Raju

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.

Bash How to Add to Array

If you want to add a new item to the end of the array without specifying an index use:

~]$ my_array=()
~]$ my_array+=("Arch")
~]$ echo ${my_array[@]}
Arch

In our previous article Bash How to Print Array, we have already seen the array creation, accessing the array elements and getting the count of array.

The array is created and can be verified as follows:

$ declare -p my_array
declare -a my_array=([0]="Arch")

Now we add another element to the my_array:

my_array+=("Debian")

declare -p my_array
declare -a my_array=([0]="Arch" [1]="Debian")

Using declare to check the BASH variables

Now in order to find out the number of elements within our array, you can use “#” to get the index count, The indexed elements count are as follows for 2 element array:

echo ${#my_array[@]}
2

To add the elements to the end of the array you can use this technique demonstrated as follows:

As your array is sequential list, To insert the element to the last index, This is done by getting the total count of elements and adding that as the index:

my_array[${#my_array[@]}]="Fedora"

Now the 3rd element “Fedora” is inserted to the end of array

echo ${my_array[@]}
Arch Debian Fedora

as you already understood that “${#my_array[@]}” gets the length of the array.

Using this technique you can append arrays and also assign them to a new array as demonstrated in the following example:

new_array=(${my_array[@]} "Ubuntu")
echo ${new_array[@]}
Arch Debian Fedora Ubuntu

This is how the elements inside the new array are stored:

declare -p new_array
declare -a new_array=([0]="Arch" [1]="Debian" [2]="Fedora" [3]="Ubuntu")

What is Fork Bomb and How to Avoid It

The fork bomb is a recursive bash function. It is a DoS attack against linux operating system. Definition of fork bomb:

:(){ :|:& };:

What do all these symbols mean?

• :() – defines function called “:”
• :|: – recursive sends output to “:”
• & – puts function to background
• ; – terminate the function definition
• : – at the end calls the function

Be careful this example may crush your computer. We can prevent against fork bomb limiting the number of processes for user (or group of users) in file /etc/security/limits.conf.

For example, we want to limit the number of process to 300:

likeIT hard nproc 30

“likeIT” is name of user. If you want to apply this limitation to the group, use “@groupName”.

There is an example of the whole configuration file:

Let’s explain some important keywords from /etc/security/limits.conf file:

[domain] [type] [item] [value]

The domain can be:

• a user name
• a group name – use @group syntax

Type can have these two values:

• soft – for enforcing the soft limits
• hard – for enforcing hard limits

Item can be:

• core – limit the core file size (KB)
• fsize – maximum filesize (KB)
• cpu – max CPU time (MIN)
• nproc – max number of processes