[SOLVED] How can I edit a command?

venom4u31 · 11-13-2011, 12:30 PM

Some commands (or most), as far as I know are stored inside files within the bin folder. My question is how can I edit such commands? For example, if I want to change what cd does, how can I do that?

Obviously, I don't want to change those basic commands, but I want to change custom commands that might be installed by me.

tronayne · 11-13-2011, 12:37 PM

Get the source, edit the source, compile the source and install it (preferably with a different name -- when you get updates for your system, they'll overwrite existing files).

The source may already be on your distribution media (Slackware is) or you'll need to find it at your distribution web site.

Hope this helps some.

sycamorex · 11-13-2011, 12:41 PM

I wouldn't recommend changing the original commands. You could create your own commands. This can be done in 2 ways:

a) Create an alias in ~/.bashrc

eg.

Code:

alias elisp='cd /home/sycamorex/data/projects/programming/lisp ; emacs -nw'
alias ffp="ps aux | grep firefox"

b) create a directory ~/bin and add it to your path by adding "export PATH=$PATH:~/bin" to ~/.bashrc
After that create shell scripts in ~/bin

For example, a program that lists all my aliases:

Code:

#!/bin/bash

sed -n '/^alias/ s///gp' ~/.bashrc

exit 0

Make it executable to be able to run it:
chmod +x ~/bin/name_of_the_file

venom4u31 · 11-13-2011, 12:41 PM

Quote:

Originally Posted by tronayne

Get the source, edit the source, compile the source and install it (preferably with a different name -- when you get updates for your system, they'll overwrite existing files).

The source may already be on your distribution media (Slackware is) or you'll need to find it at your distribution web site.

Hope this helps some.

So what you're saying is that I cannot reverse engineer an installed command to its source code so I can edit it? I might not have direct access to the source.

venom4u31 · 11-13-2011, 12:44 PM

Quote:

Originally Posted by sycamorex

I wouldn't recommend changing the original commands. You could create your own commands. This can be done in 2 ways:

a) Create an alias in ~/.bashrc

eg.

Code:

alias elisp='cd /home/sycamorex/data/projects/programming/lisp ; emacs -nw'
alias ffp="ps aux | grep firefox"

b) create a directory ~/bin and add it to your path by adding "export PATH=$PATH:~/bin" to ~/.bashrc
After that create shell scripts in ~/bin

For example, a program that lists all my aliases:

Code:

#!/bin/bash

sed -n '/^alias/ s///gp' ~/.bashrc

exit 0

Make it executable to be able to run it:
chmod +x ~/bin/name_of_the_file

I understand what you're saying. I know I can combine commands in shell scripting, but I wonder how I can change complex commands (that are separately installed).

Doc CPU · 11-13-2011, 12:44 PM

Hi there,

Quote:

Originally Posted by venom4u31

Some commands (or most), as far as I know are stored inside files within the bin folder. My question is how can I edit such commands? For example, if I want to change what cd does, how can I do that?

actually, most of these "commands" like ls or cat are in fact nothing else than small programs. So how do you change them? Normally you don't.

Quote:

Originally Posted by venom4u31

Obviously, I don't want to change those basic commands, but I want to change custom commands that might be installed by me.

Then we're talking of programs you'll have to write. You've got to learn a common programming language like for instance C, C++ or Java, you'll need the programming tools like a compiler, and a lot of knowledge of how these things work.

For simple tasks, script languages like Perl or PHP may be sufficient; these programs aren't compiled (i.e. translated into machine code), but instead remain human-readable, and the so-called interpreter translates them on-the-fly each time they're executed.

What's really suitable for you depends very much on what you want to achieve, and what programming skills and experience you already have.

[X] Doc CPU

venom4u31 · 11-13-2011, 12:48 PM

Quote:

Originally Posted by Doc CPU

Hi there,

actually, most of these "commands" like ls or cat are in fact nothing else than small programs. So how do you change them? Normally you don't.

Then we're talking of programs you'll have to write. You've got to learn a common programming language like for instance C, C++ or Java, you'll need the programming tools like a compiler, and a lot of knowledge of how these things work.

For simple tasks, script languages like Perl or PHP may be sufficient; these programs aren't compiled (i.e. translated into machine code), but instead remain human-readable, and the so-called interpreter translates them on-the-fly each time they're executed.

What's really suitable for you depends very much on what you want to achieve, and what programming skills and experience you already have.

[X] Doc CPU

Programming languages aren't a problem. I want to know how can I reverse engineer the executables (or the sheer programs) to their source code and edit them from there.

sycamorex · 11-13-2011, 12:58 PM

Quote:

Originally Posted by venom4u31

Programming languages aren't a problem. I want to know how can I reverse engineer the executables (or the sheer programs) to their source code and edit them from there.

Well, if you're at the level where you are actually skilled enough to make those modifications, I don't understand your question. Grab the sources and start coding

Depending on a distribution, the sources might already be installed on the system. If not, it's easy to install them.

venom4u31 · 11-13-2011, 12:59 PM

Quote:

Originally Posted by sycamorex

Well, if you're at the level where you are actually skilled enough to make those modifications, I don't understand your question. Grab the sources and start coding

Depending on a distribution, the sources might already be installed on the system. If not, it's easy to install them.

Unfortunately I only have the programs, not the sources. I wanted to ask if I can generate the sources from the installed programs.

Doc CPU · 11-13-2011, 01:03 PM

Hi there,

Quote:

Originally Posted by venom4u31

Programming languages aren't a problem. I want to know how can I reverse engineer the executables (or the sheer programs) to their source code and edit them from there.

why? If we're talking about GNU/Linux, you do have access to the source code. So why make it harder than necessary?
Besides, yes, it is possible to reverse-engineer the machine code and reconstruct the source to a certain extent. But you have no identifiers (symbols, variable or function names) because they're left behind during compilation. You have no comments that tell you how a few lines of code are supposed to work.

So that hard work isn't worth the effort, unless you're talking about closed-source software and you have a very strong motivation to disclose how it works.

[X] Doc CPU

sycamorex · 11-13-2011, 01:04 PM

Which distro are you using and what programs are we talking about?
If these are not open-source programs then you've got a problem

tronayne · 11-13-2011, 01:53 PM

Quote:

Originally Posted by venom4u31

So what you're saying is that I cannot reverse engineer an installed command to its source code so I can edit it? I might not have direct access to the source.

Why in the world would you want to reverse-engineer instead of just getting the blasted source and edit from there? You, in fact, do have access to everything in Linux, the kernel source, all the programs and utilities source; it's free and open source.

If you're trying to reverse-engineer a proprietary package, well, that's different (and you're on your own for that one -- don't be naughty).

The key phrase is "free and open source;" that literally means you can get the source for anything that is free and open source, just go look for it (Google is your fiend here). You've obviously got internet access: Want the source code for VirtualBox -- go download it; what the source for LibreOffice -- go download it; want the source for cd -- go download it.

Hope this helps some.

venom4u31 · 11-13-2011, 01:55 PM

Quote:

Originally Posted by tronayne

Why in the world would you want to reverse-engineer instead of just getting the blasted source and edit from there? You, in fact, do have access to everything in Linux, the kernel source, all the programs and utilities source; it's free and open source.

If you're trying to reverse-engineer a proprietary package, well, that's different (and you're on your own for that one -- don't be naughty).

The key phrase is "free and open source;" that literally means you can get the source for anything that is free and open source, just go look for it (Google is your fiend here). You've obviously got internet access: Want the source code for VirtualBox -- go download it; what the source for LibreOffice -- go download it; want the source for cd -- go download it.

Hope this helps some.

It does. Thanks. I didn't fully understand how this system works.

Stephen Morgan · 11-14-2011, 05:25 AM

I believe cd is a shell built-in command, rather than a programme with its own source code.

MTK358 · 11-14-2011, 07:41 AM

Quote:

Originally Posted by Stephen Morgan

I believe cd is a shell built-in command, rather than a programme with its own source code.

I don't know why anyone marked this as unhelpful, because it's true.

In fact, it's impossible to create a separate program that changes the current directory of the shell, since each process (including the shell and command) has its own separate current directory.