Data Streams and the UNIX Shell - Saving Streams into Variables
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Now you can play around with the input and the output as long as they are files. The question is, how can we can save the data stream of a command inside a variable? A variable is a data stream store that lives as long as your program/script. You can save the output of a command to a variable in two ways.
There is an older, more traditional syntax, and a new one which is more modern. You can use either of them as you wish. The traditional syntax is as follows:
variable=`command line`
Here we use the back tick to surround our command line. The modern syntax is just a little different:
variable=$(command line)
The result is the same whatever you use. However, the second one is easier to read, so I recommend that you that. For example, we can save the first two lines of a file with the following command:
#traditional
firstLines='head -2 alfa.txt'
#modern
firstLines=$(head -2 alfa.txt)
The shell will rewrite every new line character to a space in order to fit everything inside the variable and into a single line. The shell will also rewrite the command line before it executes it, if there are any meta-characters. Besides the one presented in my previous article, there are also the following related to the directories:
*
It will replace it with the names of the directories inside the working directory
~
The path to the users home directory ( for example /home/gaborj)
If we want to use them in their literal meaning, you will need to surround it with the forward tick or add a backslash before them. This is all you need to learn for now about streams inside the shell. Rate my articles if you consider that they were worth your attention. Do not hesitate to ask your questions in the blog following the article.
In the future, we will write more and more scripts inside the shell. Therefore, I need to ask you to make yourself familiar with one of the text editors inside the terminal. If you want something easy to use and learn, I recommend Joe. Nevertheless, if you want to learn to use something more advanced, a tool from which you can benefit in the future, VI Improved is the way to go. You can read a nice article about it here. We will continue next week with some control mechanisms of the shell. Until that arrives, remember to Live With Passion!
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