virtual memory

http://duartes.org/gustavo/blog/post/anatomy-of-a-program-in-memory

Work harder today!

I will work harder today and take a rest tomorrow. That will be a very good
deal to me.

assign output to variable

a=$(echo $(wc -l ))

sed editor

1. sed 's/[a-zA-Z]* //3' new
Here /2 means we change the second word to empty
for the sentence: will this black car work?
echo "will this black car work?" |sed 's/[a-zA-Z0-9][a-zA-Z0-9]*\s//3' will output
> will this car work?
\s matches the space and tab

去掉所有的空白行:“:%s/\(\s*\n\)\+/\r/”。这回多了“\(”、 \)”、 \n”、 \r”和
“ “ “
“*”。“*”代表对前面的字符(此处为“ \s”)匹配零次或多次(越多越好;使用“ \*”表
示单纯的“*”字符),“\n”代表换行符,“ \r”代表回车符,“\(”和“\)”对表达式进
行分组,使其被视作一个不可分割的整体。因此,这个表达式的完整意义是,把连续的换行符
(包含换行符前面可能有的连续空白字符)替换成为一个单个的换行符。唯一很特殊的地方是,
在模式中使用的是“ \n”,而被替换的内容中却不能使用“ \n”,而只能使用“\r”。原因是
历史造成的,详情如果有兴趣的话可以查看“:help NL-used-for-Nul”。

Flag -n

The "-n" option will not print anything unless an explicit request to print is found. I mentioned the "/p" flag to the substitute command as one way to turn printing back on. Let me clarify this. The command

sed 's/PATTERN/&/p' file

acts like the cat program if PATTERN is not in the file: e.g. nothing is changed. If PATTERN is in the file, then each line that has this is printed twice. Add the "-n" option and the example acts like grep:

sed -n 's/PATTERN/&/p' file

Nothing is printed, except those lines with PATTERN included.

sed -f scriptname

If you have a large number of sed commands, you can put them into a file and use

sed -f sedscript new

where sedscript could look like this:

# sed comment - This script changes lower case vowels to upper case
s/a/A/g
s/e/E/g
s/i/I/g
s/o/O/g
s/u/U/g

When there are several commands in one file, each command must be on a separate line.

Also see here

Quoting multiple sed lines in the Bourne shell

The Bourne shell makes this easier as a quote can cover several lines:

#!/bin/sh
sed '
s/a/A/g
s/e/E/g
s/i/I/g
s/o/O/g
s/u/U/g' new

the debugging of makefile from Dr. Dobb's

This
http://www.ddj.com/development-tools/197003338?pgno=2

installation

Install a package
rpm –ivh packagename
upgrade a package
rpm –Uvh packagename

create a tar file
tar –cvf myfiles.tar mydir/
(add z if you are dealing with or creating .tgz (.tar.gz) files)

standard install from source
tar –xvzf Apackage.tar.gz
cd Apackage
./configure
make
make install

Makefile note-findstring

$(findstring find,in)

Searches in for an occurrence of find. If it occurs, the value is find; otherwise, the value is empty. You can use this function in a conditional to test for the presence of a specific substring in a given string. Thus, the two examples,

$(findstring a,a b c)
$(findstring a,b c)

produce the values `a' and `' (the empty string), respectively.

You can write a conditional that tests make command flags such as `-t' by using the variable MAKEFLAGS together with the findstring function (see section Functions for String Substitution and Analysis). This is useful when touch is not enough to make a file appear up to date.

The findstring function determines whether one string appears as a substring of another. If you want to test for the `-t' flag, use `t' as the first string and the value of MAKEFLAGS as the other.

For example, here is how to arrange to use `ranlib -t' to finish marking an archive file up to date:

archive.a: ...
ifneq (,$(findstring t,$(MAKEFLAGS)))
       +touch archive.a
       +ranlib -t archive.a
else
       ranlib archive.a
endif

The `+' prefix marks those command lines as "recursive" so that they will be executed despite use of the `-t' flag. See section Recursive Use of make.