Conditions and loops

if with [[ ]] and (( )), case patterns, for over words, a range or a C-style counter, and while.

Runs in Zerethon IDE · Bash 5.2 Official Bash documentation ↗

main.sh

for n in {1..5}; do
  if (( n % 2 == 0 )); then
    echo "$n even"
  elif [[ $n -eq 5 ]]; then
    echo "$n last"
  else
    echo "$n odd"
  fi
done

i=3
while (( i > 0 )); do
  (( i-- ))
  [[ $i -eq 1 ]] && continue
  echo "i = $i"
done

for (( k = 0; k < 2; k++ )); do
  echo "k = $k"
done

file="photo.png"
case "$file" in
  *.png | *.jpg) echo "image" ;;
  *.txt) echo "text" ;;
  *) echo "other" ;;
esac

[[ -f main.sh ]] && echo "main.sh exists"
[[ "abc" < "abd" ]] && echo "abc sorts first"

Output

1 odd
2 even
3 odd
4 even
5 last
i = 2
i = 0
k = 0
k = 1
image
main.sh exists
abc sorts first

(( ... )) compares numbers with <, ==, %; inside [[ ... ]] numbers use -eq, -lt, -gt, and == / < compare text.
Spaces are required inside the brackets: [[ $n -eq 5 ]], not [[$n -eq 5]].
File tests: -f file exists, -d folder exists, -z text is empty, -n text is not empty.
cmd && other runs other only if cmd succeeded; cmd || other only if it failed.

Conditions and loops

Related topics

Official references