If command

The GNU bash manual has a great description of how the if-command works:

if test-commands; then
  consequent-commands;
[elif more-test-commands; then
  more-consequents;]
[else
  alternate-consequents;]
fi

This page contains examples of how to use each form of the if command.

Simple conditional

In the simplest form, the if-command conditions the execution of a command on the succesful execution of another:

if command1; then
  command2;
fi

This is just an arguably more readable version of:

command1 && command2

The condition is that the first command returns status code "OK" (0):

if ls ~/.emacs; then
  cat ~/.emacs;
fi

Explicit test conditional

Sometimes, the condition is not just the return code of a command:

if test 1 -ne 2; then
  echo "1 is not 2"
fi

I find the explicit test command very natural to read - the exception being matching strings using [[ string =~ regexp ]] and BASH_REMATCH.

Careful:

• the test command should end with a ";".
• bash will perform all sorts of preprocessing on the "test" arguments. Always quote them, unless you really want this.

[ condition ] and [[ condition ]] Syntax

In my opinion, this syntax is too obscure to be worth the couple of characters saved:

if [ 1 -eq 5 ]; then...
if [[ 1 -eq 5 ]]; then...

This is less readable to me than:

if test 1 -eq 5; then...

I always prefer as little special syntax as possible - which is, of course, a matter of personal preference.

Multiple condition "if"

Use elif to treat multiple cases in one if-statement:

if test 1 -eq 2; then
  echo "first case"
elif test 1 -eq 1; then
  echo "second case"
fi

Default case (if condition then ... else ...)

Use else to execute commands only if none of the conditions were satisfied:

if test 1 -eq 2; then
  echo "condition OK"
else
  echo "condition KO"
fi

Testing that multiple expressions are true ("and")

Use && to test that a number of expressions are all true:

[[ "a" =~ a && "b" =~ b && "c" =~ c ]];

Each expression tests that a string matches a regular expression.

If any of the expressions evaluates to false, subsequent expressions are not evaluated:

# outputs nothing
false && echo "Hello world!"

Testing that one of multiple expressions is true ("or")

Use || to test that at least one of a number of expressions is true:

[[ "a" =~ a || "b" =~ b || "c" =~ c ]];

If one of the expressions evaluates to true, subsequent expressions are not evaluated:

# outputs nothing
true || echo "Hello world!"

Subexpressions

Consider the following expression:

if test -n "$a" && test -n "$b" || test -n "$c" &&
test -n "$d"; then
  echo true;
fi

Note: test -n "$a" is true if the variable a is not empty.

Question: Which of the variables a, b, c, and d have to be non-empty for the expression to evaluate to true?

Answer: Don't risk putting yourself or your fellow coders in a position where they would have to know the bash rules of precedence well enough to answer this question!

Use parentheses to clarify your intent:

# "and" takes precedence
if (test -n "$a" && test -n "$b") || (test -n "$c"
&& test -n "$d"); then
  echo true;
fi

# "or" takes precedence
if test -n "$a" && (test -n "$b" || test -n "$c")
&& test -n "$d"; then
  echo true;
fi

The test Command

The builtin command test provides various checks:

# Does the file/directory exist?
if test -e .;         then echo "File or directory exists!";     fi
if test -f file;      then echo "File exists!";                  fi
if test -s file;      then echo "File exists and is not empty!"; fi
if test -d directory; then echo "Directory exists!";             fi

# Permissions
if test -r .; then echo "File exists and is readable!";   fi
if test -w .; then echo "File exists and is writable!";   fi
if test -x .; then echo "File exists and is executable!"; fi

# Variables
if test -v PATH; then echo "Shell variable is set!"; fi

# String checks
if test -z "";          then echo "String is empty!";       fi
if test -n "Not empty"; then echo "String is not empty!";   fi
if test "A" =  "A";     then echo "Strings are equal!";     fi
if test "A" == "A";     then echo "Strings are equal!";     fi
if test "A" != "B";     then echo "Strings are not equal!"; fi

# Arithmetic
if test 4 -gt 3; then echo "4 is greater-than 3"; fi
if test 2 -lt 3; then echo "2 is less-than 3";    fi
if test 2 -eq 2; then echo "2 is equal-to 2";     fi
if test 1 -ne 2; then echo "1 is not-equal-to 2"; fi

The bash documentation contains an excellent description of conditional expressions.

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