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Bash How to Pass Array to Function

To pass all array items as arguments to the function using the following syntax:

my_function "${array[@]}"

To get values from the array in function use:

array_in_function=("$@")

If you would use “$1” instead of (“$@”) you get the first item from the array, “$2”-second item and etc.

Here is an example of the function “array_echo” which writes all items of the array.

function array_echo() {
arr=("$@")
for i in "${arr[@]}"; do
echo -n "$i "
done
}

array=(1 2 3 4 5 )
array_echo "${array[@]}"

Note: The “n” option in the echo command doesn’t output the trailing newline.

Actually, it is not passing an array as a variable, but as a list of its elements. In our example, you can not change values of array items from function array_echo().

Bash How to get Result of Command

To get the result of the command you need to use command substitution (bash feature). Command substitution provides executing a bash command/commands and store its output to a variable.

You can use special backticks (“). Everything what you write between backticks is executed by the shell before the main command is executed. Sometimes people call them back quotes. Surely you know that they’re under [Esc] key.

In this example, we get date into variables:

DATE=`date -I`
echo $DATE

If you expect multi-line output, and you would like to display it, it is good practice to use double quotes. In this example, we get all files in /etc into variable FILES:

FILES=$(ls /etc -l)
echo "$FILES"

What if you want to use backticks into backticks? It is a little trouble. The solution is to use a dollar with brackets $(). This example does the same:

DATE=$(date -I)
echo $DATE

It is also possible to use the inner way. In the next example, the output of date will be used as the first echo argument:

echo `date -I`

Bash How to Format Date

The easiest way to get a date in YYYY-MM-DD is as follows:

date -I

Output: 2016-06-07

The parameter “I” tells the date command, the ISO date format is required.

Of course, you can also use your own definition. %Y means year, %m means month, and %d means day:

date +%Y-%m-%d

Output: 2016-06-07, the same.

If you want to display also hours:minutes:second, let’s do:

date +%Y-%m-%d:%H:%M:%S

Output: 2016-06-07:17:02:19

You fill the variable date as follows:

DATE=`date +%Y-%m-%d:%H:%M:%S`

Here are some examples of format control:

• %u – day of week(1..7), 1 is Monday
• %w – day of week(0..6), 0 is Sunday
• %U – week number of year, with Sunday as the first day of the week (00..53)
• %V – week number of year, with Monday as the first day of the week (00..53)
• %s – seconds since 1.1.1970 UTC
• %Y – year
• %y – last two digits of year(00..99)
• %r – locale’s 12-hour clock time
• %R – 24-hour hour and minute

Bash How to Add Numbers

Bash provides a lot of methods how to add numbers. First of all is use $(())

echo $((2+2+1))

Output: 5

If we have a variable which has an integer value, we can use “$var”:

var=5
echo $((6+$var))

Output: 11

We can assign to variable counted numbers:

var=$((3+6))

The value of “var” is 9 now.

Another arithmetic expansion:
var=$((num1 + num2))
var=$(($num + $num2))
var=$((num1 + 10 + 20))
var=$[num1+num2]

Bash How to get Yesterday’s Date

Date command has d parameter, that displays time described after d parameter:

date -d yesterday '+%Y-%m-%d'

%Y means year, %m means month, and %d means day. You can change formatting if you want:

date -d yesterday '+%Y:%m:%d'

What if you want get several days, for example, day before 5 days?

date +%Y-%m-%d -d "5 day ago"

Would you take yesterday’s date to the variable?

YESTERDAY=`date -d yesterday '+%Y-%m-%d'`
echo $YESTERDAY

Alternative way is using hours:

date -d "24 hours ago" '+%Y-%m-%d'

Another way:

date --date='-1 day' '+%Y-%m-%d'

Bash How to Quit a Script

The command “exit” terminate the script and return value:

Every command returns an exit status, sometimes it is called exit code or return status.

0: Script was executed with success.

1 and greater: Script was executed with error. The non-zero return value is interpreted as an “error code”.

If you want to get the exit value last executed script use $?.

./script.sh
echo $?

Sometimes you can see exit $? which is equivalent to exit.

Bash Vs KSH

Linux and Unix have various shells. Two kinds of these numerous shells are KSH and BASH.

KSH (The Korn Shell) was developed many years before the BASH. Ksh has associative arrays and handles loop syntax better than bash. Also, ksh’s command print is better than bash’s echo command. In other way, ksh does not support history completion, process substitution, and rebindable command-line editing.

Bash has more added extension than ksh. Bash has tab completion and an easier method to set a prompt in order to display the current directory.

Compared to ksh, bash is newer and more popular.

Example of difference ksh and bash in condition test. First bash:

if [ $i -eq 3 ]

and condition test in ksh:

if (($i==3))

Bash can handle exit codes from pipes in a cleaner way. Bash and KSH are both Bourne=compatible shells, they share common functions and features and can be interchangeable to use.

Bash How to Check If File Exists

What is the definition file in Linux? The file is almost everything – keyboard, and disk, and the regular file. Here is an example of a regular file: document.odt or /etc/passwd.

If you want the script to find out if there is any file (eg. tile.txt or /dev/sda), you can do the following:

if [ -e /root/file.txt ]; then
echo "File found";
fi

So, we tested if any kind of file (named /root/file.txt) exists.

If you want to take into consideration just regular files (not /dev/sda but just /root/file.txt), you can use -f parameter instead of -e parameter:

if [ -f /root/file.txt ]; then
echo "Regular file found";
fi

If you want to check, if a regular file not exists, you can use not(!) in test command []:

if [ ! -f /root/file.txt ]; then
echo "Regular file not found";
fi

We could tune the last example. You can also use short way how to write it:

[ ! -f /root/file.txt ] && echo "Regular file not found"

It is my favorite if you have just one command after the test command.

Here are some examples of file test operators:

Bash How to Trim String

You can use a bash parameter expansion, sed, cut and tr to trim a string.

Let’s use bash parameter expansion to remove all whitespace characters from the variable foo:

foo="Hello world."
echo "${foo//[["space:]]/}"

Output: Helloworld.

“${foo// /}” removes all space characters, “$foo/ /” removes the first space character.

To remove only space characters before and after string use sed:

foo=" Hello world. "
echo "${foo}" | sed -e 's/^[[:space:]]*//'

Output: Hello world.

Easy to use and remember the way how to remove whitespaces before and afterword:

echo " text. text2 " | xargs

xargs remove all whitespaces before “text.” and let one space between text. and text2.

If you want to delete only the last character from the variable:

foo="hello"
echo "${foo::-1}"

Output: hell

Bash How to Sum Variables

Let’s define two variables:

FILES_ETC=1416
FILES_VAR=7928

If you want to get total value, you can do:

TOTAL_FILES=$((FILES_ETC+FILES_VAR))
echo $TOTAL_FILES

For 100% compatibility with old shells, you can use external command expr. In modern systems (ec. Centos 7 64bit), in fact, expr is not an external command. It is an internal shell function. So there is not a significant difference in speed between the above and this command:

TOTAL_FILES=`expr $FILES_ETC + $FILES_VAR`
echo $TOTAL_FILES

Funny thing is that there must be spaces between arguments. In fact, expr is a command, $FILES_ETC is the first argument, + is the second argument and $FILE_VAR is the third argument.

If you want to add numbers with decimal points, you can use bc. bc is a very useful external command for making arithmetical operations:

TOTAL_FILES=$(echo $FILES_ETC + $FILES_VAR | bc)
echo $TOTAL_FILES

Common mistakes are spaces before or after ‘=’ sign. However bc and expr commands expect spaces between arguments and operators, so in these cases don’t forget to spaces.

Difference between a Kernel and Shell

A shell is a command interpreted, way to communicate with the operating system and kernel using the command line.

Consider what would happen if we have only kernel and no shell ?. We would have a device with OS but there is no method to use it. We need to have an interface between OS and humans. That’s shell’s purpose

A kernel is a low-level program interfacing with the hardware in privileged mode. It is an essential part of the system. Any requests from the shell is processed by the kernel.

It is impossible to have a shell without a kernel. Without a kernel, we can not execute commands.

A kernel is the lowest level program running on computers. The kernel does task scheduling, handles filesystems, I/O handling, memory management.