Table of Contents

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.

Syntax

[ 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:

Test

To test a conditional expression, see the test command 

test conditional_expression

Expression

Boolean Expression (And/Or)

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

Example:

  • Double Bracket And
[[ -n $var && -f $var ]] && echo "$var is a file''
  • Double Bracket Or
[[ -b $var || -c $var ]] && echo "$var is a device"
  • Single Bracket And
if [ ${PATH_FILE:0:1} != "/" -a ${PATH_FILE:0:1} != "." ] 
then
   PATH_FILE="`pwd`/${PATH_FILE}";
fi

Date Comparison

How to use the date / time variable in Bash?

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

Double Bracket example 

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

Comparison Operator

Ordinal Comparison operator. They works on string and integer

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"

File System Path

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"

Exit Status / Command

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

We can then use it in a conditional statement.

Example:

  • With the command.
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