• The special variable ? has the exit/return value of the last program/command that ran.

if [ $? -eq 0 ]; then
    echo "All is well."
    exit 0
elif [ $? -eq 1 ]; then 
    echo "Doh!"
    exit 1
else
   echo "Really?!"
   exit 2
fi

if [$? -eq 0]; then echo "OK"; else echo "Error"; fi

for i in alpha beta gamma
do
    printf "$i\n"
done
 
a=alpha
b=beta
c=gamma
 
for j in "$a" "$b" "$c"; do
    printf "$j\n"
done

for i in $( ls ); do
  echo item: $i
done
 
LIST="$(ls *.html)"
for i in "$LIST"; do
   echo $1
done

COUNTER=0
 
while [  $COUNTER -lt 10 ]; do
  echo The counter is $COUNTER
  let COUNTER=COUNTER+1 
done

• Command substitution will run the specified command and replace the command with the output from the command in the location where it is specified as part of input to some other command.

$(<command expression>)

text=$(cat alpha.txt)

`<command expression>`