Bash - Conditional Expression

About

A predicate or boolean expression is known in Bash as a Conditional Expression.

It is a command that returns an exit status of 0 or 1.

The conditional expression is part of the compound expression.

[ conditional_expression ]
# or without word spliting and pathname expansion
[[ conditional_expression ]]
# or

where:

[] defines a conditional expression evaluation
[[]] defines a conditional expression evaluation where Word splitting and pathname expansion are not performed
conditional_expression is a conditional expression
Return a status of 0 or 1

Note:

tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed.
Conditional operators such as -f must be unquoted to be recognized as primaries.

To test a conditional expression, see the test command

test conditional_expression

Name	Double Bracket operator	Single Bracket Operator
And	&&	-a (**)
Or	\|\|	-o (**)

Example:

[[ -n $var && -f $var ]] && echo "$var is a file''

[[ -b $var || -c $var ]] && echo "$var is a device"

if [ ${PATH_FILE:0:1} != "/" -a ${PATH_FILE:0:1} != "." ] 
then
   PATH_FILE="`pwd`/${PATH_FILE}";
fi

Name	Double Bracket operator	Single Bracket Operator
RegularExpression matching	=~	(not available)

Double Bracket example

[[ $(date) =~ ^Fri\ ...\ 13 ]] && echo "It's Friday the 13th!"

Double Bracket operator	Single Bracket Operator	Example
-lt	-lt	[[ 8 -lt 9 ]] && echo "8 is less than 9"
-ge	-ge	[[ 3 -ge 3 ]] && echo "3 is greater than or equal to 3"
-gt	-gt	[[ 5 -gt 10 ]] \|\| echo "5 is greater than than 10"
-le	-le	[[ 3 -le 8 ]] && echo "3 is less than or equal to 8"
-eq	-eq	[[ 5 -eq 05 ]] && echo "5 equals 05"
-ne	-ne	[[ 6 -ne 20 ]] && echo "6 is not equal to 20"

On the Path works on file and directory path

Name	Double Bracket operator	Single Bracket Operator	Example
entry (file or directory) exists	-e	Na	[[ -e $config ]] && echo "config file exists: $config"
file is newer/older than other file	-nt / -ot	Na	[[ $file0 -nt $file1 ]] && echo "$file0 is newer than $file1"
two files are the same	-ef	Na	[[ $input -ef $output ]] && { echo "will not overwrite input file: $input"; exit 1; }
negation	!	Na	[[ ! -u $file ]] && echo "$file is not a setuid file"

The exit status is a boolean 0 for success and 1 for error.

We can then use it in a conditional statement.

Example:

if ! (command args); then
    echo "The command has error"
fi

commando

# The output
if [ $? != 0 ]; then
	echo "Commando goes bad"
	else
	echo "Commando goes good"
fi