(Note: I have plenty of python and unix shell experience, but fairly new to ipython -- using 7.5)
I'm trying to replicate a UNIX shell function that I use all the time, so that it works in the ipython shell.
The requirement is that I want to type something like to myproj, and then have ipython process the resulting text by doing a cd to the directory that comes back from to. (This is a quick-directory-change utility I use in unix)
The way it works in unix is that a shell function invokes an external command, that command prints its result to stdout, and the shell function then invokes the internal cd to the target dir.
I've been trying to wrap my head around %magic and macros and aliases in ipython, but so far I don't see how to get this done. Any ideas?
Related
Question is in the title.
I know that % usually denotes a "magic variable" in IPython. That's not a concept I'm terribly familiar with yet, but I have read about it.
However, today I saw a tutorial where someone was using it to run a shell command. Normally I have seen and used !.
Is there a difference? Both seem to be doing the same thing when I try them.
The difference is this:
When you run a command with !, it directly executes a bash command in a subshell.
When you run a command with %, it executes one of the magic commands defined in IPython.
Some of the magic commands defined by IPython deliberately mirror bash commands, but they differ in the implementation details.
For example, running the !cd bash command does not persistently change your directory, because it runs in a temporary subshell. However, running the %cd magic command will persistently change your directory:
!pwd
# /content
!cd sample_data/
!pwd
# /content
%cd sample_data/
!pwd
# /content/sample_data
Read more in IPython: Built-in Magic Commands.
I'm relatively new to NumPy/SciPy and IPython.
To execute a python script in the python interactive mode, we may use the following commands.
>>> import os
>>> os.system('executable.py')
Then the print outputs can be seen from the python prompt.
But the same idea doesn't work with IPython notebook.
In [64]:
import os
os.system('executable.py')
Out[64]:
0
In this case, I cannot see any print outputs. The notebook only tells weather execution was successful or not. Are there any ways to see the outputs when I use IPython notebook?
Use the magic function %run:
%run executable.py
This properly redirects stdout to the browser and you will see the output from the program in the notebook.
It gives you both, the typical features of running from command line plus Python tracebacks if there is exception.
Parameters after the filename are passed as command-line arguments to
the program (put in sys.argv). Then, control returns to IPython's
prompt.
This is similar to running at a system prompt python file args,
but with the advantage of giving you IPython's tracebacks, and of
loading all variables into your interactive namespace for further use
(unless -p is used, see below).
The option -t times your script. With -d it runs in the debugger pdb. More nice options to explore.
I need to source configuration file 'eg.conf' to terminal though perl script. I am using system command but its not working.
system('. /etc/eg.conf')
Basically I am writing script in which later point it will use the environment variable (under conf file) for execute other process.
It is not clear what you are trying to achieve, but if you want to make the config available from within Perl AND your config file is valid Perl code you can use do or require (see perldoc for more information).
What you are doing in your code is to spawn a shell with system, include the config inside this shell (which must be in shell syntax) and then exit the shell again which of course throws all the config away on close. I guess this is not what you intend to do, but your real intention is not clear.
What is your goal? Do you need to source eg.conf to set up further calculations from within a perl controlled shell, or are you trying to affect the parent shell that is running the perl script?
Your example call to system('. /etc/eg.conf') creates a new shell subprocess. /etc/eg.conf is sourced into that shell at which point the shell exits. Nothing is changed within the perl script nor in the parent process that spawned the perl script.
One can not modify the environment of a parent process from a child process, without the assistance of the parent process[1]. One generally returns code for the parent shell to source or to eval.
1: ok, one could theoretically affect the parent process by directly poking into its memory space. Don't do that.
Is it possible to interact with an IPython interactive session (or with a kernel) from a Bash script? Ideally, I'd like to do something like this, within a shell script (I'm aware that the send subcommand probably doesn't exist like this):
# do stuff in Bash ...
# start a kernel and get its Id
KERNEL=`ipython init --command="print(__KERNELID__)"`
# do something inside the kernel
ipython send --kernel=KERNELID --command="mylist = [0,1,2]"
Then, ideally, the command
ipython send --kernel=KERNELID --command="print(mylist)"
would output
[0, 1, 2]
In the end, I would need to destroy the kernel somehow:
ipython --kernel=KERNELID --command="sys.exit()"
Probably, there is already a mechanism to do what I'd like,
right? Unfortunately, I wasn't able to find it ...
There are quite a number of ways around this problem. Since you are having to use python you might as well use python for the whole thing. Python programs can take command line arguments like mylist and do whatever you want with them.
Since you are sending commands to be evaluated make sure you are the one controlling the inputs. Don't let someone start typing "import os" and "os.unlink([your hard drive here])" for example.
For other options: Check out expect for your interactive needs http://expect.sourceforge.net/
or just the python version check out the pexpect module http://pexpect.sourceforge.net/pexpect.html
I have a script that opens up different files at the same time.
My problem here is that when I run system() on the alias I've defined in bash which points to /home/user1/Software/nc, perl tells me that it can't execute the alias because there is no file/directory at the current location.
I know that the alias works because when I invoke it directly in a shell, it opens fine.
Funny enough, I can do system("firefox") within my script fine, but not the alias. How do I use this alias in this script without it breaking?
Perl won't run bash, it will execute the command directly. You can call
bash -c your_command
instead of calling the command itself in Perl.
As it is, this doesn't load your aliases. You need to open an interactive shell, as in #MortezaLSC's answer. There supposedly is a way of loading aliases correctly in a non-interactive shell, but I can't figure it out.
But why don't you just use the command you have aliased to directly in perl? The only reason I could see not to do this, is if your alias is going to change in the future, but you will still want to run whatever command it points to. This seems weird and dangerous to say the least.
Aliases are designed to reduce the typing you do if you invoke commands with the same options etc all the time. They're not all-purpose macros for bash. Bash has functions for doing more complicated stuff, but why would you want to call non-trivial bash code from a perl script? It doesn't seem like you really need this here. Keep the complexity, and the potential for modification and failure, in one place (the perl script).
See a couple of answers to similar questions:
https://unix.stackexchange.com/a/1499/41977
https://superuser.com/a/183980/187150
If you're smart, you made it so your alias is only defined for interactive shells, so you'll have to launch bash and specify that you want an interactive shell using -i.
system('bash', '-i', '-c', 'shell command');
Is it working?
system 'bash -i -c "your alias parameter"';