Foundations Updated 2026-09 View as Markdown

Decision Making

As in popular programming languages, the shell also supports logical decision making.

As in popular programming languages, the shell also supports logical decision making.

The basic conditional decision making construct is:

if [ expression ]; then

code if ‘expression’ is true

fi

NAME="John"
if [ "$NAME" = "John" ]; then
  echo "True - my name is indeed John"
fi

It can be expanded with ‘else’

NAME="Bill"
if [ "$NAME" = "John" ]; then
  echo "True - my name is indeed John"
else
  echo "False"
  echo "You must mistaken me for $NAME"
fi

It can be expanded with ‘elif’ (else-if)

NAME="George"
if [ "$NAME" = "John" ]; then
  echo "John Lennon"
elif [ "$NAME" = "George" ]; then
  echo "George Harrison"
else
  echo "This leaves us with Paul and Ringo"
fi

The expression used by the conditional construct is evaluated to either true or false. The expression can be a single string or variable. A empty string or a string consisting of spaces or an undefined variable name, are evaluated as false. The expression can be a logical combination of comparisons: negation is denoted by !, logical AND (conjunction) is denoted by &&, and logical OR (disjunction) is denoted by ||. Conditional expressions should be surrounded by double brackets [[ ]].

Types of numeric comparisons#

comparison    Evaluated to true when
$a -lt $b    $a < $b
$a -gt $b    $a > $b
$a -le $b    $a <= $b
$a -ge $b    $a >= $b
$a -eq $b    $a is equal to $b
$a -ne $b    $a is not equal to $b

Types of string comparisons#

comparison    Evaluated to true when
"$a" = "$b"     $a is the same as $b
"$a" == "$b"    $a is the same as $b
"$a" != "$b"    $a is different from $b
-z "$a"         $a is empty

• note1: whitespace around = is required

• note2: use “” around string variables to avoid shell expansion of special characters as *

Logical combinations#

if [[ $VAR_A -eq 1 && ($VAR_B = "bee" || $VAR_T = "tee") ]] ; then
    command...
fi

case structure#

case "$variable" in
    "$condition1" )
        command...
    ;;
    "$condition2" )
        command...
    ;;
esac

simple case bash structure#

Note in this case $case is variable and does not have to be named case - this is just an example

mycase=1
case $mycase in
    1) echo "You selected bash";;
    2) echo "You selected perl";;
    3) echo "You selected phyton";;
    4) echo "You selected c++";;
    5) exit
esac

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#!/bin/bash
# change these variables
NUMBER=16
APPLES=16
KING="LUIS"
# modify above variables
# to make all decisions below TRUE
if [ $NUMBER -gt 15 ] ; then
  echo 1
fi
if [ $NUMBER -eq $APPLES ] ; then
  echo 2
fi
if [[ ($APPLES -eq 12) || ("$KING" = "LUIS") ]] ; then
  echo 3
fi
if [[ $(($NUMBER + $APPLES)) -le 32 ]] ; then
  echo 4
fi