lsof/ ulimit/ ps

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1. lsof
1.1 File Descriptor in Linux
1.2 What does lsof do?
2. ulimit
2.1 Overview
2.2 Common Commands
3. ps
3.1 Overview
3.2 Some common commands
Reference

1. lsof

1.1 File Descriptor in Linux

Linux consider everything as a file
- pipes, sockets, directories, devices, etc

1.2 What does lsof do?

lsof means — List Open Files
some columns need to understand
- FD
  - stands for file descriptor
  - possible values
    - cwd — current working directory
    - rtd — root directory
    - txt — program text
    - mem — memory mapped file
    - (number)(parameter)
      - r for read acccess
      - w for write
      - u for both read and write
- TYPE
  - DIR — directory
  - REG — Regular file
  - CHR — Character special file
  - FIFO — First In First Out

// show long listing of open files, show cols like Command, PID, USER, FD, TYPE
# lsof 

// display the list of all opened file of uder leilei
# lsof -u leilei

// find process running on specific port
# lsof -i TCP:22

// list only IPv4 & IPv6 Open Files 
# lsof -i 4
# lsof -i 6

// list open files of TCP port ranges from 11 - 1023
# lsof -i TCP:1-1023

// exclude user with ^
# lsof -i -u^leilei

// list all network connections 
# lsof -i

// search by PID 
# lsof -p PID 

// kill all activities of particular user 
# kill -9 `lsof -t -u leilei`

2. ulimit

2.1 Overview

Set or report the resource limit of the current user.
Use with ulimit requires admin access, it only work on systems that allow control through the shell
Types of resource limitation
- hard limit
  - define the physical limit that the user can reach
- soft limit
  - manageable by the user, its value can go up to the hard limit
system resources are defined in a configuration file located at /etc/security/limits.conf

2.2 Common Commands

// print all the resource limits for the current user 
# ulimit -a

// check the value of max core file size 
# ulimit -c 

// check the max data seg size
# ulimit -d

// check the max stack size of current user 
# ulimit -s

// check the max number of user processes 
# ulimit -u

// check the max number of threads 
# ulimit -T 

// check the size of virtual memory 
# ulimit -v 

// check time each process is allowed to run for 
# ulimit -t 

// check how many file descriptors a process can have 
# ulimit -n

3. ps

3.1 Overview

Linux is a multi-tasking and multi-user system
so it allows multiple processes to operate simultaneously without interfering with each other
A process is an executing instance of a program and carry out different tasks within the operating system
PS command help us to review information related with the processes on a system
- used to list the currently running processes and their PIDs along with some other information depends on different options

3.2 Some common commands

Some common columns you should know what it means
- PID - the unique process ID
- TTY - terminal type that the user is logged into
- TIME - amount of CPU in minutes and seconds that the process has been running
  - sometimes you see TIME as 00:00:00, merely means the total accumulated CPU utilization time for any process currently is 0
- CMD - name of the command that launched the process
- C - the CPU utilization in percentage
- STIME - the start time of the process

# ps 
PID TTY          TIME CMD
12330 pts/0    00:00:00 bash
21621 pts/0    00:00:00 ps

// view all the running processes 
# ps -A 
# ps -e

// view all processes associated with the terminal 
# ps -T 

// view all the running processes 
# ps -r 

// view all the processes owned by you 
# ps -x 

// print all the processes within the system 
# ps -e

// More detailed output by using -f option 
# ps -e -f 

// Search for a particular process 
# ps -C systemd