Monday, January 31, 2011

Unix spoken here (4)

There are a just few more Unix commands I should mention before we exhaust everyone's patience. (Three posts here, here, and here).

I'm sure you're familiar with sudo, which stands for superuser, do once. If not, you can see wikipedia. Normally you will use this to write (install) or even delete files in /usr/bin and the like.


> mv /usr/bin/znew ~/Desktop/znew
mv: rename /usr/bin/znew to /Users/telliott_admin/Desktop/znew: Permission denied


Don't forget to read the man page on this one. There is always something interesting in the manual.

Many Unix commands appear simple but have numerous options and arguments that make them more like a Swiss Army Knife than a pocketknife. Output on the command line using man can be broken up into chunks or streamed using the down arrow, but maybe you want to study it or keep it as a reference.

For some reason you need to format the output from man in a special way before directing it to a file, for example:


man ls | col -bx > results.txt
head -n 8 results.txt
..
SYNOPSIS
ls [-ABCFGHLOPRSTUW@abcdefghiklmnopqrstuwx1] [file ...]


Nearly the whole alphabet! But doing this without the col leads to selective stuttering:


LS(1)                     BSD General Commands Manual                    LS(1)

N NA AM ME E
l ls s -- list directory contents

S SY YN NO OP PS SI IS S
l ls s [- -A AB BC CF FG GH HL LO OP PR RS ST TU UW W@ @a ab bc cd de ef fg gh hi ik kl lm mn no op pq qr rs st tu uw wx x1 1] [_ f_ i_ l_ e _ ._ ._ .]

D DE ES SC CR RI IP PT TI IO ON N
For each operand that names a _ f_ i_ l_ e of a type other than directory, l ls s


head and tail are useful for looking at the first few lines or last few lines of a file. less is like what you get when you just do man some_command ---it gives chunks or streams.

Try man cat to see what else that can do.

There is also split but I couldn't figure out how to make it take my favorite separator ('\n\n').

diff does what you might expect. The use case might be that you have some good edits but also some bad edits to a file, so you want to revert to the old one, and then try the edits one by one.


> diff old.txt new.txt > results.txt
> cat results.txt
28a29
> my edit
50,51c51
<
<
---
> deleted a line here


Or drop results.txt onto TextMate for a little color (graphic).



There's a command to find out information about different processes with finer grained control than Activity Viewer. For example:


ps -u username
ps -U userid
ps -p processid


Say:


> ps -p 48068 -O %mem
PID %MEM TT STAT TIME COMMAND
48068 1.1 ?? S 0:00.91 /Applications/R64.app/Contents/MacOS/R -psn_0_2404939


The man page on ps is extensive.

find is tricky. I need a lot more practice to understand it. Here's a simple one, and then a fun one though:


> find . -name "old.txt" -print
./old.txt

> find -f ~/* | wc
118791 235608 11536780


I "find" all the files under my home directory and feed the results to wc (word count), discovering that there are 118,791 lines in the output---that's the number of files! This link seems to have good info on find.

Sometimes I just want to use Python as a calculator or get an ascii value, but without calling up the interpreter (and having to dismiss it):


> python -c 'print 3.0/17'
0.176470588235
> python -c 'print ord("a")'
97
> python -c 'print chr(101)'
e
> python -c 'import math; print math.e'
2.71828182846



> hexdump -C post.txt
00000000 4d 61 6e 79 20 55 6e 69 78 20 63 6f 6d 6d 61 6e |Many Unix comman|
00000010 64 73 20 61 70 70 65 61 72 20 74 6f 20 62 65 20 |ds appear to be |
00000020 73 69 6d 70 6c 65 20 62 75 74 20 68 61 76 65 20 |simple but have |



Finally, here are a few more that either I've found useful or I'm sure they will be, but I don't feel expert enough to try to explain them. Have fun exploring:


alias
awk
curl
hash
md5
openssl sha1
set
tar
top
touch
unset


If you get bored here; or try to check out how Greg's email address gets generated!

And there's always ssh. But that really does deserve a post of its own.

1 comment:

keith-hughitt said...

For people on Linux, there is also a pretty good graphical diff viewer called meld that is worth checking out.