Quick Reference: The Cheat Sheet

Variable Operations at a Glance

var="hello"              # Assignment (no spaces!)
echo "$var"              # Use variable
echo "${var}"            # Safe access (use when ambiguous)
echo "${var:-default}"   # Use default if unset (doesn't set var)
echo "${var:=default}"   # Set and use default if unset (sets var)
echo "${#var}"           # String length
echo "${var^^}"          # All uppercase
echo "${var,,}"          # All lowercase
echo "${var^}"           # Capitalize first letter
echo "${var,}"           # Lowercase first letter
echo "${var#prefix}"     # Remove shortest prefix
echo "${var##prefix}"    # Remove longest prefix (greedy)
echo "${var%suffix}"     # Remove shortest suffix
echo "${var%%suffix}"    # Remove longest suffix (greedy)
echo "${var/old/new}"    # Replace first match
echo "${var//old/new}"   # Replace all matches

Test Operators Quick Reference

# Files
[-f, -d, -e, -r, -w, -x, -s, -L  # (file, dir, exists, readable, writable, executable, non-empty, symlink)](https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html)

# Strings
-z  # empty
-n  # non-empty
==  # equal
!=  # not equal
<   # less (lexicographic, inside [[ ]])
>   # greater (lexicographic, inside [[ ]])
[=~  # regex match (inside [[ ]])][https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html]

# Numbers (inside [ ] or [[ ]])
-eq  # equal
-ne  # not equal
-lt  # less than
-le  # less than or equal
-gt  # greater than
-ge  # greater than or equal

Common One-Liners

# Find and kill a process by name
[kill $(pgrep process_name)](https://man7.org/linux/man-pages/man1/pgrep.1.html)

# Monitor a command continuously
[watch -n 5 df -h](https://man7.org/linux/man-pages/man1/watch.1.html)

# Show disk usage, sorted
du -sh /* 2>/dev/null | sort -rh | head -20

# Find largest files
find . -type f -exec du -sh {} + | sort -rh | head -10

# Count lines in all Python files
find . -name "*.py" | xargs wc -l | tail -1

# Extract all links from an HTML file
grep -E -o 'href="[^"]+"' page.html | sed 's/href="//' | sed 's/"$//'

# Rename files: replace spaces with underscores
for f in *\ *; do mv "$f" "${f// /_}"; done

# Generate a random password
tr -dc 'a-zA-Z0-9!@#$%' < /dev/urandom | head -c 16; echo

# Watch memory usage live
watch -n 1 'free -h'

# Find files modified in the last 24 hours
find . -type f -mtime -1

[# Find files modified since a specific date (GNU find)
find . -type f -newermt '2024-01-01'](https://man7.org/linux/man-pages/man1/find.1.html)

{% element elem_scenario_17_4_28b1e8 %}


# Tail multiple log files simultaneously
tail -f /var/log/app.log /var/log/nginx/error.log

# Add date prefix to all jpg files in current directory
DATE=$(date +%Y-%m-%d); for f in *.jpg; do mv "$f" "${DATE}_${f}"; done

This cheat sheet will serve you well as a quick reference while you develop your scripting fluency. Pin it somewhere useful.

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Now go build something. The terminal is waiting.