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Old 01-17-2012, 12:36 AM   #1
zx_
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copy file and append string in the process


Hi,

I use this command:

Code:
cp input-file.ext /tmp/tmp.ext && echo -e '\nfinishing line' >> /tmp/tmp.ext
to append a line to a file and copy it to temp folder without changing input file.

Is there better way to do the same?
 
Old 01-17-2012, 01:00 AM   #2
chrism01
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Well, possibly
Code:
# instead of
... echo -e '\nfinishing line' ...

# you can just do
... echo "finishing line" ...
ie you'll get a newline anyway that way, unless you actually wanted a blank line inserted as well?
 
Old 01-17-2012, 01:27 AM   #3
zx_
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OK, thanks, that's slight improvement

I was thinking more if there is some command so that I could append line while copying file

It's nothing critical, I'm just curious if I'm missing some more elegant way, as I'm new to Linux and I imagine there is terminal command for any imaginable scenario
 
Old 01-17-2012, 01:40 AM   #4
Dark_Helmet
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If all you want is a non-traditional linux-shell-trick kind of command, then you can use sed instead of cp. For instance:

Code:
sed '$ a finishing line' input-file.ext > /tmp/tmp.ext
EDIT:
I guess I should explain what's happening. I don't know how familiar you are with sed (if at all). So please excuse me if you know any of this already.

The sed command modifies a stream of text. It does so by looking for matching patterns and executing commands when a pattern is matched. In this particular case, sed is told to look for the end of the stream of text (represented by the '$'), append text when found (represented by the 'a'), and the text to append (represented by the 'finishing line').

The sed command reads the stream of text from input-file.ext. The normal behavior of sed is to print the modified stream to the screen/stdout. The output redirector ('>') tells the shell to send sed's output to the file /tmp/tmp.ext instead.

Keep in mind, the above sed command is very basic. There's much more sed can do, but it might take a while for you to become familiar with the patterns/commands.

Last edited by Dark_Helmet; 01-17-2012 at 01:50 AM.
 
Old 01-17-2012, 01:50 AM   #5
zx_
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Thanks I was looking for that kind of simplification
sed seems so flexible...
 
Old 01-17-2012, 03:26 AM   #6
Tinkster
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Or another way:
Code:
(cat test2;echo "finishing line")>test3
 
Old 01-17-2012, 03:06 PM   #7
colucix
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And don't miss the awk way!
Code:
awk '1;END{print "finishing line"}' input-file.ext > /tmp/tmp.ext
As you can see there are a lot of ways to accomplish the same task in *nix: it's a matter of taste, style, performance, compatibility, etc. etc.
 
Old 01-18-2012, 10:13 AM   #8
David the H.
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Quote:
Originally Posted by chrism01 View Post
Well, possibly
Code:
# instead of
... echo -e '\nfinishing line' ...

# you can just do
... echo "finishing line" ...
ie you'll get a newline anyway that way, unless you actually wanted a blank line inserted as well?
Actually both versions suffer from the same problem. echo, on its own, only inserts a newline after the string, not before it.

So if the file you're modifying does not have a final newline, then the first command above will supply it before inserting the text. But if it does have a newline already, then you'll end up with an extra blank line.

The second command does the opposite, of course. It will come out properly if there's already a newline, but if not then the new string will be concatenated with the final line of text.

To safely avoid this problem you should ensure that any trailing newlines are removed from the text before the operation, and then supply your own.

Here's one way, using command substitution and printf.
Code:
printf '%s\n%s\n' "$(<input-file.ext)" "finishing line" > newfile
One of the side-effects of $(..) is that it removes trailing newlines from the substituted text if it finds one. And bash lets you use < inside of them to redirect the contents of a file directly (in other shells you can use cat). Finally, quoting the whole thing means printf sees the entire contents of the file (minus trailing newlines) as a single input argument.

I probably wouldn't use it if the file is exceptionally big though, as the whole thing is, I believe, temporarily copied to shell memory. Use the previously supplied sed or awk commands in that case.
 
  


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