Bash Scripting Basics

Line 1 should be #!/bin/bash (the shebang). It tells the kernel which interpreter to use when you run ./script.sh. Without a shebang, some environments may still run the file with a default shell — but you should not rely on that. Be explicit. #!/usr/bin/env bash is another common form that searches PATH for bash.

Without execute permission, the kernel will not launch ./script.sh that way — chmod +x script.sh adds the execute bit for the owner (on a file you own under your home). You can always bash script.sh instead, which ignores the need for +x but still needs a readable file. Prefer chmod +x only on scripts you wrote and reviewed. Never chmod +x a mystery download "so it runs."

NAME="analyst" stores a string. echo "User: $NAME" expands it. Quotes around the assignment keep spaces inside one value. When you expand, prefer "$NAME" so the result stays one word even if it contains spaces. Unquoted expansions undergo word splitting and globbing: a variable holding * can turn into a list of filenames in the current directory — a surprise you do not want in automation.

Positional arguments are $1, $2, and so on; $# is the count; "$@" is all arguments as separate quoted words when used correctly. A script that runs rm $1 (unquoted) is a classic accident: if $1 is My Report.txt, rm sees two arguments; if someone passes a glob or extra words, you may touch files you never named. Always "$1" unless you have a precise, documented reason not to. This course will not teach you to rm user input in the lab — quoting still matters for echo, mkdir, cat, and test.

Default values help: TARGET="${1:-$HOME/cyberlium-lab}" uses $1 if provided, otherwise a safe lab path. That pattern keeps scripts usable with zero arguments without inventing dangerous fallbacks like /.

Every command returns an exit status: 0 usually means success, non-zero means failure. echo $? prints the last status. Scripts should exit 0 when they did their job and a non-zero code when they cannot — so other scripts, cron wrappers, and CI can detect failure. Silent "success" on failure hides bugs until production.

if [ -d "$TARGET" ]; then … fi tests whether a path is a directory. [ is a command (test); the spaces around [ and ] are required. Quote "$TARGET" inside [ ] so a path with spaces is one operand. An unquoted empty variable can make [ ] parse as a broken test and behave surprisingly. Common tests: -f (regular file), -d (directory), -e (exists), -z (string empty), -n (string non-empty). Learn them slowly on files you own.

set -e near the top makes the script exit when a command fails (with some caveats around if and pipelines). It is not magic armor, but it beats continuing after a failed mkdir and writing into the wrong place. Combine with clear messages on stderr: echo "error..." >&2; exit 1.

for f in "$HOME/cyberlium-lab"/*.txt; do echo "$f"; done is a readable pattern for iterating files you already created. Quote "$f" inside the loop. Avoid for loops that expand untrusted globs over system directories. Keep loops inside your lab tree. Clever one-liners that parse ls are fragile; prefer globs or find you understand, and never pipe find into a destructive command for "practice."

Write this under $HOME/cyberlium-lab only on YOUR VM or WSL. Do not cat or rm paths from strangers. No destructive commands in the script. If ./hello.sh says Permission denied, check chmod +x before inventing SUID "fixes."

mkdir -p "$HOME/cyberlium-lab"
cd "$HOME/cyberlium-lab"

cat > hello.sh << 'EOF'
#!/bin/bash
set -e
TARGET="${1:-$HOME/cyberlium-lab}"
echo "Hello from Cyberlium"
echo "User: $(whoami)"
echo "PWD: $(pwd)"
if [ -d "$TARGET" ]; then
  echo "Directory exists: $TARGET"
  # Tiny safe loop over .txt files if any exist
  shopt -s nullglob
  for f in "$TARGET"/*.txt; do
    echo "Found text file: $f"
  done
  exit 0
else
  echo "Not a directory: $TARGET" >&2
  exit 1
fi
EOF

chmod +x hello.sh
./hello.sh "$HOME/cyberlium-lab"
echo "exit code was $?"

demo='two words'
echo unquoted: $demo
echo quoted: "$demo"

You can write a shebang script, set the execute bit, branch on a test, quote expansions so arguments stay data, and exit with a status callers can trust. Next — Lab — Mini Automation Script — you will put whoami, pwd, and date into $HOME/cyberlium-lab/sysinfo.sh and run it.