Bash syntax: Conditions

Sometimes you want a command to be executed or not...

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1. Conditional expressions

A conditional expression si similar to an Yes-no question in the real life, it has to return a boolean value (true or false). In bash programming language the value of true is 0 (that means success) and false is a non-zero value.

You can use, as a conditional expression, every commands that return an exit status that can be interpreted as a boolean.

For example: In C programming language, the exit status is the value returned by the function main(). If we omit the standard and error outputs of a C program, like the project ft_ls made at 42 school, we can use its exit status as a conditionnal expression because it should return 0 when its execution is successful, and a non-zero value when an error occurs.

./ft_ls -Rl / &>/dev/null   # '&/dev/null' suppress outputs (standard and error)
                            # returns 0 if success -> true
                            # returns non-zero     -> false

In bash scripts, the builtin command test is commonly used in conditional expressions. It enables you to make comparison expressions and file tests:

test -f "filename"          # true if the file exists
test -d "dirname"           # true if the directory exists
test -n "$MY_VAR"           # true if the value of MY_VAR is not null
test "$MY_VAR" == "Yes"     # true if the value of MY_VAR is equal to "Yes"
test "$MY_VAR" -eq 1        # true if the arithmetic value of MY_VAR is equal to 1
test "$EXAM_RATE" -ge 75    # true if the arithmetic value of EXAM_RATE is greater than or equal to 75

Use the alias [ of the builtin command test for a simpler code (Newer versions of Bash require the closing tag ]). These are the equivalents of the preceding commands:

[ -f "filename" ]
[ -d "dirname" ]
[ -n "$MY_VAR" ]
[ "$MY_VAR" == "Yes" ]
[ "$MY_VAR" -eq 1 ]
[ "$EXAM_RATE" -ge 75 ]

Arithmetic comparisons can also be performed with a more familiar syntax (as in C language) thanks to the builtin command let or its equivalent ((...)) construct. These following commands are similar:

test "$EXAM_RATE" -ge 75
[ "$EXAM_RATE" -ge 75 ]
let "$EXAM_RATE >= 75"
(( "$EXAM_RATE" >= 75 ))

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2. If-then-else

The if-then-else construct requires a conditional expression. If the result of the expression is true, the commands that follow the then are executed. Otherwise the script execution continues in the optional else block or, if it does not exist, continues after the keyword fi which ends the if-then-else construct:

if (( "$MY_EXAM_GRADE" >= 75 ))
then
    echo "I validated my exam"
else
    echo "I did not validate my exam"
fi

You can also use the keyword elif followed by a conditional expression when a single conditional expression does not handle all the possibilities you want:

if (( "$MY_EXAM_GRADE" < 75 ))
then
    echo "I did not validate my exam"
elif (( "$MY_EXAM_GRADE" <= 100 ))
    echo "I validated my exam"
else
    echo "I validated my exam with additional bonus"
fi

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3. Case-in

Rather than creating a long sequence of if and elif when you want a single value to be compared with several patterns, use the case-in statement whose closing tag is esac:

case "$PROJECT_NAME" in
    "libft")     echo "This is the libft project"         ;;
    "ft_ls")     echo "This is the ft_ls project"         ;;
    "ft_printf") echo "This is the ft_printf project"     ;;
    *)           echo "This is not a 42 school's project" ;;
esac

The value surrounded by the keywords case and in is compared with each pattern. If a pattern matches, the commands following the keyword ) are executed until ;; is encountered, then the program execution continues after the keyword esac. The symbol * is a wildcard that matches with all sequence of characters, even an empty string, allowing you to set a default statement when any pattern does match.