Linux Cent

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.

Best Shell IDE

There is no shell scripting ide. There are some examples of text editors that help you to write code.

1. Sublime Text

Sublime Text has many powerful features that make coding painless. It has all basic features mentioned on this page and many others like multi-select (hold CTRL and put mouse cursor in another line), creating your own snippets(lines of code that repeat), minimap(zoomed view of entire file).

2. Atom

Atom is developed by Github so it supports Github integration. It is often called the “hackable IDE of 21st century”, so you could easy customize almost everything.

Cool visual extension is “Power mode”, every time you hit a key, editor does a little move, like you hit the screen.

3. Geany

Geany is a lightweight IDE, aims to provide fast development environment. Features: filebrowser, save actions( autosave, instantsave, backupcopy), split window. In ubuntu you can install Geany using apt:

sudo apt-get install geany

4. Kate

It is a pre-installed text editor in Kubuntu. Some of the useful features: embedded terminal, SQL plugin, find and replace, syntax highlighting, bracket matching, auto backup, auto-completion with argument handling.

Bash How to Rename Directory

Use the “rename” command. Syntax of rename command:

rename [options] expression replacement file

If we want to rename directory “old-name-dir” to “new-name-dir”:

rename 'old-name-dir' new-name-dir old-name-dir

To preview change type the “ls” command.

You can rename the directory with the “mv” command:

mv old-name-dir/ new-name-dir

It will rename old-name-dir to new-name-dir. If old-name-dir contains any files, it is good advice to add option -R after mv.

Bash How to Escape Single Quote

Enclosing characters or variables in single quotes (‘) represents the literal value of characters. Example:

$foo="Hello world"
echo '$foo'

Output: $foo

If you want to write the content of variable $foo in ‘ use the character \ to escape a single quote.

echo \'$foo\'

Output: ‘Hello world’

In single quotes, escaping a single quote is not possible, thus you can’t include a single quote into single quotes

Instead of single quotes use double quotes (“):

echo "Hello'world"

Output: Hello’world

Single quote may not occur between single quotes, even when preceded by a backslash, but this works:

echo $'I\'m a linux admin.'

Output: I’m a linux admin.