Practical Linux Tricks for the Aspiring Hacker

This is a collection of commands I've picked up over the last few years, which I've found immensely useful. My favourite ones are probably:

• less: search/filter on a file or long text
• ^r: reverse search
• !$: last argument of previous command

By "favourite", I mean I've used these commands a lot, and they've drastically increased my productivity.

Cool Stuff

Control (^) Commands

^c # Duh. https://xkcd.com/416/
^d # Exit / EOF.

^r # Reverse search: for those long commands stashed in history.

Ternary Expression

[ 1 -eq 1 ] && echo 'true' || echo 'false'
# true

Clear screen. Useful for graphical hiccups.

reset

Run shell script without chmod +x.

. ~/.zshrc  # Dot command.
source ~/.zshrc

Tree view of files.

tree

Strings

Double-Quotes vs. Single-Quotes

• Double-quotes allow variable expansion and command substitution.
• Single-quotes don't. Prefer single-quotes for simple strings.

echo "$((1+1))" "$SHELL"
# 2 /bin/zsh

echo '$((1+1))' '$SHELL'
# $((1+1)) $SHELL

Multi-Line / Escape
Prefix the string with $.

echo $'...'

Escape Single-Quotes

echo $'1\n2\n3'
# 1
# 2
# 3

echo -n $'can\'t,don\'t,I\'ll,I\'m,won\'t' | awk -vRS=, $'$0 ~ /\'t/'
# can't
# don't
# won't

Previous-Command Tricks

• $?: exit code of previous command
  • By convention, 0 means no error. Non-0 implies an error occurred.
• !!: previous command
• !$ or $_: last argument of previous command

mkdir /var/tmp/lol
# Permission denied.
sudo !!
# Yay!

ls long/path
cd !$
# → cd long/path

cat long/path/file.txt
mv "!$" "$(dirname !$)/booyah.md"
# → mv long/path/file.txt long/path/booyah.md

Other Useful Commands (stolen from here)

• !!:n - nth argument from previous command
• !^ - first argument (after the program/built-in/script) from previous command
• !* - all arguments from previous command
• !n - command number n from history
• !pattern - most recent command matching pattern
• !!:s/find/replace - last command, substitute find with replace

Redirection

< out.txt # Read from file (to stdin).
> out.txt # Write to file (from stdout).
>> out.txt # Append to file (from stdout).
2> out.txt # Write to file (from stderr).
&> out.txt # Redirect all output.
&> /dev/null # Redirect everything into the void.
2>&1 # Redirect stderr to stdout.

>& # Same as `&>`.

Powerful Utilities

• awk: filter lines, filter columns, math, scripting, etc.
• sed: filter/replace text
• grep: filter lines
• cut: filter columns
• tr: replace/remove characters
• wc: count characters/bytes/words
• find: find files in folder, execute command for each file with -exec
• xargs: feed arguments into commands, simple cmdline multi-processing

I won't cover too much of these commands here, as tons of articles already cover them. And you can browse examples online or in their man pages.

awkward things

awk - Cut

awk '$0=$3' # Cut third field.
awk '{print $3}' # Print third field. (Pretty much same as the command above.)

# Use ',' as field delimiter, e.g. for CSVs.
awk -F, '{print $3}'
# or use the script variable `FS` (Field Separator).
awk -v FS=, '{print $3}

awk - Filtering

awk '$1 == 1' # Filter lines with first field = 1.
awk '$0 ~ /^foo/' # Filter lines with regex.

awk - Math

awk '{$1 += 5}1' # Add 5 to the first arg, then print the line.

seq 1 3 | awk '{$1 += 5}1'
# 6
# 7
# 8

awk - Scripting

awk '{print "booyah",$1,"yahoo"}'
# awk also has variables, if, for, while, arrays, etc.

Script variables. (Useful for configuring row/column delimiters.)

• RS: Record Separator (rows)
• FS: Field Separator (columns)
• ORS: Output Row Separator
• OFS: Output Field Separator
• NR: Record Number (current row, 1-indexed) [read-only]
• NF: Number of Fields [read-only]

grep

Useful Flags

# case-insensitive
-i
# regex
-E
# non-match (inVert)
-v

xargs

xargs is a versatile command-line utility that allows efficient execution of commands from, making it a powerful tool for automation and batch processing.

Interesting options:

-P <n> # max procs
-n <n> # num args
-I {}  # pattern to insert into command

echo $'1\n2\n3' | xargs 
# Output: 1 2 3 (no newline)

cat files.txt | xargs -P 4 -n1 ./do-something-to-file.sh

seq 1 1000 | xargs -P 50 -I{} proxychains4 -q nmap -p {} -sT -Pn --open -n -T4 --oN nmap.txt --append-output 192.168.101.10

Other Utilities

basename ~/.bashrc # .bashrc     # Filename, without path.
dirname ~/.bashrc  # /home/bob/  # Path to file.

Directory Stack

pushd # Push current directory, for easier returning.
popd  # Return to directory on top of stack.

cd ~/a/b/c
pushd deep/nested/directory
# Jump to `deep/nested/directory`, push `~/a/b/c` into the stack.
cd ../../jump/around/some/more
cd ../../and/a/little/more
popd  # Return to `~/a/b/c`.

less

less is a powerful text viewer (read-only), with capabilities to navigate, search, and filter lines in a file or long text.

less - Nice Options

less file.txt

# Renders ANSI colors.
less -R file.txt

# Pager search becomes case insensitive.
less -I file.txt

# Line numbers.
less -N file.txt

less - Navigation

j # Line down.
k # Line up.
f # Page down.
b # Page up.
d # Half-page down.
u # Half-page up.

g # Go to start of file.
G # Go to end of file.

<n> g # Go to nth line.

# Go to the n% line of the file.
<n> p
20p 40p 50p 80p

# What's the current line?
^g

less - Search / Filtering

# Search (regex enabled).
/ <pattern>
# For case-insensitive search, use `less -I`.

# Navigate search results: next/prev result.
nN

# Filter lines by search.
& <pattern>
# Filter NON-MATCHING lines
& ! <pattern>
# Clear filter.
& <enter>

less - Scrolling

Personally, I prefer less+F over tail -f.
Use ^c to exit the feed.

# Continuous feed (e.g. for streams of data)
F

less - Working with Multiple Files

less also works with multiple files passed in the command line, e.g. less *.txt.

# Next file.
:n
# Previous file.
:p

More commands in man less.

Processes

fg/bg - "I'll be back."

Shells allow you to move processes between the foreground (which accepts interactive input) and background (to run things which don't require input).

^z # Push process to background (and pause it).
bg # Start background process.
fg # Bring most recent background process into foreground.
fg 2 # Bring job 2 into foreground.
jobs # View background jobs.

^c # Good ol' ctrl-c stops the process in fg.
kill <pid> # Kill process with given process ID.

# Start a command in the background.
<cmd> &

python -m http.server 8080 &
# [1] 17999

fg
^c
# or
kill 17999

View Running Procs

ps aux

Combine with grep/less for filtered results.

Networking

Information

# IP
ifconfig
ifconfig tun0

# Open Ports/Sockets
netstat -anp
# -a: all sockets
# -n: numeric addresses
# -p: associated processes

Listen/Connect

# Initiate a connection.
nc 192.168.1.1 8080

# Listen for a connection.
nc -nlvp 4444

# Listen persistently (keep listening after disconnect).
nc -nklvp 4444

Download Files

wget

# Quick download.
wget <url>

# Save to specific location.
wget <url> -O filename.txt

# Download silently (no progress), and display in less.
wget <url> -s | less

curl

curl <url> -s | less

# Use jq to format JSON.
curl some.api.site/api/v1/users/ -s | jq | less

Upload Files

Server

python -m uploadserver

• By default, uploadserver starts a server at port 8000.
• Get our IP from ifconfig.

Client

curl -F files=@file1.txt -F files=@file2.txt 192.168.45.179:8000/upload

git gud

Git commands for completeness.

# Make new branch.
git checkout -b <name>

# Checkout commits in tree before HEAD.
git checkout HEAD~1  # 1 commit before.
git checkout HEAD~10 # 10 commits before.

# Checkout commit from parent.
git checkout HEAD^  # 1 commit before (from parent 1, base).
git checkout HEAD^2 # 1 commit before (from parent 2, target).

# Store changes locally.
git stash
git stash pop

# Clean edited files.
git reset [--hard]
# --hard removes unstaged files.

# View changes.
git diff | less
git diff <file> # See change in specific file.

# Jump through commits (to find, say, the cause of a bug).
git bisect [start|reset|good|bad|skip]

git tree

# Command-line git tree from git log.
git log --graph --abbrev-commit --decorate --format=format:'%C(bold blue)%h%C(reset) - %C(bold green)(%ar)%C(reset) %C(white)%s%C(reset) %C(dim white)- %an%C(reset)%C(bold yellow)%d%C(reset)' --all

# More detailed git-tree 
git log --graph --abbrev-commit --decorate --format=format:'%C(bold blue)%h%C(reset) - %C(bold cyan)%aD%C(reset) %C(bold green)(%ar)%C(reset)%C(bold yellow)%d%C(reset)%n''          %C(white)%s%C(reset) %C(dim white)- %an%C(reset)' --all

# Add them as git aliases in ~/.gitconfig or script aliases in ~/.bashrc.
# See https://stackoverflow.com/a/9074343/10239789.

Fun watch: So You Think You Know Git?

vim

Haha. Nope.

Not covering that here.

Hacky Hack Hack

Generate Bytes

Buffer overflow for fun and profit.

echo

echo -n '\x01\x02'

echo -n '\x41' | xxd
00000000: 41                     A

perl (good for repetitive sequences)

perl -e 'print "\x41"x4 . "\x42\x43"' | xxd
00000000: 4141 4141 4243         AAAABC

assert '\xc0'.encode() == b'\xc3\x80'