Decisions and Loops

if, elif, else

if runs a command and branches on its exit status: 0 means “then,” anything else means “else” (module 6) ¹:

if grep -q "^$USER:" /etc/passwd; then
    echo "you're a local user"
elif [[ -n "${SUDO_USER-}" ]]; then
    echo "running under sudo"
else
    echo "who are you?"
fi

then must start the block; fi (“if” backwards) ends it. Put then on the same line after a semicolon, as Google’s style guide does ².

Tests with [[ ]]

Most conditions are written with [[ ... ]], bash’s test command. It succeeds or fails, so it works with if, &&, and || ^{1 3}. Spaces inside the brackets are required.

Combine with && and ||, and negate with ! ¹:

if [[ -f "$file" && ! -s "$file" ]]; then
    echo "$file exists but is empty"
fi

The older single-bracket form, [ ... ] (the same as the test command), works everywhere sh does but trips over empty variables and unquoted words. In bash scripts, Google’s guide prefers [[ ]] ².

For arithmetic, (( )) reads like math and needs no $ on variable names ^{1 3}:

count=$(ls | wc -l)
if (( count > 100 )); then
    echo "that's a lot of files"
fi

case

case matches one value against several wildcard patterns, top to bottom, and runs the first that matches ¹:

read -rp "Continue? [y/N] " answer
case "$answer" in
    y|Y|yes)  echo "continuing" ;;
    n|N|no|"") echo "stopping"; exit 0 ;;
    *)        echo "please answer y or n" >&2; exit 1 ;;
esac

| separates alternatives, *) catches everything else, and ;; ends each branch. It’s cleaner than a long chain of elifs, and it’s how scripts handle options and file types:

case "$file" in
    *.tar.gz|*.tgz) tar -xzf "$file" ;;
    *.zip)          unzip "$file" ;;
    *)              echo "don't know how to unpack $file" >&2 ;;
esac

for loops

for runs its body once for each word in a list ¹:

for name in alex sam kids; do
    echo "making a folder for $name"
    mkdir -p "/srv/household/$name"
done

for i in {1..5}; do echo "round $i"; done          # a range (brace expansion)

for (( i = 1; i <= 10; i += 3 )); do echo "$i"; done  # C style: 1 4 7 10

Looping over files

Let the shell’s wildcards produce the list, not ls. Each match arrives as one word, spaces and all, as long as you quote "$f" inside the loop ¹:

for f in ~/photos/*.jpg; do
    echo "resizing $f"
    # convert "$f" -resize 50% "${f%.jpg}-small.jpg"
done

${f%.jpg} removes .jpg from the end of the value (# would remove from the start); these are bash’s string-trimming expansions ³.

If nothing matches, the pattern is left as the literal text ~/photos/*.jpg and the loop runs once on a file that doesn’t exist. Two fixes: check [[ -e "$f" ]] || continue at the top of the loop, or set shopt -s nullglob so an unmatched pattern expands to nothing ³.

Never write for f in $(ls): ls output is split at every space, so Summer 2026.jpg becomes two files ². For whole trees, use find (module 4) and read its output in a while loop.

while and until

while repeats as long as a command succeeds; until repeats until it does ¹:

until ping -c1 -W1 router > /dev/null 2>&1; do
    echo "waiting for the network..."
    sleep 5
done

The most useful while reads a file line by line ¹:

while read -r line; do
    echo "> $line"
done < notes.txt

find ~/photos -name '*.jpg' -print0 | while IFS= read -r -d '' f; do
    echo "found: $f"
done

-r stops read from treating backslashes as escapes, and IFS= keeps leading spaces. The second form pairs with find -print0 to survive any file name (module 4).

break leaves a loop early; continue skips to the next round ¹.

Your turn

So

if branches on exit status, and [[ ]] asks the questions: files, strings, patterns, numbers. case picks among patterns. for loops over words, ranges, and wildcard matches (never $(ls)); while read -r walks a file line by line; break and continue steer. Quote "$f" everywhere, and plan for no matches.