bashconditionals

#!/bin/bash

echo "Control execution, evaluating [ 1 -eq 0 ], which is not true:"
if [ 1 -eq 0 ]; then
    echo "The expression evaluates as true."
else
    echo "The expression evaluates as false."
fi

# In the following examples, I unset boolval before setting it with
# a new statement to avoid carryover from a previous experiment
# when a statement has no effect on the variable.

echo
echo "Experiment 1: Setting boolval=false, boolval should be false."
unset boolval
boolval=false
echo -n "boolval is '$boolval' "
if [ boolval ]; then
   echo "boolval evaluates true"
else
   echo "boolval evaluates false"
fi

echo
echo "Experiment 2: Setting boolval=1, boolval should be false."
unset boolval
boolval=1
echo -n "boolval is '$boolval' "
if [ boolval ]; then
   echo "boolval evaluates true"
else
   echo "boolval evaluates false"
fi

echo
echo "Experiment 3: Setting boolval=\$( [ 1 -eq 0 ] ), boolval should be false."
unset boolval
boolval=$( [ 1 -eq 0 ] )

echo -n "boolval is '$boolval' "
if [ boolval ]; then
   echo "boolval evaluates true"
else
   echo "boolval evaluates false"
fi

echo
echo "Experiment 4: Setting boolval=\$( [ 1 -eq 0 ]; echo \$? ), boolval should be false."
unset boolval
boolval=$( [ 1 -eq 0 ]; echo $? )

echo -n "boolval is '$boolval' "
if [ boolval ]; then
   echo "boolval evaluates true"
else
   echo "boolval evaluates false"
fi

echo
echo "None of the experiments resulted in a variable that directly evaluates"
echo "as false.  Evaluating a variable will "

loop_with_lambda()
{
    local -i limit=10
    local -i ndx=0
    keep_looping() { (( ndx++ )); [ "$ndx" -le "$limit" ]; }

    while keep_looping; do
        echo "ndx = $ndx"
    done
}

loop_with_lambda