Dan's notes, etc

Bash conditionals

bash_conditional_statement

Basic forms for conditionals

Multi line

if test -f '/etc/profile'
then
    echo 'profile exists'
fi
# Or
if test -f '/etc/profile'; then
    echo 'profile exists'
fi
# Or
if [ -f '/etc/profile' ]
then
    echo 'profile exists'
fi

One liner

if [ -f '/etc/profile' ]; then echo 'profile exists'; fi
# Or
[ -f '/etc/profile' ] && echo 'profile exists'
# Or
if test -f '/etc/profile'; then echo 'profile exists'; fi

test vs square brackets etc

[ is an alias for test builtin

if test -f /etc/profile; then
    echo 'The file /etc/profile exists'
fi
# Is the same as
if [ -f /etc/profile ]; then
    echo 'The file /etc/profile exists'
fi

Just think of the brackets as a call to test. Worth noting is that test expects two operands, and will fail when given only one e.g.

if [ $MY_SET_VAR = $MY_UNSET_VAR ]
then
    echo 'I won't print'
else
    echo 'I won't either'
fi

What about [[

if [[ -f filename ]]; then echo 'filename exists!'; fi

[[ is the new upgraded version of test. It’s from ksh, and bash, zsh, yash, busybox sh also support it. This form is needed for checking if a string matches a wildcard pattern (but not on busybox sh). It is also recommended by many experienced programmers. It is not a call to test, and consequentially supports unary tests. It also allows for more C-like syntax with >, <, >=, <=, !=, ==, &&, and || operators.

[[ is not supported on dash :-(

More about the differences between [ and [[ here

And there are still more

if (command); then echo 'Truth!'; fi

Runs command in a subshell.

if command; then echo 'Truth!'; fi

Runs command and responds to exit code. This is the first form shown on this page. The only difference is test was replaced by command to show any command which sets an exit code can be used.

For every form listed above exit code 0 is treated as true.

The weird one

if (( arithmatic )); then echo 'Math!'; fi
# e.g.
if (( 1+1 )); then echo 'True! exit code 0'; fi

This is in bash and zsh. However, because it is not posix, it is not as portable. As the result of the arithmetic, an exit code is set and the if statement acts accordingly. It returns an exit code of zero (true) if the result of the arithmetic calculation is nonzero.

Test the value of a variable

if [[ "$XDG_SESSION_TYPE" == "wayland" ]]; then
  export term=kitty
fi

Some of the operators

-eq	 # Returns true if two numbers are equivalent
-lt	 # Returns true if a number is less than another number
-gt	 # Returns true if a number is greater than another number
==	 # Returns true if two strings are equivalent
!=	 # Returns true if two strings are not equivalent
!	 # Returns true if the expression is false
-d	 # Check the existence of a directory
-e	 # Check the existence of a file system object (directory, file, link, etc).
-f   # Check the existence of a file
-r	 # Check the existence of a file and read permission
-w	 # Check the existence of a file and write permission
-x	 # Check the existence of a file and execute permission
-n   # String is not null
-z   # String has zero length, i.e. string is null
-L   # True if param is a symbolic link. Only works when the fso does not have a trailing '/'

     # From man dash(1)
-b   # True if file exists and is a block special file.

Operator reference

Examples

# -f does not pass if param is a directory
if [ -f /etc/profile.d ]; then
    echo "This won't be printed"
fi

if [ -d /etc/profile ]; then
    echo "This won't be printed, either"
fi

TODO: tests with -x and directories

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