Shell Programming and Scripting

Hi,

Could you please explain me the below statement -- phrase wise.

sed -e :a -e '$q;N;'$cnt',$D;ba' abc.txt > xyz.txt


if suppose $cnt contains value: 10
it copies last 9 lines of abc.txt to xyz.txt

why it is copying last 9 rather than 10.
and also what is ba and $D over there in command.

reply me as early as possible.

Thanks,
Subbu

A more lisible format of the command (with variable substitition made)

Code:

sed -e '
:a
$q
N
10,$D
ba
' abc.txt > xyz.txt

:a

(0):label Marks a branch point to be referenced by the b and t subcommands.
This label can be any sequence of eight or fewer bytes.
Defines label a

$q

(1)q Branches to the end of the script. It does not start a new cycle.

$ means last line of input.
If last line of input, print the line and stop processing.

N

(2)N Appends the next line of input to the pattern space with an embedded
new-line character (the current line number changes). You can use this to search for patterns that are split onto two lines.

10,$D

(2)D Deletes the initial segment of the pattern space through the first new-line character and then starts the next cycle.

From line 10 to end, removes the oldest line (added by N command) at the begining of the buffer.
So the buffer contains all time at most 9 lines.

ba

(2)b[label] Branches to the : command bearing the label variable. If the label variable is empty, it branches to the end of the script.

Branch to label a


The command is equivalant to :

Code:

tail -f 9 abc.txt > xyz.txt

Jean-Pierre.

Hi Jean-Pierre,

Thank you very much for your quick response.

Is this correct way to copy all the lines of abc.txt, except the first line to xyz.txt using sed command as following....

cnt=`cat abc.txt | wc -l`
sed -e :a -e '$q;N;'$cnt',$D;ba' abc.txt > xyz.txt

or, is there any effective way of doing this using sed command?
Pls, give me solution using sed command only.

Thanks,
Subbu.

The simplest manner, erases the first line and prints the others

Code:

sed -e '1d' abc.txt > xyz.txt

Why solution using sed command only ?


Jean-Pierre.

Hi Jean-Pierre,

why I have asked you only sed command is .... I don't want to use tail command (it's truncating some part of data as it handles in only 20 k buffer). Let me know any other options available except tail.

Thank you very much,
Subbu.