Linux Tips

Cool stuff

How can I, a regular citizen, install Linux

  • Flash image to USB and boot
  • Dual booting
  • VM is an option
  • Some devices ship with linux by default (not many)

Quick CLI Review

ls - list the files in current directory
cd - change current directory
mkdir - make a new directory
nano - open nano editor
touch - create a file or update its modification time
chmod - change access permissions
rm - remove (permanently delete) a file
rmdir - remove a directory
pwd - print working directory

CLI tips

  • !!
    • replaced with last command
  • !$
    • Replaced with last argument of last command
  • cd -
    • cd back to the previous location you were
  • man
    • read manual for a command
    • man -a intro (intro to linux)
  • sudo
    • run command as root
    • sudo !! (run previous command as root if you forgot sudo)

Keyboard Shortcuts

  • Ctrl+C → kills running program
  • Ctrl+Z → pauses running program
  • Ctrl+D → send EOF
  • Ctrl+S & Ctrl+Q → start & stop text stream
  • Ctrl+R → to search for previous commands (fuzzy finder)
  • Ctrl+W → erase current word up to cursor
  • Ctrl+V → verbatim input

Useful tools

  • Text Editor
    • Nano or even vim
  • Cscope
    • Tool for browsing source code
  • Man
    • Read the docs (manual) for every BASH command/C library

sed - a text processing utility - Stream EDitor



$ sed -i -e ‘s/ *$//’ -e ‘s/[[:space:]]*$//’ -e ‘/^$/d’

  1. Delete all spaces at the end of every line in a file
  2. Delete all tabs at the end of every line in a file
  3. Delete all empty lines in a file
    ^ is the beginning of a line
    $ is the end of a line

vim/cscope - text based search utility

  • cscope -R (in a parent directory to create a cscope database)
  • CTRL+D (to exit when the database is generated)
  • Open vim in the directory where cscope.out resides and you can search for text within the tree
  • Using cscope you can traverse the files that include the text you searched for

grep - command line util to find text

You can pipe the output of a command into grep or give a path to an input file
dmesg | grep "text" or grep "text"
If you want to find file.txt in your home directory
ls ~ | grep file.txt #not entirely correct
. → any character in regex
ls /home | grep -F file.txt #-F means no regex, just text
^ → start of line
$ → end of line

awk - a text processing utility


Basic Scripting

Quick Variable Review

$0...9 - $0 is filepath of the script and $1...9 are the arguments
$# - the number of arguments
$@ - an array of arguments
$? - process’ exit code

Arithmetic Operations

$x+$z #3
$z-$x #1
$x*$z #2
$z/$x #2
$x%$z #1
[ $x == $z ] #false
[ $x != $z ] #true

Relational Operations

[ $x -eq $z ] #false (they aren’t equal)
[ $x -ne $z ] #true (they are unequal)
[ $x -gt $z ] #false (x is not greater than z)
[ $x -lt $z ] #true (x is less than z)
[ $x -ge $z ] #false (x is not greater than or equal to z)
[ $x -le $z ] #true (x is less than or equal to z)

Boolean Operations

[ ! true ] #false
[ $x -gt $z -o $x -ne $z ] #true
[ $x -ne $z -a $x -lt $z ] #true

String Operations

[ $x == $z ] #false
[ $x != $z ] #true
[ -z $x ] #false (“1” is not of length zero)
[ -n $x ] #true (“1” is of length non-zero)
[ $x ] #true ($x is not an empty string)

File Operations

$ ls -l secrets.txt
--w------- 1 kdlp kdlp 106 Aug 22 04:01 secrets.txt
[ -d $file ] #false ($file is not a directory)
[ -f $file ] #true ($file is a regular file)
[ -r $file ] #false ($file is not readable)
[ -w $file ] #true ($file is writable)
[ -x $file ] #false ($file is not executable)
[ -s $file ] #true ($file has size greater than zero)
[ -e $file ] #true ($file exists)

Getting set up

$ touch
$ chmod 744 (chmod changes the permissions that people have for this file)
-rwxr--r-- (new permissions after chmod, now you can execute
r: read w: write x: execute
$ ls -l
Run ^ to check permissions in cwd
First permissions index is a directory marker
Then 1-3: owner permissions 4-6: group permissions 7-9: other user permissions

Getting set up

To write a bash script you can execute direction, you will need to include a shebang line to indicate that bash should be used to interpret the program
On the first line you write #!/bin/bash
Now you can use all commands you can use in your CLI in combination with conditionals, loops, and more

Command-line Arguments

You can reference the arguments passed to your script with $i where i is the index of the argument when the script is run
$ ./ dog cat
$0 #./
$1 #dog
$2 #cat

If statements: integers and strings

if statements




Functions in a bash script are declared with
function foo(){...}
Function arguments come after calling the function, like passing arguments to a script e.g. foo arg1 arg2 ...
Within foo() → $1=arg1, $2=arg2, ...

A && echo "true" || echo "false"

‘echo true’ fires if the prior command succeeds (echo $? = 0)
‘echo false’ fires if the prior command fails
What is echoed?

Debugging / Accessing Linux systems

Remote Access

TTL cable

  • Used to connect to serial console ports
  • Microcontrollers, Raspberry Pi, WiFi routers ...
  • Useful for low level debugging

ttl cable

ttl wiring

Tracing and bpftrace

  • Tracing is most often used for debugging purposes. In the kernel, debugging can become overwhelming without the proper tools.
  • bpftrace is a tracing language which relies on the Linux BPF system and built-in Linux tracing capabilities, such as: kprobes(kernel level), uprobes(user-space level), and tracepoints
  • Which are all types of breakpoints in a stream of processes

Tracing and bpftrace