flow-control.md

Flow Control

• if
  • if/else
  • if/elif
• for..in
  • for..in + Sequence
• for
• while
• until
• break and continue
• case

if

if [ $# -eq 0 ]; then
    echo 'Pass at least 1 argument'
    exit 1
fi

if/else

if [ "$1" = "-s" ]; then
    cat file | cut -c-50
else
    cat file
fi

if/elif

if [ "$1" = "$2" -a "$2" = "$3" ]; then
    echo "equal words: 1, 2 and 3"
elif [ "$1" = "$2" ]; then
    echo "equal words: 1 and 2"
elif [ "$1" = "$3" ]; then
    echo "equal words: 1 and 3"
elif [ "$2" = "$3" ]; then
    echo "equal words: 2 and 3"
else
    echo "no equal words"
fi

for..in

for ch in a b c d
do
    echo $ch | tr a-d A-D
done

for file in *
do
    if [ -d "$file" ]; then
        echo $file
    fi
done

for..in + Sequence

• {start..end[..step]}

for n in {0..10..2}; do
    echo -n "$n "
done

• seq start [step] end - for older versions of Bash

for n in $(seq 0 2 10); do
    echo -n "$n "
done

for

for (( x=0; x<=10; x+=2 )); do
    echo -n "$x "
done

for (( x=1; x<=4; x++ )); do
    echo -n "$x "
done

while

number=0

while [ "$number" -lt 10 ]; do
    echo -n "$number "
    (( number += 1 ))
done

until

n=0

until [ $n = 10 ]; do
    echo $n
    (( n += 2 ))
done

secret=werewolf
pass=nothing

echo 'Guess the password:'
until [ "$pass" = "$secret" ]; do
    read -p "Type password: " pass
done
echo 'You guessed the password!'

break and continue

All the loops can use break and continue keywords.

for index in 1 2 3 4 5 6 7 8 9 10; do
    if [ $index -le 3 ]; then
        echo 'skip'
        continue
    fi

    echo $index

    if [ $index -gt 8 ]; then
        echo 'stop'
        break
    fi
done

output:

skip
skip
skip
4
5
6
7
8
stop

case

read -p "Type a letter:" ch

case "$ch" in
    a)
        echo 'lowercase A'
        ;;
    A)
        echo 'uppercase A'
        ;;
    b|B)
        echo 'just B'
        ;;
    *)
        echo 'different letter'
        ;;
esac

Special characters in a pattern:

• * - any string (typically used as default case)
• ? - any single character
• [...] - characters class (e.g., [a-z])
• | - separator of patterns