linux_troubleshooting

# Linux troubleshooting tools cheat sheet
# v 1.0.0
# author: Melita Mihaljevic
# date: Dec, 2012
# comment: please see manual pages for more info

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# system (CPU, memory, disk I/0)

** dmesg ** - prints the message buffer of the kernel 
$ dmesg -c   			- clear message buffer
$ dmesg |grep -i [filter] 	- use with grep for filtering output	
$ dmesg |less			- use with less for easier reading


** uptime ** - how long the system has been running and load average
$ uptime - returns: the current time, how long the system has been running, 
	            how many users are currently logged on, and the system 
		    load averages for the past 1, 5, and 15 minutes


** w ** - show who is logged in and what they are doing
$ w	- displays information about the users currently on the machine, and their processes 


** vmstat ** - reports information about processes, memory, paging, block IO, traps, disks and cpu activity
$ vmstat [delay]  - virtual memory statistics repeated after delay 
$ vmstat -a 	  - active/inactive memory
$ vmstat -s	  - memory statistics - total, active, inactive, swap, buffer,...
$ vmstat -d	  - disk statistics
$ vmstat -D	  - summary statistics about disk activity - totaly read, written, merged, ...
$ vmstat -p [partition] - partition statistics 


** top ** - real time view of running system
$ top - running processes sorted by CPU usage plus overall CPU, memory view 


** procinfo ** - system statistics gathered from /proc
$ procinfo


** mpstat ** - processors related statistics
$ mpstat 	        - statistics for all processors summarized 
$ mpstat - A 		- statistics for all processors separated


** free ** - Display amount of free and used memory in the system
$ free 		- displays the total amount of free and used physical and swap memory in the system
$ free -s 5 	- continuously displayed statistics after a delay of 5 seconds


** slabtop ** - display kernel slab cache information in real time
$ slabtop     - displays a listing of the top caches sorted by one of the listed sort criteria


** ipcs **  - provide information on ipc facilities
$ ipcs - 	- information about shared memory segment, semaphores and message queue
$ ipcs -m 	- shared memory segment
$ ipcs -u	- summary information


** lsof ** 	- list of open files
$ lsof 		- complete list of open files

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# application/process related

** ps **	- displays information about a selection of the active processes
$ ps -aux 	- see every process in the system 
$ ps -axj	- list of daemon processes


** cat /proc/<PID>/status - specific process status 


** strace ** - traces the system call that program makes while executing
$ strace ls -l 			- system calls while executing ls -l
$ strace -c ls -l 		- summary for system calls while executing ls -l
$ strace -p [PID]		- attach to specific process [PID] and track system calls


** ltrace ** - traces all calls that app makes to library
$ lstrace ls -l 		- library calls while executing ls -l
$ ltrace -c ls -l 		- summary for library calls while executing ls -l
$ ltrace -p [PID]		- attach to specific process [PID] and track lib calls


** time ** - run programs and summarize system resource usage
$ time ls -l 	- execution time, user time and system time of ls -l

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# network

** ifconfig ** 	- configure network interfaces and output configuration information 
$ ifconfig 	- prints configuration information for each interface


** ip ** - show / manipulate routing, devices, policy routing and tunnels
$ ip link 	- displays information on network devices makes a typescript of everything printed on your terminal 
$ ip route	- displays routing table
$ ip addr	- displays address information for each network device


** netstat ** - Print network connections, routing tables, interface statistics
$ netstat	- displays a list of open sockets
$ netstat -t 	- displays a list of TCP sockets
$ netstat -u 	- displays a list of UDP sockets
$ netstat -r 	- displays routing table
$ netstat -s 	- displays protocols statistics

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# other helpful commands

** uname ** - prints system information
$ uname -a 	- prints all system information in order 
		 (kernel name, node name, kernel version, machine, operating system,..) 


** ldd ** -	print shared library dependencies
$ ldd ./a.out - prints shared libraries for a.out


* a typescript of everything printed on your terminal watch ** - execute a program periodically, showing output
$ watch -n 2 uptime 		- executes uptime every 2 seconds