Bash

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

Where do I Find Bash

You can find bash as an executable program located in standard binary directories of your operating system.

If you are using an operating system which does not contain bash pre-installed (FreeBSD, Windows), you can download and build source code from gnu.org. Windows users can use Cygwin.

Bash supports two distinct operation modes: interactive and non-interactive mode. In interactive mode, the bash waits for you for entering commands. In non-interactive mode, the bash executes commands from the script file without waiting for the user’s commands.

Assuming you have bash installed, you can run bash from the terminal. Most terminals are pre-configured to start the shell program. To find out where is your bash located enter the following command:

echo $SHELL

Output: /bin/bash

Another way how to get path to bash:

echo $BASH

Output: /bin/bash

If you are not sure if you are using bash, enter:

echo $0

Output: -bash

$0 prints the program name, in our case it is actually running shell.