Bash Scripting Advanced

Filters :

These are shell commands, which :

• Take input from standard input
• Send output to standard output
• Transform input data to output data
• Examples are wc, cat, more, head, sort
• Filters can be chained together

Pipes :

Pipe command - |

• For chaining filter commands   :  commands1 | commands2
• Here the Output of commands1 is the input of commands2

Summary :

• Shell variables are variables that are limited in scope to the shell in which they were created. Accordingly, shells cannot “see” each other’s shell variables.
• Filters are shell commands or programs such as wc, cat, sort , etc. that transform the input provided to them.
• The pipe operator can be used to create a pipeline of chained filter commands, whereby output of one command becomes the input of another command.
• Shell variables can be assigned values with a simple equal sign, and listed using the ‘set’ command.
• Environment variables are shell variables with scope extended to all child processes of the shell.  They are created using the ‘export’ command and listed using the ‘env’ command.

Metacharacters :

Metacharacters are special characters that have meaning to the shell.

Quoting :

Quoting is used to specify whether the shell should interpret special characters as metacharacters,
or ‘escape’ them

Input/Output, or ‘I’ ‘O’ redirection :

Refers to a set of features used for redirecting either the standard input, which is the keyboard, or the standard output, which is the terminal.

Command line arguments 

These are arguments used by a program that are specified on the command
line. In particular, they provide a way to pass arguments to a shell script, which is itself
a program

Command substitution is used to replace a command with its output

Command substitution is a feature of the shell, which helps save the output generated by a command in a variable.

It can also be used to nest multiple commands , so that the innermost command's output can be used by outer commands.
The inner command is enclosed in $() and will execute first.

Store the output of the command hostname -i in a variable named $myip

myip=$(hostname -i)
echo $myip

Print the following message on screen:

"Running on host : host_name" ,

Where 'host_name' should be your current hostname.

echo "Running on host: $(hostname)"

Command substitution can be done using the backquote syntax also.

ls -l `which cat`

The output of command which cat is the path to the command cat. This path is sent to ls -l as an input.

Batch vs Concurrent mode: