single quote and double quotes

Use double quotes to keep the shell from filename globbing when the string/variable contains an asterisk (*, wildcard) or a question mark (?, single character) but embedded variables ($xxx) should be expanded nevertheless.

Single quotes keep the shell from interpreting metacharacters of any kind, also including double quotes.

Double as well as single quotes around a string containing spaces keep the shell from splitting up the string into single words.

Try

echo *

echo "*"

echo "* $USER"

echo '* $USER'

A=a b c
echo $A

A="a b c"
echo $A

Quotes are basically to group words together. If you use double quotes, then any variables within the quotes will still ne interpreted.
ie echo "this machine is $hostname"
will result in displaying the following text:
this machine is servera

but with single quotes, the variable is not interpreted
ie echo 'this machine is $hostname'
will result in displaying
this machine is $hostname

You also have the backtick ` character. this will interpret a command
ie echo "current user is `whoami` "
will give :
current user is mylogin

why i am not getting the the whole word

[root@rhel5 scripts]# grep '#define BLCKSZ' /usr/local/src/postgresql-8.2.10/src/include/pg_config_manual.h
#define BLCKSZ  8192

[root@rhel5 scripts]# grep '#define BLCKSZ 8192' /usr/local/src/postgresql-8.2.10/src/include/pg_config_manual.h
[root@rhel5 scripts]#  ---->>> not getting any output. why?

How many spaces before the 8192 ?

Because there are two or more spaces between BLCKSz and 8192 and you're searching for just one space.

Try

 egrep "#define BLCKSZ {1,}8192" filename

{1,} means "match one or more occurrences of the preceeding character" (here: a space)

"egrep" (the same as "grep -E") means "interpret the pattern as an extended regular expression"

i did count the space and tried. two spaces between BLCKSZ and 8192.

not working.

[root@rhel5 include]# egrep "#define BLCKSZ {1,}8192" pg_config_manual.h
[root@rhel5 include]#

grep '#define BLCKSZ  8192' /usr/local/src/postgresql-8.2.10/src/include/pg_config_manual.h
Or, if there are other invisible characters we are not seeing
grep '#define BLCKSZ' /usr/local/src/postgresql-8.2.10/src/include/pg_config_manual.h | od -c

Could be one or more <TAB>, or one or more <TAB> + one or more <SPACE>. or one or more <SPACE> + one or more <TAB>

grep -P "#define BLCKSZ[ \t]{1,}8192" pg_config_manual.h

"grep -P"  means "interpret PATTERN as a Perl regular expression"

thanks. that what is it.

if I am using sed to change the value should use this way? it was not working. please advise.

sed 's/#define BLCKSZ[ \t]{1,}8192/#define BLCKSZ[ \t]{1,}32768/' pg_config_manual.h

sed -r 's/#define BLCKSZ[ \t]{1,}8192/#define BLCKSZ\t32768/' pg_config_manual.h

"-r" tells "sed" to accept extended regular expressions.

Use "sed -ri ..." to change the file in place or redirect the output to a new file.

sed 's/#define BLCKSZ[[:space:]]\{1,\}8192/#define BLCKSZ 32768/' pg_config_manual.h

I tried did't work.

Which one of the above suggestions do you mean?

when i run this command  why i am getting many outputs rather than the one changed. is that the behavior of the sed command?

sed -r 's/#define BLCKSZ[ \t]{1,}8192/#define BLCKSZ\t32768/' pg_config_manual.h

If "sed -r" is not supported:

sed  's/#define BLCKSZ\([ \t]\{1,\}\)8192/#define BLCKSZ \1 32768/' pg_config_manual.h

And please note that without "-i" the result is just written to stdout, the file remains unchanged.

>> when i run this command  why i am getting many outputs <<

You see the whole file with the relevant parts changed as desired.

To see just the changes use "sed -nr ..." (first suggestion) or "sed -n ..." (second suggestion).

[root@rhel5 include]# sed -ir 's/#define BLCKSZ[ \t]{1,}8192/#define BLCKSZ\t32768/' pg_config_manual.h
[root@rhel5 include]#
[root@rhel5 include]#
[root@rhel5 include]# grep "#define BLCKSZ" pg_config_manual.h
 

it didn't change

Sorry, must be "sed -ri ..."

[root@rhel5 include]# sed -i 's/#define BLCKSZ\([ \t]\{1,\}\)8192/#define BLCKSZ \1 32768/' pg_config_manual.h
[root@rhel5 include]# diff pg_config_manual.h /tmp/file
26c26
< #define BLCKSZ         32768
---
> #define BLCKSZ        8192

I notice the change now. is the new value with the tab or with spaces.

Check if there is a new file pg_config_manual.hr

I assume there is, and if so it should contain the original data - in case you want to start over ...