System Command on Linux

# Monitor the system:

pstree	Processes and parent-child relationships
top	Show top processes
iostat	Report CPU statistics and input/output statistics for devices and partitions.
ps -auxw	process status
uname -a	print system information
cat /proc/version	Display Linux kernel version in use.
cat /etc/redhat-release	Display Red Hat Linux Release. (also /etc/issue)
uptime	Tell how long the system has been running. Also number of users and system's load average.
w	Show who is logged on and what they are doing.
/sbin/lsmod	List all currently loaded kernel modules. Same as cat /proc/modules
/sbin/runlevel	Displays the system's current runlevel.
hostname	Displays/changes the system's node name. (Must also manually change hostname setting in /etc/sysconfig/network. Command will change entry in /etc/hosts)
service	Red Hat/Fedora command to display status of system services. Example: service --status-all Help: service --help

# Process management :
The basic Linux monitoring commands such as pstree and ps -auxw and top will inform you of the processes running on your system. Sometimes a process must be terminated. To terminate a process:

1. Identify the process:
   • pstree -p
     OR
   • ps -auxw
     OR
   • top
2. Kill the process:
   • kill <process-id-number>
   • killall <command-name>

# Memory Usage.

Linux Commands to Monitor Memory Usage:

vmstat	Monitor virtual memory
free	Display amount of free and used memory in the system. (Also: cat /proc/meminfo)
pmap	Display/examine memory map and libraries (so). Usage: pmap pid
top	Show top processes
sar -B	Show statistics on page swapping.
time -v date	Show system page size, page faults, etc of a process during execution. Note you must fully qualify the command as "/usr/bin/time" to avoid using the bash shell command "time".
cat /proc/sys/vm/freepages	Display virtual memory "free pages". One may increase/decrease this limit: echo 300 400 500 > /proc/sys/vm/freepages
cat /proc/meminfo	Show memory size and usage

 #Filesystems and Storage Devices. 

df -k	report filesystem disk space usage. (-k reports in Kbytes)
du -sh	Calculates file space usage for a given directory. (and everything under it) (-s option summarizes)
mount	Displays all mounted devices, their mount point, filesystem, and access. Used with command line arguments to mount file system.
cat /proc/filesystems	Display filesystems currently in use.
cat /proc/mounts	Display mounted filesystems currently in use.
showmount	Displays mount info for NFS filesystems.
cat /proc/swaps	Displays swap partition(s) size, type and quantity used.
cat /proc/ide/hda/any-file	Displays disk information held by kernel.

Adding an extra hard drive: (See commands and dialog of adding a second IDE hard drive)

1. fdisk /dev/<drive> - Allocate drive space and register info on the partition table.(Option "n"/"p", then "w" to write.)
   
2. mkfs -t ext3 /dev/<drive> - Create file system. (RH 7.1 and earlier use ext2, RH 7.2-8.0 use ext3)
3. mount -t ext3 /dev/<drive's device name> /<home2 or some suitable directory> - Mount the drive
   Mount a raw ISO file: mount -t iso9660 -o loop /home/user1/RedHat-9.0-i386-Disk1.iso /mnt/iso-1
   (Fstab entry: /home/user1/RedHat-9.0-i386-Disk1.iso /mnt/iso-1 iso9660 loop,ro 0 3)

///////////////////////System Users////////////////////////
# UserInformation:

who	Displays currently logged in users. Use who -uH for idle time and terminal info.
users	Show all users logged in.
w	Displays currently logged in users and processes they are running.
whoami	Displays user id.
groups	Display groups you are part of. Use groups user-id to display groups for a given user.
set	Display all environment variables in your current environment.
id	Display user and all group ids. Use id user-id to display info for another user id.
last	Listing of most recent logins by users. Show where from, date and time of login (ftp, ssh, ...) Also see lastlog command. Show last 100 logins: last -100
history	Shell command to display previously entered commands.