These are some really useful commands to know in the Linux terminal.
System Command
Reboot the System
Reboot the system immediately:
shutdown -r now
Reboot the system after 10 minutes:
shutdown -r +10
Reboot the system after 10 minutes and send a message to all user:
shutdown -r +10 "The system is rebooting in 10 minutes. Please, save your work."
Check last reboot time:
last reboot
Display information about running processes
ps command is used to display information about the processes that are running in the system.
ps
The output will show:
- PID = the process ID that identify the running process
- TTY = the Terminal type
- TIME
- CMD
You can add the -ef option to get the full list (-e option show all the processes, -f option show full details):
ps -ef
or add -eF to view more and more details:
ps -eF
Show the top running processes
The command top show the top running process in your system.
top
The output show information like the user, memory usage, CPU usage, running time, command lunched… You can sort the list pressing SHIFT+O.
top - 16:13:47 up 6 days, 18:56, 1 user, load average: 0.00, 0.00, 0.00
Tasks: 130 total, 1 running, 129 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.3 us, 0.3 sy, 0.0 ni, 99.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem : 949572 total, 120876 free, 140968 used, 687728 buff/cache
KiB Swap: 102396 total, 101380 free, 1016 used. 729356 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
362 stefano 20 0 399000 73304 9344 S 1.0 7.7 65:40.04 java
6395 stefano 20 0 8100 3180 2704 R 1.0 0.3 0:00.10 top
3070 mongodb 20 0 372996 41672 13452 S 0.3 4.4 37:33.36 mongod
1 root 20 0 28108 6116 4884 S 0.0 0.6 0:16.58 systemd
2 root 20 0 0 0 0 S 0.0 0.0 0:00.16 kthreadd
3 root 20 0 0 0 0 S 0.0 0.0 0:20.76 ksoftirqd/0
5 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 kworker/0:+
7 root 20 0 0 0 0 S 0.0 0.0 0:45.95 rcu_sched
8 root 20 0 0 0 0 S 0.0 0.0 0:00.00 rcu_bh
9 root rt 0 0 0 0 S 0.0 0.0 0:00.11 migration/0
10 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 lru-add-dr+
11 root 20 0 0 0 0 S 0.0 0.0 0:00.00 cpuhp/0
12 root 20 0 0 0 0 S 0.0 0.0 0:00.00 cpuhp/1
13 root rt 0 0 0 0 S 0.0 0.0 0:00.09 migration/1
14 root 20 0 0 0 0 S 0.0 0.0 0:13.24 ksoftirqd/1
16 root 0 -20 0 0 0 S 0.0 0.0 0:00.00 kworker/1:+
17 root 20 0 0 0 0 S 0.0 0.0 0:00.00 cpuhp/2
Start or stop a service
To check the current status of a service (with the service name in SERVICE_NAME) run:
service SERVICE_NAME status
For example, to check the status of samba service:
service samba status
To run a service use:
service SERVICE_NAME start
or to stop:
service SERVICE_NAME stop
or to restart:
service SERVICE_NAME restart
If you want to check set status of all running processes:
service --status-all
Know free memory
To know free memory in your system type:
free
The output is displayed in bytes:
total used free shared buff/cache available Mem: 949572 140332 121768 13716 687472 729976 Swap: 102396 1016 101380
Search for a given string
The grep (general regular expression print) command allow to search for a given string in a file or output of another command.
To search a string in a given file type:
grep -I "STRING_TO_SEARCH" FILE_TO_BE_SCANNED
For example, to search the word “express” (case insensitive, option -i) in the file app.js:
grep -i "express" app.js
The output will be:
var express = require("express");
session = require("express-session"),
var app = express();
app.use(express.static(path.join(__dirname, "public")));
The grep command is very useful. Please, consider to read the full article here.
Show system environmental variables
To show the system environmental variables use the command export:
export
The output can be really confusing. If you need to search for a given string, combine the export command with grep:
export | grep JAVA
Export a system environmental variable
Use again the export command, with the variable to export:
export VARIABLE_NAME=VALUE
For example to export the JAVA_HOME:
export JAVA_HOME="/usr/lib/jvm/jdk-8-oracle-arm32-vfp-hflt/jre/