETECTED
gz@aix# errupdate
=BFE4C025:
Report=False
0 entries added.
0 entries deleted.
1 entries updated.
gz@aix# errpt -t -F Report=0  
Id       Label               Type CL Description
1850F542 MINIDUMP_INFO       INFO O  NVRAM WAS RE-INITIALIZED
45E4E066 DUPCHECK_OFF        TEMP O  Duplicate checking turned off
86DC0701 DUPCHECK_ON         TEMP O  Duplicate checking turned on
B0787F02 NVRAM_ERRDATA       UNKN U  CORRUPT DATA DETECTED
[color=green]BFE4C025 SCAN_ERROR_CHRP     PERM H  UNDETERMINED ERROR[/color]

How to enable the reporting of the ERRLOG_OFF event (error ID 192AC071) in case it was turned off:

gz@aix# errupdate <Enter>
=192AC071: <Enter>
Report=True <Enter>
<Ctrl-D>
<Ctrl-D>

How to delete the ERRLOG_OFF event (error ID 192AC071) from the Error Record Template Repository:

gz@aix# errupdate <Enter>
-192AC071:
<Ctrl-D>
<Ctrl-D>

How to add an entry to the errorlog:

gz@aix# /usr/bin/errlogger "The desired message"

How to dispay the HMC information (hmc ip is on HostName field of "resource N" stanza):

gz@aix# lsrsrc "IBM.ManagementServer"
or
gz@aix# lsrsrc IBM.MCP

Sometimes you may get error on the above command, like:
/usr/sbin/rsct/bin/lsrsrc-api: 2612-010 Resource class IBM.ManagementServer is not defined.[/code]
In this case, you can search for the hostname or ip of hmc in the file:
/var/ct/cfg/ctrmc.acls

How to list partition information:

gz@aix# lparstat -i
gz@aix# lsattr -El sys0

How to display the processors available in the partition:

gz@aix# bindprocessor -q

How to turn off SMT:

gz@aix# smtctl -m off

How to turn on SMT:

gz@aix# smtctl -m on

How to display SMT configuration:

gz@aix# smtctl

How to set the number of hardware threads per core:

gz@aix# smtctl -t N

How to turn on SMT:

gz@aix# smtctl -m on

Details volume group info for the hard disk:

gz@aix# lqueryvg -Atp /dev/hdisk

Find a file after the inode number:

gz@aix# find /usr -xdev -inum INODE -print
gz@aix# ncheck -i INODE /dev/hd2 <- warning: the path starts with the FS (eg /usr/bin/ksh = /bin/ksh in our example)

How to list the kernel processes:

gz@aix# ps -ekl

How to list the threads of a given process:

1. get the pid of sshd (for example):
gz@aix# ps -ef|grep sshd
2. list the threads of sshd process:
gz@aix# ps -lmp <PID>

How to get the serial number of a vio disk from the lpar client (DS disk in this example):

gz@aix# odmget CuAt | grep -p hdisk25 | grep -p "unique_id" | grep value
        value = "2343243243243[color=red]75992817EE8[/color]07210790003IBMfcp05VDASD03AIXvscsi"

or better,
for DS disks:

gz@aix# odmget -q "name=hdisk0 and attribute=unique_id" CuAt | awk '/value/ {print substr($3,14,11)}'
75992817EE8

for SVC disks:

gz@aix# odmget -q "name=hdisk0 and attribute=unique_id" CuAt | awk '/value/ {print substr($3,11,32)}'
6005076801810514A800000000000853

How to check the error log size:

gz@aix# /usr/lib/errdemon -l

How to display the current control key settings:

gz@aix# stty -a

How to put a system in maintenance mode without reboot:

gz@aix# telinit m

HMC Led Codes:

0c54 = An AIX install is in progress
0552 = varyon of the root volume group failed
0c31 = waiting for user to define console type vty0

How to get the last modification date of a file:

gz@aix# $ istat loop.pl
Inode 120 on device 10/8        File
Protection: rwxr-xr-x   
Owner: 205(florian)             Group: 1(staff)
Link count:   1         Length 74 bytes

Last updated:   Tue May 15 03:55:23 2012
Last modified:  Tue May 15 03:55:23 2012
Last accessed:  Tue May 15 04:06:31 2012

How to show terminal name:

gz@aix# tty

How to apply the changes made to the inittab file without a system reboot:

gz@aix# init q

How to check the size of memory:

#bootinfo -r
or
prtconf -m

How to use keyboard shortcuts in AIX terminal:

Backspace : to correct mistakes.
<ctrl+c> : terminates the correct command and return to the shell.\
<ctrl+d> : end of transmission or end of file
<ctrl+s> : temporary stops the output to the screen
<ctrl+q> : resumes output which was stopped by ctrl+s
<ctrl+u>: erases the entire line.