Introduction to Unix

5.12. Control Constructs

To allow users to automate tasks using shell script programs, BASH provide some basic control constructs for expressing program logic.

5.12.1. if

See also

test

if [ test expressiony ]; then
    statements
fi

if [ test expressiony ]; then
    statements
else
    other statements
fi

if [ test expressiony ]; then
    statements
elif [ another test ]; then
    alternate statements
else
    other statements
fi

5.12.2. case

The case construct allows for a multi-branch selection statement.

SYNOPSIS

case word in
    "pat1")
        commands1
        ;;
    "pat2"|"pat3")  # or operation
        commands2
        ;;
    *)             # default case
        commands3
        ;;
esac

5.12.3. for loop

The for loop is used to iterate over a set of items. The set of items might be obtained by the shell listing files, by reading lines from a file, or by running a command with command substitution.

SYNOPSIS

for name in set
do
    commands
done

EXAMPLES

for file in *.txt    # set is a list of file produced by the shell
do
    grep pattern $file
done

for i in $(cat file)
do
    echo $i | sed 's/goat/cat/g'
done

for i in $(jot 1 5)
do
    touch file$i.txt
done

5.12.4. while loop

SYNOPSIS

while test
do
    commands
done

5.12.5. until loop

The until loop behaves opposed of the while loop in terms of the test to continue executing the loop.

SYNOPSIS

until test
do
    commands
done