Learn to use the command-line interface (cli)
Read Introduction to the command line
You can start by reading the book Introduction to the command line. You can find a pdf copy of this book there: https://static.fsf.org/nosvn/gnu-press/source-files/books/command-line.pdf.
I keep a printed copy on my desk for everyday usage; I find that consulting this book provides me with more reliable and in-depth answers than searching the web.
The book is published under GNU Free Documentation License.
You can also use man. man is short for system’s manual pager. Most programmes available on the command-line have a man page.
You can display a man page in your terminal by running:
$ man *nameofthecommand*
$ man ls
will display the manual of the command ls.
You can navigate a man page using the arrow keys, search a man page for a keyword by typing
/keyword. Press q to close a man page.
Man pages are a bit dry to read at first, but one gets use to it. It is worth it; most of what you need will be in there. Look at the Example section of man pages. Over time, it becomes much more efficient (and reliable) to form the habit of consulting man pages (or Introduction to the command line, pdf or print) rather than searching the web.
You do not need to download man pages. Man pages are already on your system.
You can see some of the commands I use the most there: https://yctct.com/log#programmes
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command-line interface (cli) learn gnu linux personal computing