Linux Notes

Linux Notes

/usr

User usable programs and data. User System Resources

• /usr/include - Contains 'header files', needed for compiling user space source code. Headers like stdio.h, stdlib.h stdint.h, etc. are here.
• /usr/lib - Contains program libraries for the normal user-programs, that mostly can be found under /usr/bin and /usr/bin
• /usr/local/lib - are the libraries for locally installed programs and packages (things you've compiled and installed from source-packages yourself)

/lib

Contains those shared libraries needed to boot the system and run the commands/programs in the root filesystem (binaries in /bin and /sbin folders). Kernel modules (device drivers) are under /lib.

/sys

• /sys/module - Its a sysfs directory hierarchy containing information on currently-loaded modules

/proc

• /proc is a "pseudo" file system that exists only in kernel memory and is used primarily for querying various per-process as well as kernel statistics, contains processes as subdirs

• /proc/iomem - This file shows you the current map of the system's memory for each physical device:

• /proc/<pid>/status: check status of a process

/etc

• /etc/shells - shows which command shells are installed on your Linux system.

Shell

• A shell is the term used to refer to a program that interprets and executes command, is used as a command line user interface
• There are built-in commands which are used by default and other programs/commands with the same name. Use command type to know how a command will be interpreted
  • type -a <command>
  • type -a echo
• For built in commands you can do help <command> to get help information
• enable -a: show built-in shell commands
• To check which terminal you are using echo $0 or echo $SHELL
• Bash ( Bourne Again Shell ) is a Unix shell and command language and its the default shell on most Linux based distros

Expansions

• brackets: mkdir -p testdirs/{test1,test2,test3}
• single character ?: ls tes?t
• any string *: ls tes*
• enclosed characters: ls test[123]
• commands substitution use $(): echo "the current dir is: $(pwd)"

grub

• To change the grub boot default option just change the GRUB_DEFAULT=0 on the /etc/default/grub file and the run update-grub

loops

for

• one-line for: for i in `seq 1 10`; do echo $i; done
• using variable as limit

  for (( i=0; i <= $upperlim; i++ )); do
     echo "$i"
  done
  

while

• decrement counter with while loop

  while [ $cnt -gt 0 ]
  do
    echo "Welcome $cnt times"
    cnt=$(( $cnt - 1 ))
  done
  

Commands

printf

• printf "Message no color, \033[0;36mMessage color\033[0m\n"

chmod

• <groups-letters> user - group - others
  • group can be seen as the group where the user belongs
• <permissions-letters> read - write - execute
• chmod <groups-letters> + <permissions-letters> <file>
  • chmod u+rwx,g+rx,o+rx test1
  • chmod 755 test1
  • chmod u+x myscript.sh

ssh

• ssh -t <user>@<host> '<bash_command_here>; <shell_name>'
  • ssh -t jose@127.0.0.1 'echo Hello; bash'

Enable SSH

• Install sudo apt-get install openssh-server
• Check status sudo service ssh status
• Restart service sudo service ssh restart
• Allow connections on port 22: sudo ufw allow 22

scp

• scp -r <path_to_local_folder> <host_user_here>@<host_dest_ip>:<host_dest_folder>
  • scp -r content/ root@10.219.85.238:/home
  • scp -P 40XX -r content/ root@10.219.85.238:/home

rsync

• rsync <source_folder> <dest_folder>
  • rsync -vP <source_file> <dest_file>
  • rsync -azhP --exclude=".*" <source_folder> <dest_folder>
  • rsync -azhP --exclude=".*" <source_folder> <dest_user>@<dest_ip>:<dest_folder>

ssh keys

• ssh-keygen -t rsa
• eval ssh-agent -s
• ssh-add -l
• ssh-add ~/.ssh/id_rsa
• ssh-keygen -i -f id_dsa_1024_a.pub >> ~/.ssh/authorized_keys

openssl

• openssl enc -aes-256-ofb -in user.md -out user.enc
• openssl enc -aes-256-ofb -d -in user.enc -out user.md
• openssl enc -pbkdf2 -in user.md -out user.md.enc
• openssl enc -pbkdf2 -d -in user.md.enc -out user.md

SHA - md5

• md5sum <path_to_file>

key

apt-get install debian-archive-keyring apt-key update

date

• date --set="YYYYMMDD HH:MM:SS"
  • date --set="20150716 14:36:00"

find

• find <path_to_search> -name <search_filename_here>
  • find /usr/include -name stdio.h
• find <path_to_search> -type <type_indicator> -iname "search_pattern_here"
  • find ./ -type f -iname "myfile"
• find / -type d \( -path <path to exclude> \) -prune -o -type <type_indicator> -iname "search_pattern_here"
  • find / -type d \( -path /root/ \) -prune -o -type f -iname "myjsonfile.json"
• Clock skew detected your build may be incomplete. File has modification time in the future problem:
  • find . -exec touch {} \;

sort

• find . -type f -name "*.md" | sort
• git ls-files -m | sort

wc

• Count files in directory or from list outputted from other command
  • ls -alt | wc -l
  • git ls-files -m | wc -l
  • find . -type f -name "*.md" | wc -l

sed

• Composite sed examples
  • find . -type f -name "*.md" -exec sed -i '' -e 's|\./img\/\(.*\.[[:alpha:]]*\)|https://firebasestorage.googleapis.com/v0/b/pepedocs-ecf95.appspot.com/o/\1?alt=media|g' {} \;

grep

• grep -rIn <pattern_to_search> <path_to_search>
  • grep -rIn "#include <stdio.h>" ./
  • grep -rIn main ./

version & architecture

• cat /etc/issue
• lsb_release -a
• uname -a
• dpkg --print-architecture
• arch

System statistics

• vmstat - reports memory-usage statistics
  • vmstat <interval> <num_reports> e.g. vmstat 1 3
• netstat - reports statistics for network interfaces
• iostat - reports I/O usage for disk

Tracing

• strace - traces system calls invoked by a process
• tcpdump - collects network packets

dmesg

• -w option to monitor (-H is just for human readable)
  • dmesg -wH >> kernel.log
  • dmesg -wH
• Clear kernel log dmesg -C

kill process

Check process ID (pid with) ps command

• Check ssh connections ps -aux | grep "sshd"
• Check tty connections ps -dN | grep pts/0
• Check user processes ps -fu
• Check real time processes top

Use kill command to kill process

• kill -l: show sigspec list
• kill <pid>
• kill 9 <pid> or kill -SIGKILL <pid>
• kill -<signal name/number> <pid>

Signals Table

Sig Name	Number	Description
SIGHUP	1	Hang-up (POSIX)
SIGINT	2	Terminal interrupt (ANSI)
SIGQUIT	3	Terminal quit (POSIX)
SIGILL	4	Illegal instruction (ANSI)
SIGTRAP	5	Trace trap (POSIX)
SIGIOT	6	IOT Trap (4.2 BSD)
SIGBUS	7	BUS error (4.2 BSD)
SIGFPE	8	Floating point exception (ANSI)
SIGKILL	9	Kill (can't be caught or ignored) (POSIX)
SIGUSR1	10	User defined signal 1 (POSIX)
SIGSEGV	11	Invalid memory segment access (ANSI)
SIGUSR2	12	User defined signal 2 (POSIX)
SIGPIPE	13	Write on a pipe with no reader, Broken pipe (POSIX)
SIGALRM	14	Alarm clock (POSIX)
SIGTERM	15	Termination (ANSI)
SIGSTKFLT	16	Stack fault
SIGCHLD	17	Child process has stopped or exited, changed (POSIX)
SIGCONT	18	Continue executing, if stopped (POSIX)
SIGSTOP	19	Stop executing(can't be caught or ignored) (POSIX)
SIGTSTP	20	Terminal stop signal (POSIX)
SIGTTIN	21	Background process trying to read, from TTY (POSIX)
SIGTTOU	22	Background process trying to write, to TTY (POSIX)
SIGURG	23	Urgent condition on socket (4.2 BSD)
SIGXCPU	24	CPU limit exceeded (4.2 BSD)
SIGXFSZ	25	File size limit exceeded (4.2 BSD)
SIGVTALRM	26	Virtual alarm clock (4.2 BSD)
SIGPROF	27	Profiling alarm clock (4.2 BSD)
SIGWINCH	28	Window size change (4.3 BSD, Sun)
SIGIO	29	I/O now possible (4.2 BSD)
SIGPWR	30	Power failure restart (System V)

Build packages

• apt-get install debhelper dpkg-dev binutils

  • dpkg-buildpackage -rfakeroot -Tclean
  • dpkg-buildpackage -b

• dnf install fedora-packager after build check -> /root/rpmbuild/RPMS/

  • rpmbuild -bb --build-in-place specs/*.spec

Keyboard

• setxkbmap -query
• alt+shift: to toggle keybords
• setxkbmap -option grp:alt_shift_toggle us,es
• cat /usr/share/X11/xkb/rules/xorg.xml

Modules

• lsmod
• insmod <module>.ko
• insmod <module>.ko <param1_name>=<param1_value> <param2_name>=<param2_value>
• rmmod <module>.ko
• modinfo <module>.ko

PCI/PCIe

• lspci -nnn

• lspci -d 8086:19f2 -x -v

• lspci -d 8086:19f2 -xxx -v

• lspci -d 8086:19f2 -vvv

• Check Errors and status (+ signs means error is set - means is clear)

  • lspci -d 8086:19f2 -vvv | grep 'UESta'
  • lspci -d 8086:19f2 -vvv | grep 'Lnk'
  • lspci -d 8086:19f2 -vvv | grep 'DevSta'
  • lspci -d 8086:19f2 -vvv | grep 'CESta'

• Read config space byte/word/long:

  • setpci -s <bus>:<dev>.<fun> <offset_hex>.<Size>
    • setpci -s 01:00.0 2.W
    • setpci -s 01:00.0 0.L
    • setpci -s 01:00.0 8.B
  • setpci -d <vid>:<did> <offset_hex>.<Size>
    • setpci -s 0101:0101 2.W (2 bytes)
    • setpci -s 0101:0101 0.L (4 bytes)
    • setpci -s 0101:0101 8.B (1 byte)

• Write config space byte/word/long is equal to read but use = and the value to write

  • setpci -s <bus>:<dev>.<fun> <offset_hex>.<Size>=<value>
  • setpci -d <vid>:<did> <offset_hex>.<Size>=<value>

dpkg

• dpkg -i <package_name>
• dpkg -r <package_name>
• dpkg --get-selections: Show installed packages.
• dpkg -l: Show installed packages.
• dpkg -l <package_name>: Show info of a package
• dpkg -c <package_name>.deb: Show files in a package
• dpkg-deb --contents <package_name>.deb: Show info and files in a package
• dpkg -S <path_to_file>: shows which package installed this file

apt

• apt-get update && apt-get upgrade: Update repos and upgrade packages
• apt-get remove <package_name>
• apt-get install <package_name>=<package_version>: Install a certain version
• apt-get search cache <package_name>: search available packages
• apt-cache show <package_name>: show available package information
• apt-cache madison <package_name>: show available versions of a package
• apt-cache pkgnames --all-names: Show available packages.
• Search package that contains a file
  • apt-get install apt-file
  • apt-file update
  • apt-file search <file>

Setup mail services

1. Update repos then install SSMTP and mail utilities

apt-get update
apt-get install ssmtp mailutils

2. Edit the SSMTP configuration file /etc/ssmtp/ssmtp.conf. Mandatory lines:

root=postmaster
mailhub=smtp.gmail.com:587
hostname=<HOSTNAME_OF_DEVICE_HERE>
AuthUser=<YOUR_GMAIL_EMAIL_HERE>
AuthPass=<YOUR_GMAIL_PASSWORD>
UseSTARTTLS=YES

You will need root user permissions to edit /etc/ssmtp/ssmtp.conf You can use the hostname command to get the hostname of your device

3. Edit the SSMTP aliases file /etc/ssmtp/revaliases add a line for each user that will send emails

root:<YOUR_GMAIL_EMAIL_HERE>:smtp.gmail.com:587
pi:<YOUR_GMAIL_EMAIL_HERE>:smtp.gmail.com:587

4. Set permissions for SSMTP config file: chmod 774 /etc/ssmtp/ssmtp.conf

5. Test email command: echo "Test text" | mail -s "Test Mail" mailuser@example.com

6. Add MAILTO variable at the top of the crontab file MAILTO="<dest_email>" add it as empty to disable cron mail service

Reference: Sending emails from the Raspberry Pi

Check Memory and HD usage

RAM

• free -m

HD

• df -h
• df -H
• df
• fdisk -l
• du -h --max_depth=1

Other

• startxfce4
• sudo shutdown -h now
• readelf --debug-dump=line <path-to-shared-lib.so>
• sudo chown -R <username>:<group> <directory>
• sudo passwd <user_name>
• aspell -c file.txt
• sudo dpkg-reconfigure tzdata

soft links / sym links / symbolic links

• ln -s -f <existing_folder_or_file> <path_and_name_of_existing_soft_link>
• ln -s <existing_folder_or_file> <path_and_name_of_soft_link>

tar

.tar.gz

• tar: tar -czvf name_for_tar.tar.gz /dir/to/targz/
• untar: tar -xzvf tar_file.tar.gz
• untar: tar -xzvf tar_file.tar.gz -C /output/dir

.tar:

• tar: tar -cvf name_for_tar.tar /dir/to/tar/
• untar: tar -xvf tar_file.tar

.gz:

• compress: gzip -9 index.php
• uncompress: gzip -d index.php.gz

.zip:

• zip: zip name_for_zip.zip folder
• unzip: unzip zip_file.zip

network

• sudo service networking stop
• sudo service networking start
• sudo service networking restart
• w
• netstat

curl (connect to URL)

• curl <url>
• curl -I <url>

nslookup

• nslookup <web_site>

traceroute

• traceroute <web_site> : response are not accumulative it measures an approx. of time from init point to the reported point

ip

• ip addr show - view a list of your network devices
• sudo ip addr add <ip_addr_here> dev <dev_name>
• sudo ip link set <dev_name> up
• sudo ip link set <dev_name> down
• ip route show

Set Static IP

1. Use nmcli to get the gateway, IP Address and DNS info

   • nmcli device show <interfacename> | grep IP4
   • nmcli device show enp0s3 | grep IP4

2. Edit the interfaces file sudo vim /etc/network/interfaces. Add the following lines with the appropriate information you got with nmcli

auto enp0s3
iface enp0s3 inet static
address 192.168.0.10
netmask 255.255.255.0
gateway 192.168.0.1
dns-nameservers 192.168.0.1

3. Edit /etc/NetworkManager/NetworkManager.conf:

[main]
plugins=ifupdown,keyfile
#dns=dnsmasq

[ifupdown]
managed=true

route

• route -n - view routing table

arp

• arp -a - view local arp table

hostname

• hostname is stored on /etc/hostname and /etc/hosts files

apache

1. Installs Apache, utilities, configuration files, and other items

   • sudo apt-get install apache2
   • apt-cache show apache2

2. Config files in /etc/apache2

   • main config file /etc/apache2/apache2.conf

redirect >, >>, <, <<

• output functions write to the stdout
• input function read from the stdin

pipe |

• stdout of the program on the left will be used as the stdin of the program on the right

xrdp

• sudo apt install xrdp

VNC vnc4server

1. install dependencies: sudo apt-get install gnome-panel gnome-settings-daemon metacity nautilus gnome-terminal

2. install VNC server:

   • vnc4server: sudo apt-get install vnc4server
   • x11vnc: sudo apt-get install x11vnc

3. Create VNC server file. vnc4server: sudo vim /etc/systemd/system/vncserver@:1.service with the following content:

[Unit]
Description=Start VNC server at startup
After=syslog.target network.target

[Service]
Type=forking
#YOUR USER HERE!
User=pepe
PAMName=login
#YOUR USER HERE!
PIDFile=/home/pepe/.vnc/%H%i.pid
#ExecStartPre=/usr/bin/vncserver -kill %i > /dev/null 2>&1
#Change your resolution here
ExecStart=/usr/bin/vncserver -depth 16 -geometry 1920x1080 %i
ExecStop=/usr/bin/vncserver -kill %i

[Install]
WantedBy=multi-user.target

4. Reload daemon, start VNC and set password using the following two commands

sudo systemctl daemon-reload
vncserver # Set password

5. Edit VNC startup file to look like this:

#!/bin/sh
export XKL_XMODMAP_DISABLE=1
unset SESSION_MANAGER
unset DBUS_SESSION_BUS_ADDRESS

[ -x /etc/vnc/xstartup ] && exec /etc/vnc/xstartup
[ -r $HOME/.Xresources ] && xrdb $HOME/.Xresources
xsetroot -solid grey
vncconfig -iconic &

gnome-session --session=ubuntu-2d &
gnome-panel &
gnome-settings-daemon &
metacity &
nautilus &
gnome-terminal &

6. Enable VNC server at startup sudo systemctl enable vncserver@:1.service
   • Manually start VNC server command: sudo systemctl start vncserver@:1.service

Desktop share native app

1. On Ubuntu open "Desktop Share" app, enable remote connections and set password
2. On the terminal execute: gsettings set org.gnome.Vino require-encryption false

Other Random Links

• Install VNC ubuntu 16.04
• USB ubuntu install freeze