3 SC2244

Joachim Ansorg edited this page 2021-11-12 19:54:09 +01:00

Table of Contents

• Prefer explicit -n to check non-empty string (or use =/-ne to check boolean/integer).
• Problematic code:
• Correct code:
• Rationale:
• Exceptions:
• Related resources:

Prefer explicit -n to check non-empty string (or use =/-ne to check boolean/integer).

Problematic code:

if [ "$1" ]
then
  echo "True"
fi

Correct code:

# Check if $1 is empty or non-empty
if [ -n "$1" ]
then
  echo "True, $1 is a non-empty value"
fi

# Check instead if $1 is true or false, as in Java
[ "$1" = "true" ]

# Check instead if $1 is non-zero or zero, as in C
[ "$1" -ne 0 ]

# Check instead if $1 is defined (even if just assigned the empty string) or undefined
[ "${1+x}" = "x" ]

Rationale:

[ "$var" ] is equivalent to [ -n "$var" ] and checks that a string is non-empty.

Users more familiar with other languages are often surprised to learn that [ "$var" ] is true when:

• var=false
• var=0
• var=null
• var=" "

Adding the explicit -n helps clarify that this is a string comparison, and not related to any concept of boolean values or "truthiness" as it is in most languages.

Exceptions:

If you are familiar with the semantics of [, you can ignore this stylistic suggestion with no ill effects.

Related resources:

• Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!

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