sed not working...

culin · 01-30-2007, 12:15 AM

Hi all,
i wanted to alter an option in the file /etc/xinetd.d/telnet i.e. disable = no to disable = yes... i have used the command...

Code:

sed -i -e 's/^disable = no/disable = yes/' /etc/xinetd.d/telnet

but this is not working .. what is the problem ??/ can anyone help me ???
thanks...

druuna · 01-30-2007, 01:05 AM

Hi,

This should work (the -e is not needed, but that isn't a show stopper).

- What error (if any) do you get?
- Does it work without the -i option (-i is sed 4.* specific)?

If the -i option is the problem, try the 'old fashion' way:

sed 's/^disable = no/disable = yes/' /etc/xinetd.d/telnet > /etc/xinetd.d/telnet.new
mv /etc/xinetd.d/telnet.new /etc/xinetd.d/telnet

Hope this helps.

raskin · 01-30-2007, 02:28 AM

Are you sure about what whitespace is used in the file?

culin · 01-30-2007, 02:32 AM

Thanks a lot for the reply druuna,
but the methods u said is not working... and it is not showing any error also...(but that option is not changing thats all) and if i use without -i it is printing the contents of the file and exiting.... now what to do.. ????

matthewg42 · 01-30-2007, 02:36 AM

Are you sure the whitespace is correct? Perhaps there's a tab after the "disable". Try it like this:

Code:

sed -i 's/^[ \t]*disable[ \t]*=[ \t]*no/disable = yes/'

Note, this is case-sensitive. It should be more robust as far as mixed spaces and tabs is concerned.

culin · 01-30-2007, 02:50 AM

wow... this is superb raskin thanks a million....

actually the command should be like this

Code:

 sed -i -e 's/^        disable         = no/        disable         = yes/' /etc/xinetd.d/telnet

now it is working...and why it is like this.....

culin · 01-30-2007, 02:52 AM

oh.. ya better way thanks matthewg42,

but why it is like that....

colucix · 01-30-2007, 02:55 AM

I looked at the files in /etc/xinetd.d and they contain some control character as ^I. We can verify this by opening the file with vi or vim and setting

Code:

:set list

this shows all the hidden characters. To strip out the ^I chars you can use sed in this way:

Code:

cat telnet | sed 's/^V^I/ /g' | sed 's/disable = no/disable = yes/'

Note that instead of literally write the ^V^I chars, you have to press Ctrl-V Ctrl-I in sequence. The line above will look like

Code:

cat telnet | sed 's/    / /g' | sed 's/disable = no/disable = yes/'

Edit: Just seen the last posts... sorry for redundancy!

culin · 01-30-2007, 03:05 AM

ya i saw that why there will be hidden charecters ?????

raskin · 01-30-2007, 03:07 AM

What do you mean "why it is like this"? Why in xinet.d tabs are used? Because they want it to be nicely formatted with slightly different parameter name length and without efforts. Why sed doesn't count ' ' as any whitespace? Because maybe you need just space. Why '[ \t]' works? Because [] means 'one of contained single characters' and '\t' means tab.

raskin · 01-30-2007, 03:10 AM

Tab is not exactly hidden - it is usual visual formatting character with non-constant display length.

culin · 01-30-2007, 03:23 AM

thanks raskin....ya i should have been more specific..

. what i mean to say was " do i need to count all the spaces every time i need to change the option..???" if say some has intended in other way then " is there a way to skip those spaces and change the options.. say if it is like this in file...
" disable = no" and while using sed, should i have to take care of these spaces ?? or else how can i make "programmer friendly "

i.e even though if i do not consider spaces it has to work ???

archtoad6 · 01-31-2007, 02:43 PM

If you want to preserve the spaces/tabs, then:

Code:

cp /etc/xinetd.d /etc/xinetd.d.000
sed -ri 's,^([ \t]*disable[ \t]*=[ \t]*)no,\1yes,' /etc/xinetd.d

I have done 5 things here:

Backed up the target file, just in case.
Used '(', ')', & '\1' to define a sub-expression in the match portion of the regex & then insert it in the replacement portion -- not only saves typing & mistakes, but preserves the formatting.
Used the "-r" option to avoid escaping those parentheses.
Put the 2 options together, but when using "-i" be careful to make it "-ri", not "-ir" -- that would make 'r' the back up file suffix.
Used ',' for the regex delimiter to make it easier to read. It's a little known, or at least rarely used, fact that even in Perl the '/' is NOT the mandatory delimiter. It can be anything you like -- sed uses the 1st character after the command (here, "s") as the delimiter. Pick whatever you find pleasing & easy to parse visually. for me, that's the comma in most cases.

sed is 1 of those basic commands w/ a lot of depth & power, & it pays to read the man page to know what it can do for you.

raskin · 01-31-2007, 02:47 PM

Even more - it pays to know that 'info sed' is more complete than 'man sed'. But only on large investments (and even larger return, though).

culin · 02-01-2007, 12:26 AM

Thanks for the reply archtoad6,
still i am in confusion.. say (in the file the indentation is like this..) if there are 10 spaces before disable and 2 spaces after "disable" and 3 spaces after "=" sign and then "no"... now if i want to replace this by "yes" do i need to take care of these spaces in "sed" command. ??? cant i do simply like this.. sed -i -e 's/^disable = no/disable = yes/' /etc/xinetd.d/telne