unix cmd doubt

wat does this cmd do..
cat /etc/shells | sed 's/bash/BASH/g'

It means that
display the content of etc/shells in which replace all 'bash' with 'BASH'

k.. fine... but wats meanin of s and g in line ==> 's/bash/BASH/g'

i guess s stands for substitute and g stands for all instance in a line. But im not sure.

s stands for substitute.It replaces the first occurence alone.g stands for global replace.It replaces all occurences!

yes christopher is correct..
's' stands for substitute and 'g' stands for global [all matching occurrences ]

cat /etc/shells |sed 'y/ab/AB/'
output:
/Bin/sh
/Bin/BAsh
/sBin/nologin
/Bin/tcsh
/Bin/csh
/Bin/ksh
it replaces all a n b wit A n B......

but wat does "y " in command mean ?

I Think the below content with help you with sed command

UNIX --> SED
-------------+----------------------------------------------------------------
cat --> sed ':'
cat -s --> sed '1s/^$//p;/./,/^$/!d'
tac --> sed '1!G;h;$!d'
grep --> sed '/patt/!d'
grep -v --> sed '/patt/d'
head --> sed '10q'
head -1 --> sed 'q'
tail --> sed -e ':a' -e '$q;N;11,$D;ba'
tail -1 --> sed '$!d'
tail -f --> sed -u '/./!d'
cut -c 10 --> sed 's/\(.\)\{10\}.*/\1/'
cut -d: -f4 --> sed 's/\(\([^:]*\):\)\{4\}.*/\2/'
tr A-Z a-z --> sed 'y/ABC/abc'
tr a-z A-Z --> sed 'y/abc/ABC/'
tr -s ' ' --> sed 's/ \+/ /g'
tr -d '\012'--> sed 'H;$!d;g;s/\n//g'
wc -l --> sed -n '$='
uniq --> sed 'N;/^\(.*\)\n\1$/!P;D'
rev --> sed '/\n/!G;s/\(.\)\(.*\n\)/&\2\1/;//D;s/.//'
basename --> sed 's,.*/,,'
dirname --> sed 's,[^/]*$,,'
xargs --> sed -e ':a' -e '$!N;s/\n/ /;ta'
paste -sd: --> sed -e ':a' -e '$!N;s/\n/:/;ta'
cat -n --> sed '=' | sed '$!N;s/\n/ /'
grep -n --> sed -n '/patt/{=;p;}' | sed '$!N;s/\n/:/'
cp orig new--> sed 'w new' orig

's' stands for substitute and 'g' stands for global [all matching occurrences ] and replace them globally without confirmation and 'cg' replace globally with confirmation.

y stands for transform. but i cant find the diff between s and y.

in 'y each character is replaced
and in 's the entire string is replaced

yup

$ cat /etc/shells |sed 's/bin/BIN/g'.............................
/BIN/sh
/BIN/bash
/sBIN/nologin
/BIN/tcsh
/BIN/csh
/BIN/ksh
$ cat /etc/shells |sed 's/bin/BIN/' .................
/BIN/sh
/BIN/bash
/sBIN/nologin
/BIN/tcsh
/BIN/csh
/BIN/ksh

whats the diff bet these two

cat /etc/shells |sed 's/bin/BIN/cg' ===> s not workin ...

how to replace globally with confirmation

'cg' option is not working with sed command on command prompt but it is working in vi editor
syntax:
:$s/word-to-find/word-to-replace/cg

k..fine .. i tried like this ...

cat /etc/shells | sed 'c/bin/BIN/g' =====>
ouput:
/bin/BIN/g
/bin/BIN/g
/bin/BIN/g
/bin/BIN/g
/bin/BIN/g
/bin/BIN/g

how does this work.. any idea?

@ aravinth

The difference between with 'g' and without 'g' is that

without g => it will find only the first occurance of the string in each line.
with g => it will find all the occurances of the string in each line

the example you tried contains only one occurance of the string 'bin' in each line. so both the commands work in the same way.

Try both the commands to the lines having more than one occurances of the string so that you can know the difference

@anand

'sed c' is the replacement tool

syntax is sed '<<line no>>c <<new string>>'

for eg
try

cat /etc/shells | sed 'c My Name is Anand'
and
cat /etc/shells | sed '3c My Name is Anand'

then U will find the answer......