Common and useful BASH comands
pwd
present working directory tells you what the directory you are working in is. Many people (like me) set up their prompt to indicate the path to the present working directory. Note the '~' symbol in my prompt. That is a symbol indicateing your home directory.
cd
lets you move to a directory_
- cd with NO argument takes you to your home directory
- cd - (minus sign) takes you to the directory where you were before
- cd takes you to where is the full or relative path to a directory (relative means everything after the directory you are already in; absolute means starting with the original directory in the file system. In this case "/home". So I went to "/home/fbianco/..." which is equivalent to "~/..."
ls
lists the content a directory
- ls NO argument lists the content of the present working directory
- ls lists the content of the directory given as argument (if it exists).
- ls lists the file given as argument (Untitled.ipynb for example). The usefulness of that resides in the ability to use it with "wildcards": caracter that can mean multiple things. For example '*' means anything. So
ls Screen*
lists all files in the pwd starting with "Screen"
- ls -l will list information about the files it lists, including permissions, ownership, size, and last modification time stamp
mkdir <dir>
creates a directory. It requires the name of the directory
as argument.
touch <file>
creates a file . The file created is empty. It also changes the timestamp of a file, without changing the file.
cp <file or dir> <file or dir>
copies a file
- the first argument is the file or directory to be copied
- if the second argument is not an existing directory it is interpreted as the destination name, and the file is copied to a file by that name, the directory is copied to a directory by that name
- if the second argument is the path to an existing directory a copy of the file/directory is deposited in that directory.
- if the first argument is a directory it requires you to add "-r" to do it recursively (even if the directory is empty!)
mv
moves a file Same behavior as cp, but removes the original file.
rm
removes a file
- the argument(s) is(are) the file(s) to be removed. THIS CANNOT BE UNDONE
- if the argument is a directory you must add "-r" to demove the content of the directory "recursively" (even if the directory is empty!)
echo <sentence>
prints to standard output (i.e. to the terminal)
- the argument is what gets printed Note: ">>" means "redirect" and if used with echo the command will then print to wherever you are redirecting the content. You need a file name after >> (exisiting or not)
less <file>
opens for vieweing in a buffer
- requires the file name as argument (you exit the less buffer by typing 'q') (less is fine if the file is small. but for large files it may need a lot of memory)
head <file>
shows the top of a file (10 lines by default)
- argument: file name
- adding "-N" will change the behavior to show N lines
tail
same as head but shows the bottom of the file
grep <something> <files or dir>
shows all files in the pwd (or a given path) containing a given string
- arguments: the string to look for
- the files where to look. e.g. "*" means everywhere in the pwd
- it prints to standard output the file name, ":", the entire line where the content is found.
**Lets move to commands that run things, instead of commands that deal with files. **
top
shows the current running processes (dynamically updated)
- no arguments: shows all processes
- arguments are allowd; for example -U fbianco shows all processes owned by user fbianco shows CPU, Memory, time running, state....
ps
shows all running processes ID and name only (static)
bg
puts a process in the background so that you can use the same shell that started the process.
- start a process
- put the procecss to sleep with Control+z. If you just type Control+Z the process becoms inactive
- type bg to make the process run in the background.
fg
puts a sleeping process or a background process back in the foreground To quit a process in the foreground you can use Control-q
Connecting to other machines and moving files.
You can work remotely on a machine connected to the internet.
ssh
secure shell, connects your terminal to another terminal
- requires the full address of the remote machine. **at CUSP to connect to a CUSP machine you have first to connect to gw.cusp.nyu.edu (gateway) then you can connect to another CUSP machine, for example compute.cusp.nyu.edu . **gw.cusp.nyu.edu is NOT A MACHINE EHRE YOU CAN WORK. It is only used as a "gateway" to the rest of the CUSP machines. (Control-d closes the connection)
To connect you need to use a few keyword when you connect. For example to enable windows to pop up ("tunneling", for example a text editor) you need the keyword -X, and depending on yur platform sometime -XY. The full command is
ssh -X -A -t gw.cusp.nyu.edu ssh -A -X compute
this version of the command assumes that the user name on your machine and on compute are the same. otherwise you need to specify it:
ssh -X -A -t fbianco@gw.cusp.nyu.edu ssh -A -X compute
An alias is a word that stands for a command in your shell. Typing that word (of your choice) will actually run the command. So an alias has to be associated to an executable, whether it is a command like "cd", or a python code that you wrote, but it has to be an executable.
An environmental variable is a variable which is known to your "environment" (i.e. basically the machine where you are working in, with the default behavors that it has when you are working in it). It is NOT an executable, but just a variable. It can be associated to an object, for example a file, or a directory. It can be for example the argument to a command: for example if your environmental variable PUI2016 is set to point to the directory /home/fbianco/UI/PUI2016_fb55 (assuming that directory exists) then you can use it to go to that directory by passing it to the command "cd" as follows:
cd $PUI2016
now notice I used the "$" sign: that tells the teminal that when I say PUI2016 I really mean the variable PUI2016, and therefore I mean what is associated to it (in this case the directory /home/fbianco/UI/PUI2016_fb55). Otherwise it will look for a directory called just PUI2016 to be in the place where I issue the command (in the present working directory) to go there, and tell me there is no such place if that directory does not exist.