Whenever we write any command on command line. There may be 3 streams:
Piping and redirection is the means by which we may connect these streams between programs and files to direct data in interesting and useful ways.
Wildcard in Linux
Wildcards are a set of building blocks that allow you to create a pattern defining a set of files or directories.
Here is the basic set of wildcards:
- * – represents zero or more characters
- ? – represents a single character
-  – represents a range of characters
Wildcards may be used with any command.
- * –
It represents zero or more characters. If you want to list all the files and directories then this wildcard is very beneficial.In simple words, whenever you use * means it will include all the character.
E.g. if you want to list all the directory which starts with D. Then you just have to write.
suppose you want to search all files with extension png.
- ? –
it means list all the file whose first letter is unknown but second character is o and I do not care about remaining characters in the file name.
it means I do not care about the file name. I care about extension only whose length consists of 3 characters.
-  –
It represents the range of characters. so, it is also called range operator.
I guess you got the basic understanding about wildcard.
The general syntax for pipes is:command_1 | command_2 [| command_3 . . . ]
What this operator does is feed the output from the program on the left as input to the program on the right.
First, we have listed all the directory using ls command. Then we have used pipe. So, all the output of ls is now as input for head -3 command.
This chain can continue for any number of commands or programs with any commands.
linux output to file
whenever we execute any command, it shows its output on the terminal window only. But, if you want this output to be saved in a file then the concept of redirection comes in picture.
It is mainly of 3 types:
> (Greater than) is used for redirecting output to a file
TO verify this you can see below screenshot.
Now appended content screenshot
< (less than) is used for redirecting input to file.
if you want to do some operation on the file. like counting number of words, lines.. etc in a file.Note the difference, when you are using redirection, your file name is not shown because during redirection file is sent anonymously.
you can also easily combine STDIN and STDOUT. e.g. you want to process a file and want to save the output into a new file then this combination is useful.
Note: you can also use 0> for STDIN and >1 for STDOUT.
If video.mp4 is not present in your file directory then you will get an error-
and this error will be saved to a new file error.txt.
ls -l video.mp4 file.txt > myoutput 2>&1