topic Re: replace a specifk line in a file with sed or awk in Operating System - Linux

replace a specifk line in a file with sed or awk

Simon Jespersen — Wed, 13 Jun 2007 07:44:55 GMT

Hey!
I want to replace line nr 220 in a file with the text "my newline"
How can that be done with se or awk.

Re: replace a specifk line in a file with sed or awk

James R. Ferguson — Wed, 13 Jun 2007 07:49:58 GMT

Hi Simon:

# sed -e '220s/.*/my newline/' file

Regards!

...JRF...

Re: replace a specifk line in a file with sed or awk

Hein van den Heuvel — Wed, 13 Jun 2007 07:51:44 GMT

awk '{if (NR==220) $0="my newline";print}' old > new

hth,
Hein.

Re: replace a specifk line in a file with sed or awk

Simon Jespersen — Wed, 13 Jun 2007 08:01:56 GMT

Thanks for thje reply, but i get strange happening

with the sed line it just behave like im doing a cat on the file
# sed -e '2s/.*/mynewline/' passwd_sijes

Here im trying to replace line 2 in the file im just getting all the file directet to stdout
a71223:
password = SyNHeFTvoiMGw
lastupdate = 1135680886
flags = ADMCHG

a74040:
password = SyNHeFTvoiMGw
lastupdate = 1157718667
flags = ADMCHG

a05243:
password = SyNHeFTvoiMGw
lastupdate = 1157718669
flags = ADMCHG

a57659:
password = SyNHeFTvoiMGw
lastupdate = 1157718670
flags = ADMCHG

can you se why this is happening.

Re: replace a specifk line in a file with sed or awk

Simon Jespersen — Wed, 13 Jun 2007 08:05:47 GMT

How to i use an ksh shell var with awk and sed.
i have the line number in an variable that i want to parse to the awk program. Se $lineNr
cat myfile | awk '{if (NR==$lineNr) $0="my newline";print}' old > new

rhanks

Re: replace a specifk line in a file with sed or awk

James R. Ferguson — Wed, 13 Jun 2007 08:14:54 GMT

Hi (again) Simon:

You must redirect the STDOUT of 'sed' to a new file:

# sed -e '220s/.*/my newline/' file file.new
# mv file.new file

If you would like to do "inplace" updates without having to move the output back over the original file, use Perl:

# perl -pi.old -e 's/.*/mynewline/ if $.==220' file

This will modify line # 220, replacing it with "mynewline". The 'file" will be modified inplace with a backup copy named 'file.old'. If you don't want the backup, do:

# perl -pi -e 's/.*/mynewline/ if $.==220' file

Regards!

...JRF...

Re: replace a specifk line in a file with sed or awk

spex — Wed, 13 Jun 2007 08:23:22 GMT

Hello,

> How to i use an ksh shell var with awk and sed.

$ LN=2
$ awk -v lineNr=${LN} '{if (NR==lineNr) $0="my newline";print}' < old > new1
$ sed "${LN}s/.*/myline/" < old > new2

PCS

Re: replace a specifk line in a file with sed or awk

James R. Ferguson — Wed, 13 Jun 2007 08:24:42 GMT

Hi (again):

> How to i use an ksh shell var with awk and sed.i have the line number in an variable that i want to parse to the awk program. Se $lineNr

cat myfile | awk '{if (NR==$lineNr) $0="my newline";print}' old > new

You pass variables to 'awk' using the '-v' switch and argument:

# awk -v lineNr=220 '{if (NR==lineNr) $0="my newline";print}' file file.new

*Notice* that the 'lineNr' variable does not have the sigil ($) when referenced within the 'awk' script, nor does it have it when it is declared. However:

# lineNR=220
# awk -v lineNr=${lineNr} '{if (NR==lineNr) $0="my newline";print}' file file.new

...is necessary since you are now using a shell variable's value.

Do not use 'cat' to open and read a file and then pipe that output to 'awk'! That's a waste of a process. 'awk' uses the commandline arguments as filenames to process.

For 'sed' use double quotes like this:

# L=220
# sed -e "${L}s/.*/mynewline/" file > file.new
# mv file.new file

Regards!

...JRF...

Re: replace a specifk line in a file with sed or awk

Simon Jespersen — Wed, 13 Jun 2007 08:24:49 GMT

cool stuff, thanks a lot.

Re: replace a specifk line in a file with sed or awk

Peter Nikitka — Thu, 14 Jun 2007 05:53:20 GMT

Hi,

one addition to JRF's sed lines:
Keep in mind, when putting such stuff in a script, to check the success of the commands.

Being not root or acting via NFS or ..., you may have write access to the directory but no read access the the file or vice versa, so something like
# L=220
# sed -e "${L}s/.*/mynewline/" file > file.new
# mv file.new file

may destroy 'file'. I suggest always
sed -e "${L}s/.*/mynewline/" file > file.new && mv file.new file

which else won't try a 'mv' when 'file.new' cannot be created.

mfG Peter