6. Condition-testing

Variables can be used to test if a certain condition is true or false, and therefore be able to take a different course of action depending on the result of the test. For example, you might want to evaluate if a file exists to decide if you can copy it into a different folder or not. For condition-testing you will use the if command. This command has the following syntax:

if [ CONDITION TO EVALUATE ]
then
    INSTRUCTIONS THAT WILL RUN IF THE CONDITION IS TRUE
elif [ OTHER CONDITION TO EVALUATE IF 1ST CONDITION IS FALSE ]
then
    INSTRUCTIONS THAT WILL RUN IF FIRST CONDITION IS FALSE BUT SECOND IS TRUE
else
    INSTRUCTIONS THAT WILL RUN IF ALL THE PREVIOUS CONDITIONS ARE FALSE
fi

The CONDITION TO EVALUATE is an expression that follows a specific syntax depending on what you want to test (checking files, string comparison, comparing numbers, or combining different expressions). Lets look at the different syntaxes used in each of these situations and at some examples that will help you understand this seemingly confusing subject.

6.1. Condition-testing to check files

In the following table, FILE refers to the path of the file or to the variable that contains the path of the file. The spaces after [ and before ] are very important. If those spaces are missing, bash will give an error.

Condition Meaning
[ -a FILE ] Tests if FILE exists.
[ -d FILE ] Tests if FILE exists and is a directory.
[ -e FILE ] Tests if FILE exists.
[ -f FILE ] Tests if FILE exists and is a regular file.
[ -g FILE ] Tests if FILE exists and its SGID bit is set.
[ -h FILE ] Tests if FILE exists and is a symbolic link.
[ -k FILE ] Tests if FILE exists and its sticky bit is set.
[ -p FILE ] Tests if FILE exists and is a named pipe (FIFO).
[ -r FILE ] Tests if FILE exists and is readable.
[ -s FILE ] Tests if FILE exists and has a size greater than zero.
[ -t FD ] Tests if file descriptor FD is open and refers to a terminal.
[ -u FILE ] Tests if FILE exists and its SUID (set user ID) bit is set.
[ -w FILE ] Tests if FILE exists and is writable.
[ -x FILE ] Tests if FILE exists and is executable.
[ -O FILE ] Tests if FILE exists and is owned by the effective user ID.
[ -G FILE ] Tests if FILE exists and is owned by the effective group ID.
[ -L FILE ] Tests if FILE exists and is a symbolic link.
[ -N FILE ] Tests if FILE exists and has been modified since it was last read.
[ -S FILE ] Tests if FILE exists and is a socket.
[ FILE1 -nt FILE2 ] Tests if FILE1 has been changed more recently than FILE2, or if FILE1 exists and FILE2 does not.
[ FILE1 -ot FILE2 ] Tests if FILE1 is older than FILE2, or is FILE2 exists and FILE1 does not.
[ FILE1 -ef FILE2 ] Tests if FILE1 and FILE2 refer to the same device and inode numbers.

Usage of [ -a FILE ]

$ FILE=/Users/MyUser/Desktop/someFile.txt
$ if [ -a "${FILE}" ]
> then
> echo "${FILE} exists"
> else
> echo "${FILE} doesn't exist"
> fi

The following example shows a common mistake. Here, the quotation marks surrounding the file path are missing. Since the path has spaces, bash gives an error:

$ if [ -a /Users/MyUser/Desktop/some file name with spaces.txt ]
> then
> echo "The file exists"
> else
> echo "The file doesn't exist"
> fi
-bash: [: too many argument

Usage of [-d FILE]

$ FILE=/Users/MyUser/Desktop/someFile.txt
$ if [ -a "${FILE}" ]
> then
> echo "The file exists. Now I will find out if it's a directory or a regular file."
> if [ -d "${FILE}" ]
> then
> echo "It is a directory."
> else
> echo "It is a regular file."
> fi
> else
> echo "The file doesn't exist."
> fi

The following example shows a common mistake. Here, the spaces before and after the expression -a "${FILE}" are missing. So, bash will give an error:

$ FILE=/Users/MyUser/Desktop/someFile.txt
$ if [-a "${FILE}"]
> then
> echo "The file exists. Now I will find out if it's a directory or a regular file."
> if [ -d "${FILE}" ]
> then
> echo "It is a directory."
> else
> echo "It is a regular file."
> fi
> else
> echo "The file doesn't exist."
> fi
-bash: [-a: command not found

Usage of [ -f FILE ]

$ FILE=/Users/MyUser/Desktop/someFile.txt
$ if [ -a "${FILE}" ]
> then
> echo "The file exists. Now I will find out if it's a directory or a regular file."
> if [ -f "${FILE}" ]
> then
> echo "It is a regular file."
> else
> echo "It is a directory."
> fi
> else
> echo "The file doesn't exist."
> fi

Usage of [ -N FILE ]

$ if [ -N "/Users/MyUser/Desktop/someFile.txt" ]
> then
> echo "The file exists and has been modified since the last time it was opened."
> else
> echo "Either the file doesn't exist, or it hasn't been modified."
> fi

6.2. Condition-testing to compare/evaluate strings

When comparing strings, it is mandatory to use the quotation marks. The spaces after [ and before ] are very important. If those spaces are missing, bash will give an error.

Condition Meaning
[ -z "STRING" ] Tests if the length of STRING is zero
[ -n "STRING" ] Tests if the length of STRING is non-zero
[ "STRING1" == "STRING2" ] Tests if the strings are equal
[ "STRING1" != "STRING2" ] Tests if the strings are not equal
[ "STRING1" \< "STRING2" ] Tests if STRING1 sorts before STRING2
[ "STRING1" \> "STRING2" ] Tests if STRING1 sorts after STRING2

Usage of [ -z "STRING" ]

$ if [ -z "" ]
> then
> echo "Empty string"
> fi
Empty string

Usage of [ -n "STRING" ]

$ VAR="Some text"
$ if [ -n "${VAR}" ]
> then
> echo "The string is not empty"
> fi
The string is not empty

In this example, bash will show an error because I forgot to put the quotation marks around ${VAR}:

$ VAR="Some text"
$ if [ -n ${VAR} ]
> then
> echo "The string is not empty"
> fi
-bash: [: Some: binary operator expected

Usage of [ "STRING1" == "STRING2" ]

$ QC="Good"
$ if [ "${QC}" == "Good" ]
> then
> echo "Quality control good"
> else
> echo "Image has bad quality"
> fi
Quality control good

Usage of [ "STRING1" != "STRING2" ]

$ QC="Good"
$ if [ "${QC}" != "Good" ]
> then
> echo "Quality control is not good"
> else
> echo "Image has good quality"
> fi
Image has good quality

Usage of [ "STRING1" \< "STRING2" ]

In the following example, I am comparing the strings "Canada" with "Colombia" and printing the one that sorts first. "Canada" will be printed because the first letter of both words is the same but the second letter in "Canada" (letter a) comes before the second letter in "Colombia" (letter o).

$ VAR1="Canada"
$ VAR2="Colombia"
$ if [ "${VAR1}" \< "${VAR2}" ]
> then
> echo ${VAR1}
> else
> echo ${VAR2}
> fi
Canada

The previous expression can be written using a shorter syntax: [ expression ] >> whattodoifexpressionistrue || whattodoifexpressionisfalse:

$ [ "${VAR1}" \< "${VAR2}" ] >> echo ${VAR1} || echo ${VAR2}
Canada

Usage of [ "STRING1" \> "STRING2" ]

$ VAR1="Canada"
$ VAR2="Colombia"
$ if [ "${VAR1}" \> "${VAR2}" ]
> then
> echo ${VAR2}
> else
> echo ${VAR1}
> fi
Canada

The previous expression can also be written using the shorter syntax:

$ [ "${VAR1}" \> "${VAR2}" ] >> echo ${VAR2} || echo ${VAR1}
Canada

6.3. Condition-testing to compare numbers

The spaces after [ and before ] are very important. If those spaces are missing, bash will give an error.

Condition Meaning
[ NUM1 -eq NUM2 ] Tests if NUM1 is equal to NUM2
[ NUM1 -ne NUM2 ] Tests if NUM1 is not equal to NUM2
[ NUM1 -lt NUM2 ] Tests if NUM1 is less than NUM2
[ NUM1 -le NUM2 ] Tests if NUM1 is less than or equal to NUM2
[ NUM1 -gt NUM2 ] Tests if NUM1 is greater than NUM2
[ NUM1 -ge NUM2 ] Tests if NUM1 is greater than or equal to NUM2

Usage of [ NUM1 -eq NUM2 ]

$ if [ 3 -eq 3 ]
> then
> echo "This makes sense, 3 equals 3."
> fi
This makes sense, 3 equals 3.

The previous expression can also be written using the shorter syntax:

$ [ 3 -eq 3 ] >> echo "This makes sense, 3 equals 3."
This makes sense, 3 equals 3.
$ A=3
$ if [ "${A}" -eq "3" ]
> then
> echo "This makes sense, ${A} equals 3."
> fi
This makes sense, 3 equals 3.

And using the shorter syntax it would be:

$ [ "${A}" -eq "3" ] >> echo "This makes sense, ${A} equals 3."
This makes sense, 3 equals 3.

Usage of [ NUM1 -ne NUM2 ]

$ A=3
$ if [ "${A}" -ne "3" ]
> then
> echo "Variable A is not equal to 3"
> else
> echo "Variable A equals 3."
> fi
Variable A equals 3.

Using the shorter syntax:

$ [ "${A}" -ne "3" ] >> echo "Variable A is not equal to 3" || echo "Variable A equals 3."
Variable A equals 3.

Usage of [ NUM1 -lt NUM2 ]

$ A=3
$ if [ "${A}" -lt "3" ]
> then
> echo "Variable A is less than 3"
> else
> echo "Variable A not less than 3."
> fi
Variable A is not less than 3.

Using the shorter syntax:

$ [ "${A}" -lt "3" ] >> echo "Variable A is less than 3" || echo "Variable A not less than 3."
Variable A is not less than 3.

Usage of [ NUM1 -le NUM2 ]

$ A=3
$ if [ "${A}" -le "3" ]
> then
> echo "Variable A is less or equal to 3"
> else
> echo "Variable A greater than 3."
> fi
Variable A is less or equal to 3.

Using the shorter syntax:

$ [ "${A}" -le "3" ] >> echo "Variable A is less or equal to 3" || echo "Variable A greater than 3."
Variable A is less or equal to 3.

Usage of [ NUM1 -gt NUM2 ]

$ A=3
$ if [ "${A}" -gt "3" ]
> then
> echo "Variable A is greater than 3."
> else
> echo "Variable A not greater than 3."
> fi
Variable A is not greater than 3.

Using the shorter syntax:

$ [ "${A}" -gt "3" ] >> echo "Variable A is greater than 3." || echo "Variable A not greater than 3."
Variable A is not greater than 3.

Usage of [ NUM1 -ge NUM2 ]

$ A=3
$ if [ "${A}" -ge "3" ]
> then
> echo "Variable A is greater or equal to 3"
> else
> echo "Variable A less than 3."
> fi
Variable A is greater or equal to 3.

Using the shorter syntax:

$ [ "${A}" -ge "3" ] >> echo "Variable A is greater or equal to 3" || echo "Variable A less than 3."
Variable A is greater or equal to 3.

6.4. Condition-testing to compare arrays

Condition Meaning
[ "${array1[*]}" == "${array2[*]}" ] Tests if array1 equals to array2
[ "${array1[*]}" != "${array2[*]}" ] Tests if array1 is different to array2

Usage of [ "${array1[*]}" == "${array2[*]}" ]

$ arr1=(a b c)
$ arr2=(a b c d)
$ arr3=(a b c)
$ if [ "${arr1[*]}" == "${arr3[*]}" ]
> then
> echo "equal"
> else
> echo "different"
> fi
equal

Using the shorter syntax:

$ [ "${arr1[*]}" == "${arr3[*]}" ] >> echo "equal" || echo "different"
equal
$ if [ "${arr1[*]}" == "${arr2[*]}" ]
> then
> echo "equal"
> else
> echo "different"
> fi
different

Using the shorter syntax:

$ [ "${arr1[*]}" == "${arr2[*]}" ] >> echo "equal" || echo "different"
different
[ "${arr1[*]}" != "${arr2[*]}" ]: $ if [ "${arr1[*]}" != "${arr2[*]}" ]
> then
> echo "different"
> else
> echo "equal"
> fi
different

Using a shorter syntax:

$ [ "${arr1[*]}" != "${arr2[*]}" ] >> echo "different" || echo "equal"
different

6.5. Combining different expressions for condition-testing

Condition Meaning
[ EXPR ] Tests if the expression EXPR is true
[ ! EXPR ] Tests if the expression EXPR is false
[ EXPR1 ] || [ EXPR2 ] Tests if EXPR1 or EXPR2 are true. You can add as many expressions as desired
[ EXPR1 ] && [ EXPR2 ] Tests if EXPR1 and EXPR2 are true. You can add as many expressions as desired

Usage of [ EXPR ] vs [ ! EXPR ]

$ if [ 3 -eq 3 ]
> then
> echo "This will be printed if the expression 3 equals 3 is true."
> else
> echo "This will be printed if the expression is false (3 is not equal to 3)."
> fi
This will be printed if the expression 3 equals 3 is true.

Using a shorter syntax:

$ [ 3 -eq 3 ] >> echo "This will be printed if the expression 3 equals 3 is true." || echo "This will be printed if the expression is false (3 is not equal to 3)."
This will be printed if the expression 3 equals 3 is true.
$ if [ ! 3 -eq 3 ]
> then
> echo "This will be printed if it is false that 3 equals 3 (so, if 3 is different than 3)."
> else
> echo "This will be printed if it is not false (it's true) that 3 equals 3."
> fi
This will be printed if it is not false (it's true) that 3 equals 3.

Using a shorter syntax:

$ [ ! 3 -eq 3 ] >> echo "This will be printed if it is false that 3 equals 3 (so, if 3 is different than 3)." || echo "This will be printed if it is not false (it's true) that 3 equals 3."
This will be printed if it is not false (it's true) that 3 equals 3.

We learned that the expression -f FILE tests if a file exists. If we want to test if a file doesn't exist, then we just need to test if -f FILE is false.

$ FILE="SomeFileThatExists.txt"
$ if [ -f ${FILE} ]
> then
> echo "The file exist"
> fi
The file exist

Using a shorter syntax:

$ [ -f ${FILE} ] >> echo "The file exist"
The file exist
$ FILE="SomeFileThatDoesntExist.txt"
$ if [ ! -f ${FILE} ]
> then
> echo "The file doesn't exist"
> fi
The file doesn't exist

Using a shorter syntax:

$ [ ! -f ${FILE} ] >> echo "The file doesn't exist"
The file doesn't exist

Usage of [ EXPR1 ] || [ EXPR2 ]

$ if [ 2 -lt 3 ] || [ 4 -lt 3 ]
> then
> echo "This will be echoed if any of the two expressions are true: 2<3 OR 4<3."
> else
> echo "This will be echoed if none of the two expressions are true"
> fi
This will be echoed if any of the two expressions are true: 2<3 OR 4<3.

Using a shorter syntax:

$ ( [ 2 -lt 3 ] || [ 4 -lt 3 ] ) && echo "This will be echoed if any of the two expressions are true: 2<3 OR 4<3." || echo "This will be echoed if none of the two expressions are true":
This will be echoed if any of the two expressions are true: 2<3 OR 4<3.

Usage of [ EXPR1 ] && [ EXPR2 ]

The following example tests if the two expressions are true. The second expression, opposite to the previous example, has the negation (!). So, the second expression is not testing if 4 is less than 3, it is testing if 4 is NOT less than 3 (if it's equal or greater than 3). So, both of the expressions are true, because 4 is not less than 3, and 2 is less than 3.

$ if [ 2 -lt 3 ] && [ ! 4 -lt 3 ]
> then
> echo "This will be echoed if the two expressions are true."
> else
> echo "This will be echoed if one of the two expressions are false, or if both are false."
> fi
This will be echoed if the two expressions are true.

Using a shorter syntax:

$ ( [ 2 -lt 3 ] && [ ! 4 -lt 3 ] ) && echo "This will be echoed if the two expressions are true." || echo "This will be echoed if one of the two expressions are false, or if both are false."
This will be echoed if the two expressions are true.

Using the and (&&) and or (||) operands to test more than two conditions

You can combine more than two expressions. In the following example, I am combining three. Because I am using the AND operator, the whole condition will test true if and only if all the three expressions are true. If one is false, then the whole expression will be false.

$ if [ 2 -lt 3 ] && [ ! 4 -lt 3 ] && [ 4 -lt 3 ]
> then
> echo "This will be echoed if all the three expressions are true."
> else
> echo "This will be echoed if any of the three expressions is false."
> fi
This will be echoed if any of the three expressions is false.

Let's take a look at why the whole expression evaluates false: [ 2 -lt 3 ]: This is true, 2 < 3. [ ! 4 -lt 3 ]: This is true, 4 is not less than 3. [ 4 -lt 3 ]: This is false, It is false that 4 be less than 3. [ 2 -lt 3 ] && [ ! 4 -lt 3 ] && [ 4 -lt 3 ]: This is false because one of the three expressions is false.

Using a shorter syntax:

$ ( [ 2 -lt 3 ] && [ ! 4 -lt 3 ] && [ 4 -lt 3 ] ) && echo "This will be echoed if all the three expressions are true." || echo "This will be echoed if any of the three expressions is false."

The following example is very similar than the previous one but instead of using the operator AND (>>), we are using the operator OR (||). So, the whole expression will be true if ANY of the three expressions is true. Since the first two expressions are true, then the result is true.

$ if [ 2 -lt 3 ] || [ ! 4 -lt 3 ] || [ 4 -lt 3 ]
> then
> echo "This will be echoed if ANY of the three expressions is true."
> else
> echo "This will be echoed if all of the three expressions are false."
> fi
This will be echoed if ANY of the three expressions is true.

Using a shorter syntax:

$ ( [ 2 -lt 3 ] || [ ! 4 -lt 3 ] || [ 4 -lt 3 ] ) && echo "This will be echoed if ANY of the three expressions is true." || echo "This will be echoed if all of the three expressions are false."
This will be echoed if ANY of the three expressions is true.

Combining && and || into one expression

When combining both operands (&&, ||), it is better to always use parenthesis to indicate the order in which you want the operations to be evaluated. In the following example we have three files. The path of the three files are saved in the variables ${FILE1} ${FILE2} and ${FILE3}. Suppose files ${FILE1} and ${FILE3} exist, but ${FILE2} doesn't exist. We want to evaluate the following condition: Does ${FILE3} and at least one of the other two files exists?

[ -f ${FILE3} ]: This condition is true because ${FILE3} exists. [ -f ${FILE2} ] || [ -f ${FILE1} ]: This condition is true because even though ${FILE2} doesn't exist,${FILE1} does exist. And with an OR (||) we only need one of the expressions to be true. ( [ -f ${FILE1} ] || [ -f ${FILE2} ] ) && [ -f ${FILE3} ]: This is true because both of expressions are true.

$ if ( [ -f ${FILE1} ] || [ -f ${FILE2} ] ) && [ -f ${FILE3} ]
> then
> echo "The condition is true"
> else
> echo "The condition is false"
> fi
The condition is true

Using a shorter syntax:

( [ -f ${FILE1} ] || [ -f ${FILE2} ] ) && [ -f ${FILE3} ] && echo "The condition is true" || echo "The condition is false"

In the following example, we want to include a subject if it is female and age less than six or male and age greater than ten:

$ GENDER="MALE"
$ AGE=23
$ if ( [ "${GENDER}" == "FEMALE" ] && [ "${AGE}" -lt "6" ] ) || ( [ "${GENDER}" == "MALE" ] && [ "${AGE}" -gt "10" ] )
> then
> echo "Include subject"
> else
> echo "Exclude subject"
> fi
Include subject