problem with redirection in cat and sed

cat littlefile
Marry had a little lamb.little lamb.little lamb
Mary had a little lamb whose fleece was white as snow
Every where that marywent,mary went,mary went.Everywhere that mary went
[@oracleclient ~]$ cat littlefile | sed -e 's/mary/naaz/g'
Marry had a little lamb.little lamb.little lamb
Mary had a little lamb whose fleece was white as snow
Every where that naazwent,naaz went,naaz went.Everywhere that naaz went
[@oracleclient ~]$ cat littlefile | sed -e 's/mary/naaz/g' > littlefile
[@oracleclient ~]$ cat littlefile
[@oracleclient ~]$
[@oracleclient ~]$ cat littlefile | sed -e 's/mary/naaz/g' > littlej
[@oracleclient ~]$ cat littlej
[@oracleclient ~]$

in the $ cat littlefile | sed -e 's/mary/naaz/g' > littlefile command
first the littlefile is opened in write mode first ! so all the content gets cleared!
so the sed command takes an empty file as the input and so it produces a empty output!

so u get an empty file at-last!

try this command

$ ls -l >abc

i mean create a file (abc) which does not exist already!

then open the file abc and see! u will see that abc is present in there !

it means first the output file is created and then the ls command is executed!

mightyganesh wrote:in the $ cat littlefile | sed -e 's/mary/naaz/g' > littlefile command
first the littlefile is opened in write mode first ! so all the content gets cleared!
so the sed command takes an empty file as the input and so it produces a empty output!

so u get an empty file at-last!

And since the littlefile is already empty, in the second attempt using littlej, the output is empty.

[335920@oracleclient Srinath]$ cat>littlefile
Sometext
Second Line.
[335920@oracleclient Srinath]$ cat littlefile | sed -e 's/Some/some/g' > little
[335920@oracleclient Srinath]$ cat little
sometext
Second Line.

if u want to store the output of sed in same file, go for '<<'. this ll append the output of sed command to the input file.

see this,

http://www.codecoffee.com/tipsforlinux/articles2/042.html

see this,

http://www.codecoffee.com/tipsforlinux/articles2/042.html

still if u want to do it, use temp file.

Code:

$ sed 's/PINK/BLUE/g' 1.txt > 1.temp ; cat 1.temp > 1.txt ; rm 1.temp

it will store the output of sed command in the same file.