The Command Line and Git

Several lessons here open a terminal and start typing. If that is unfamiliar territory, this is the short version — enough to follow the rest without being a Linux course.

Nothing here is specific to radio astronomy. Skip it if you already live in a terminal.

The ones you actually need

Command	What it does
`man [command]`	Show the manual for a command. The one to remember — every command below has a manual page.
`pwd`	Where am I?
`ls [options]`	What is in here?
`cd /directorypath`	Go somewhere
`mkdir directory`	Make a directory
`cp source destination`	Copy
`mv source destination`	Move, or rename
`rm [options] file`	Delete a file. There is no undo and no recycle bin.
`rmdir directory`	Delete an empty directory
`locate filename`	Find a file by name

The next ones you meet

Command	What it does
`cat [filename]`	Print a file to the screen
`less [filename]`	Read a file one page at a time. `q` to quit.
`tail [filename]`	The last 10 lines — useful for logs
`head [filename]`	The first 10 lines
`grep pattern [file]`	Find lines matching a pattern
`find [path] [expression]`	Find files matching a pattern
`file filename`	What kind of file is this?
`chmod mode filename`	Change a file’s permissions
`chown owner filename`	Change who owns a file
`df [options]`	How much disk is left
`du [options]`	How much space is this using
`free -m`	How much memory is in use
`top`	What is running. `q` to quit.
`ps [options]`	A snapshot of running processes
`kill pid`	Stop a process. `kill -9 pid` if it will not go.
`date`	The system date and time
`touch filename`	Create an empty file
`clear`	Clear the screen
`ssh user@machine`	Log in to another machine. `exit` to leave.
`tar [options] filename`	Pack and unpack `.tar`, `.tar.gz`, `.tgz`
`passwd`	Change a password
`su [user]`	Switch user
`who`	Who is logged in

There are printable one-page versions of this all over the internet. Search for “Linux command cheat sheet”, print one, and put it next to the telescope.

Git, enough to keep the software updated

Most of what these lessons ask of git is “get a copy” and “get the updates”.

Get a copy of a repository

Open a terminal, move to wherever you want the copy to live, and:

git clone https://github.com/WVURAIL/gr-radio_astro.git

That makes a directory named after the repository, containing everything.

Get the updates

Move into that directory and:

git pull

One rule worth following

If you change a file in a cloned repository, copy your version somewhere else before you pull. Otherwise git has to reconcile your edits with the incoming ones, and that is a conversation you did not want to have today.

When `git pull` refuses

You will eventually see this:

Updating d0ebfc5..0b10e81
error: Your local changes to the following files would be overwritten by merge:
	examples/spectrometer_w_cal.grc
Please commit your changes or stash them before you merge.
Aborting

git has noticed you changed a file and will not overwrite your work without being told to. If you want to keep your changes, copy the file elsewhere first. Then:

git stash

which puts your changes aside so the pull can proceed.

The first time you run that, git may stop and ask who you are:

*** Please tell me who you are.

Run

  git config --global user.email "you@example.com"
  git config --global user.name "Your Name"

Do exactly that, with your own name and email, then run git stash and git pull again.

Updating the GNU Radio blocks

The full sequence for picking up new versions of gr-radio_astro:

cd gr-radio_astro
git pull
cd build
cmake ..
make
sudo make install

The installation lesson covers the first install, including what to do when cmake cannot find something.

If you want to go further

Pro Git — the standard book, free online
GitHub’s Get Started guides