Bash - Tests

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Objectives: In this chapter you will learn how to:

work with the return code;
test files and compare them;
test variables, strings and integers;
perform an operation with numeric integers;

linux, script, bash, variable

Knowledge:
Complexity:

Reading time: 10 minutes

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Upon completion, all commands executed by the shell return a return code (also called status or exit code).

• If the command ran correctly, the convention is that the status code will be zero.
• If the command encountered a problem during its execution, its status code will have a non-zero value. There are many reasons for this: lack of access rights, missing file, incorrect input, etc.

You should refer to the manual of the man command to know the different values of the return code provided by the developers.

The return code is not visible directly, but is stored in a special variable: $?.

mkdir directory
echo $?
0

mkdir /directory
mkdir: unable to create directory
echo $?
1

command_that_does_not_exist
command_that_does_not_exist: command not found
echo $?
127

Note

The display of the contents of the $? variable with the echo command is done immediately after the command you want to evaluate because this variable is updated after each execution of a command, a command line or a script.

Tip

Since the value of $? changes after each command execution, it is better to put its value in a variable that will be used afterwards, for a test or to display a message.

ls no_file
ls: cannot access 'no_file': No such file or directory
result=$?
echo $?
0
echo $result
2

It is also possible to create return codes in a script. To do so, you just need to add a numeric argument to the exit command.

bash # to avoid being disconnected after the "exit 2
exit 123
echo $?
123

In addition to the correct execution of a command, the shell offers the possibility to run tests on many patterns:

• Files: existence, type, rights, comparison;
• Strings: length, comparison;
• Numeric integers: value, comparison.

The result of the test:

• $?=0 : the test was correctly executed and is true;
• $?=1 : the test was correctly executed and is false;
• $?=2 : the test was not correctly executed.

Testing the type of a file

Syntax of the test command for a file:

test [-d|-e|-f|-L] file

or:

[ -d|-e|-f|-L file ]

Note

Note that there is a space after the [ and before the ].

Options of the test command on files:

Option	Observation
-e	Tests if the file exists
-f	Tests if the file exists and is of normal type
-d	Checks if the file exists and is of type directory
-L	Checks if the file exists and is of type symbolic link
-b	Checks if the file exists and is of special type block mode
-c	Checks if the file exists and is of special type character mode
-p	Checks if the file exists and is of type named pipe (FIFO)
-S	Checks if the file exists and is of type socket
-t	Checks if the file exists and is of type terminal
-r	Checks if the file exists and is readable
-w	Checks if the file exists and is writable
-x	Checks if the file exists and is executable
-g	Checks if the file exists and has a set SGID
-u	Checks if the file exists and has a set SUID
-s	Tests if the file exists and is non-empty (size > 0 bytes)

Example:

test -e /etc/passwd
echo $?
0
[ -w /etc/passwd ]
echo $?
1

An internal command to some shells (including bash) that is more modern, and provides more features than the external command test, has been created.

[[ -s /etc/passwd ]]
echo $?
1

Note

We will therefore use the internal command for the rest of this chapter.

Compare two files

It is also possible to compare two files:

[[ file1 -nt|-ot|-ef file2 ]]

Option	Observation
-nt	Tests if the first file is newer than the second
-ot	Tests if the first file is older than the second
-ef	Tests if the first file is a physical link of the second

Testing variables

It is possible to test variables:

[[ -z|-n $variable ]]

Option	Observation
-z	Tests if the variable is empty
-n	Tests if the variable is not empty

Testing strings

It is also possible to compare two strings:

[[ string1 =|!=|<|> string2 ]]

Example:

[[ "$var" = "Rocky rocks!" ]]
echo $?
0

Option	Observation
=	Tests if the first string is equal to the second
!=	Tests if the first string is different from the second one
<	Tests if the first string is before the second in ASCII order
>	Tests if the first string is after the second in ASCII order

Comparison of integer numbers

Syntax for testing integers:

[[ "num1" -eq|-ne|-gt|-lt "num2" ]]

Example:

var=1
[[ "$var" -eq "1" ]]
echo $?
0

var=2
[[ "$var" -eq "1" ]]
echo $?
1

Option	Observation
-eq	Test if the first number is equal to the second
-ne	Test if the first number is different from the second
-gt	Test if the first number is greater than the second
-lt	Test if the first number is less than the second

Note

Since numeric values are treated by the shell as regular characters (or strings), a test on a character can return the same result whether it is treated as a numeric or not.

test "1" = "1"
echo $?
0
test "1" -eq "1"
echo $?
0

But the result of the test will not have the same meaning:

• In the first case, it will mean that the two characters have the same value in the ASCII table.
• In the second case, it will mean that the two numbers are equal.

Combined tests

The combination of tests allows you to perform several tests in one command. It is possible to test the same argument (file, string or numeric) several times or different arguments.

[ option1 argument1 [-a|-o] option2 argument 2 ]

ls -lad /etc
drwxr-xr-x 142 root root 12288 sept. 20 09:25 /etc
[ -d /etc -a -x /etc ]
echo $?
0

Option	Observation
-a	AND: The test will be true if all patterns are true.
-o	OR: The test will be true if at least one pattern is true.

With the internal command, it is better to use this syntax:

[[ -d "/etc" && -x "/etc" ]]

Tests can be grouped with parentheses ( ) to give them priority.

(TEST1 -a TEST2) -a TEST3

The ! character is used to perform the reverse test of the one requested by the option:

test -e /file # true if file exists
! test -e /file # true if file does not exist

Numerical operations

The expr command performs an operation with numeric integers.

expr num1 [+] [-] [\*] [/] [%] num2

Example:

expr 2 + 2
4

Warning

Be careful to surround the operation sign with a space. You will get an error message if you forget. In the case of a multiplication, the wildcard character * is preceded by \ to avoid a wrong interpretation.

Option	Observation
+	Addition
-	Subtraction
\*	Multiplication
/	Division quotient
%	Modulo of the division

The typeset command

The typeset -i command declares a variable as an integer.

Example:

typeset -i var1
var1=1+1
var2=1+1
echo $var1
2
echo $var2
1+1

The let command

The let command tests if a character is numeric.

Example:

var1="10"
var2="AA"
let $var1
echo $?
0
let $var2
echo $?
1

Warning

The let command does not return a consistent return code when it evaluates the numeric 0.

let 0
echo $?
1

The let command also allows you to perform mathematical operations:

let var=5+5
echo $var
10

let can be substituted by $(( )).

echo $((5+2))
7
echo $((5*2))
10
var=$((5*3))
echo $var
15

Author: Antoine Le Morvan

Contributors: Steven Spencer, Ganna Zhyrnova