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Dec 25th, 2008

Scripts and editing files

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hello,
i'm new with scripts and linux and need a little help.

i made a text file called colors using a vi editor and contains as follows:
red
green
blue
gold

now i want to delete the 2nd line with the word green using a script file. well i know how to do it while being in the vi editor of that text, simply use the dd on the 2nd line .. but how can i do that using a script ? what commands to write there?

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Blackeagle

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Dec 25th, 2008

Re: Scripts and editing files

Do you want to delete it because it's line 2, or because it's green?

Read up on the sed command.

For example
sed '2d' file > newfile
or
sed '/green/d' file > newfile

Salem

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Posting Sage

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Dec 25th, 2008

Re: Scripts and editing files

yeah i want to delete the line with word green.

your command did work well but it doesnt change the original text file, it rather sends the new edited text to the newfile.
is there any way i can change the same file i'm using and save the cahnges?

and may i ask something else? suppose i want to add a new color to the end of the file, how is that possible?

Blackeagle

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Dec 25th, 2008

Re: Scripts and editing files

Nothing works by editing the file 'in place', even your vi editor.

It's all write the new file, delete the old file, then rename. It's just vi hides those steps from you. On very rare occasions when something crashes, you'll see (and be glad of it) that the old file was preserved and the new file was broken.

Do the same thing in any script you write.

Salem

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Dec 25th, 2008

Re: Scripts and editing files

aha .. i see .. i understand and it work fine with me.

just one more and last question, how can i add to the end of the file a certain row? can i use the sed command as before?
i tried to write: sed 'Gi someText' file > newfile
but it didnt work at all.

Last edited by Blackeagle; Dec 25th, 2008 at 9:36 am.

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Dec 25th, 2008

Re: Scripts and editing files

i used
echo text >> file

and it adds to the end of file .. Thank you for helping Salem. this thread is solved.

Thx again

Blackeagle

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Dec 25th, 2008

Re: Scripts and editing files

Click to Expand / Collapse Quote originally posted by Salem ...

sed '/green/d' file > newfile

Salem, what if the user enters a certain color like this:
echo enter color
read choice

and if i write the following:
sed '/$choice/d' file > newfile

it doesnt really work!
how can i put the value of choice in that command?

Blackeagle

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Re: Scripts and editing files

Be mindful of how the various shell quoting systems work.

Eg, try
sed '/'$choice'/d' file > newfile

But beware of users typing in a /

Salem

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Re: Scripts and editing files

[edited]

Last edited by Aia; Dec 25th, 2008 at 6:46 pm.

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Dec 30th, 2008

Re: Scripts and editing files

Click to Expand / Collapse Quote originally posted by Blackeagle ...

yeah i want to delete the line with word green.

your command did work well but it doesnt change the original text file, it rather sends the new edited text to the newfile.
is there any way i can change the same file i'm using and save the cahnges?

and may i ask something else? suppose i want to add a new color to the end of the file, how is that possible?

Not all previous answers were exactly correct. You can edit a file in-place.

Here's some semi-advanced shell scripting. It demonstrates

reading from the terminal in a loop
deleting 'dangerous' characters from entered data
the case statement
piping commands to ex(1)
appending to a file
and the use of control-D as end of input (EOF)

Shell Scripting Syntax (Toggle Plain Text)
#! /bin/sh
 
echo "Enter CMD COLOR, where CMD is either a (add) or d (delete), and COLOR is the alphanumeric name of a color."
echo
echo "Press <CTRL/D> when finished."
while read cmd color; do
  # First, drop all non-alphanumeric characters
  safecolor=`echo $color|sed -e 's=[^a-zA-Z0-9]==g'`
  # Now take action
  case $cmd in
    a) echo $safecolor >> colors;;
    d) echo -e "/$safecolor/d\nw" | ex colors;;
    *) echo "Unknown command '$cmd' given!";;
  esac
done
#! /bin/sh

echo "Enter CMD COLOR, where CMD is either a (add) or d (delete), and COLOR is the alphanumeric name of a color."
echo
echo "Press <CTRL/D> when finished."
while read cmd color; do
  # First, drop all non-alphanumeric characters
  safecolor=`echo $color|sed -e 's=[^a-zA-Z0-9]==g'`
  # Now take action
  case $cmd in
    a) echo $safecolor >> colors;;
    d) echo -e "/$safecolor/d\nw" | ex colors;;
    *) echo "Unknown command '$cmd' given!";;
  esac
done

Two things to note here:

Color names are limited to only alphanumerics
In shells, stuff inside single quotes prevents expansion of shell variables, while stuff inside double quotes allows variable expansion
You can pipe commands to ex(1) to edit a file 'in place'.

Last edited by Fest3er; Dec 30th, 2008 at 10:06 pm.

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