Best way to learn UNIX and shell Programming Post: 302552669

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Top Forums Shell Programming and Scripting Best way to learn UNIX and shell Programming Post 302552669 by kermit on Monday 5th of September 2011 05:35:05 AM

09-05-2011

Registered User

Disclaimer: I am not a professional UNIX guy - I work in a completely different field.

From my perspective, the number one most helpful thing you can do is to be constantly coding. This is particularly helpful if you have a need (or at least desire) some particular behaviour in your system. For example, I had a buggy program, and sometimes would need to kill several instances of it. It was a pain to do a `ps -A` every time, and manually kill each instance of the process. So I made a one-line script that would do a ps -A, grep for the name of the particular process, parse and grab the pid from the first field, and then do a kill on those numbers. Over time, I polished and extended the script, adding functionality, and making the output a little more informative/easier to read.

Now there a few things to note here:

At the time, I did not know of pkill and friends, so I was basically reinventing the wheel. As I look back, that was OK, because I learned something while doing it. In fact, it is a decent way to learn. You basically take a known utility that you normally use, eg., pkill, or whatever, and re-implement it yourself. At first, your new program likely won't have all of the functionality of the full blown version on your system, but you can add that over time. This concept is also helpful in other areas, such as C programming. It is very instructive, for example, to make your own versions of the string library functions. Not for actual use necessarily, but for learning. (It is hard to beat the system versions, in terms of outright speed. If you already know enough to improve on the standard library, then my points here are basically moot.)
If the process of learning how to shell program seems to be coming slowly, remember to be patient with yourself, and stick with it. Everybody's situation is different. For my part, I am a slow learner (it seems), and I have been hacking away for years and am not nearly as far as others have gotten in the same time. If I was doing this for a living, I know I would be farther along, because a), I would be immersed in it, and b) I would (ostensibly) have other professionals to guide me when I was doing something that was not quite right. This is true of mostly any profession I think. There is a huge learning curve with this sort of thing - lots of nooks and crannies. Be patient!

So that is my two cents worth. Write lots of code. It would be preferable to be writing some useful little stuff which improves some particular situation of your own (as opposed to toy exercises, which initially can be somewhat helpful, but are ultimately limited in value). If you are like me, you likely want to grab an O'Reilly book on scripting, or something like that as well. Ask lots of questions as well.

kermit

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sh

sh(1)							      General Commands Manual							     sh(1)

NAME

       sh - Shell, the standard command language interpreter

DESCRIPTION

       [Tru64  UNIX]  Tru64  UNIX  provides two command interpreters with the name sh.	The XCU5.0 and POSIX.2 compliant command interpreter sh is
       available in the file /usr/bin/posix/sh and is described in the sh(1p) reference page.  The Bourne shell,  historically	known  as  sh,	is
       available in the file /usr/bin/sh and is described in the sh(1b) reference page.

       [Tru64 UNIX]  Your initial, or login, shell is determined by your entry in the file /etc/passwd. This file can be changed only by your sys-
       tem administrator. You must use whatever procedures are in place at your location to have this entry changed.

       [Tru64 UNIX]  If available on your system, you may use the passwd -s or the chsh commands to change your login shell.

									  Note

       This option is not available if your site manages passwords through the Network Information Service (NIS) facility.  Check with your system
       administrator.

       [Tru64 UNIX]  Subsequent shells spawned from the initial shell depend on the value in the environment variable BIN_SH.  If this variable is
       set to xpg4, the POSIX shell is started. If this variable is set to svr4, an SVR4 compliant version of the shell is started. If this  vari-
       able  is  unset,  the  Bourne shell is started. If this variable is set to any other value, an error is reported and the results are unpre-
       dictable. See the EXAMPLES section for information on setting this variable.

NOTES

       [Tru64 UNIX]  With Tru64 UNIX Version 4.0 the Korn shell, /usr/bin/ksh is the same as the POSIX shell /usr/bin/posix/sh.

RESTRICTIONS

       [Tru64 UNIX]  The file /etc/shells must include entries for both the POSIX shell /usr/bin/posix/sh and the Bourne  shell,  /usr/bin/sh.	If
       this file is incorrect, see your system administrator.

EXAMPLES

       Using  the  Bourne,  Korn,  or  POSIX  shell, to set the variable BIN_SH to use the POSIX/ XCU5.0compliant shell, enter: BIN_SH=xpg4 export
       BIN_SH Using the Bourne, Korn, or POSIX shell, to set the variable BIN_SH to use the SVR4 compliant shell, enter: BIN_SH=svr4 export BIN_SH
       Using the Bourne, Korn, or POSIX shell, to unset the variable BIN_SH, enter: unset BIN_SH Using the C/ shell, to set the variable BIN_SH to
       use the POSIX/XCU5.0 compliant shell, enter: setenv BIN_SH xpg4 Using the C/ shell, to set the variable BIN_SH to use  the  SVR4  compliant
       shell, enter: setenv BIN_SH svr4 Using the C/ shell, to unset the variable BIN_SH, enter: unsetenv BIN_SH

FILES

       User profile.  Contains user information, including the login shell name.  Contains the names of available and permitted shells.

SEE ALSO

       Commands:  csh(1), ksh(1), Bourne shell sh(1b), POSIX shell sh(1p), passwd(1)

       Files:  passwd(4), shells(4)

       Standards:  standards(5)

																	     sh(1)