Shell Scripting Control Structures

What you’ll practice

• Write conditionals
• Loop over values/files
• Use case statements

The mental model

• Quote variables in tests when possible.
• Spaces around [ and ] matter.
• Loops repeat commands predictably.

Try it in your terminal

cd ~/linux-practice/scripts
cat << 'EOF' > if_demo.sh
#!/usr/bin/env bash
num=${1:-10}
if [ "$num" -gt 10 ]; then
  echo "Greater than 10"
elif [ "$num" -eq 10 ]; then
  echo "Exactly 10"
else
  echo "Less than 10"
fi
EOF
chmod +x if_demo.sh
./if_demo.sh 25

cd ~/linux-practice/scripts
cat << 'EOF' > for_demo.sh
#!/usr/bin/env bash
for fruit in apple banana cherry; do
  echo "I like $fruit"
done
EOF
chmod +x for_demo.sh
./for_demo.sh

cd ~/linux-practice/scripts
cat << 'EOF' > case_demo.sh
#!/usr/bin/env bash
answer=${1:-yes}
case "$answer" in
  yes|y|YES|Y) echo "You agreed." ;;
  no|n|NO|N) echo "You disagreed." ;;
  *) echo "Invalid input" ;;
esac
EOF
chmod +x case_demo.sh
./case_demo.sh yes

Put it into practice

1. Write a script that accepts a filename and says whether it exists.

Common mistakes

• You type `if [$num -gt 10]` and get 'command not found.' Bash needs spaces inside the brackets: `if [ $num -gt 10 ]`. The brackets are actually a command, and spaces are how it parses its arguments.
• You store a filename with spaces in a variable, use it unquoted, and watch your script explode. `myfile=$'my file.txt'` then `rm $myfile` becomes `rm my file.txt` — two files, not one. Always quote: `rm "$myfile"`.

Check your understanding

Explain these in your own words. If you’re unsure, revisit the examples before moving on.

1. Which statement handles many possible text values?
2. Why should variables often be quoted?