Editing And Deleting A Line From A Text File Using Shell Script

shiftshifter · 07-10-2012, 03:41 AM

Hi,

I'm new in using shell script. I'm trying to write a script where you can add, edit or delete information saved in a text file. I have already managed to adding part but I'm having difficulty with the other two. I tried online samples as basis for my scripts but I can't seem to make them work. Especially sed and awk. From my readings, I think they are the ones I need to use. So here is my problem:

Suppose I have a file named "changeletter.sh" and a text file named "letters.txt" which contains the following:
a d f s w a s x d f c x s
r f t a s w a d f r e d s

After running the script, I would then input the letter I want to change and what the new letter will be. Say change the letter a's to z. Or to delete all the letter f's.

Any help would be greatly appreciated.

xerox · 07-10-2012, 08:18 AM

Hi ShiftShifter!

I have given a script that does the basic requirements you asked for. Please include all the other error handling you require.

Code:

#!/bin/sh 
echo "Enter letter to be changed and letter to be changed with"
read a b
cat letter.txt | sed 's/'$a'/'$b'/g' > temp.txt
echo "Enter the letter to be deleted"
read c
cat temp.txt | sed 's/'$c'//g' > letter.txt
cat letter.txt

Please let me know incase of any concern.

Xerox

David the H. · 07-10-2012, 09:19 AM

There's no simple answer to this. Each operation really needs to be addressed separately, and some tools are better suited to certain operations than others. Changing all of one character to another, for example, or deleting them, is really best done with tr, although sed is capable of it.

Xerox's example above covers the basic substitution feature of sed, but there are a couple of weaknesses with it I need to address.

First, Useless Use Of Cat. sed can (and should) read files directly.

Second, the quoting. He's attempted to let the shell variables expand by unquoting them in the expression. This isn't safe. The usual way to handle it is to simple double-quote the whole string.

To explain in more detail, you should never leave the quotes off a parameter expansion unless you explicitly want the resulting string to be word-split by the shell (globbing patterns are also expanded). This is a vitally important concept in scripting, so train yourself to do it correctly now. You can learn about the exceptions later.

http://mywiki.wooledge.org/Arguments
http://mywiki.wooledge.org/WordSplitting
http://mywiki.wooledge.org/Quotes

Third, you can apply multiple expressions at once using the -e option, and you can edit a file in place with the -i option (and optionally save the original to a backup file), so you don't need a temp file.

Code:

sed -i.bckp -e "s/$a/$b/g" -e "s/$c//g" letter.txt

Here are a few useful sed references:
http://www.grymoire.com/Unix/Sed.html
http://sed.sourceforge.net/grabbag/
http://sed.sourceforge.net/sedfaq.html
http://sed.sourceforge.net/sed1line.txt

Here are a few useful awk references:
http://www.grymoire.com/Unix/Awk.html
http://www.gnu.org/software/gawk/man...ode/index.html
http://www.pement.org/awk/awk1line.txt
http://www.catonmat.net/blog/awk-one...ined-part-one/

For other tools like tr, start by calling up "info coreutils". There are also info pages for grep and sed.

Regular expressions are a core function of many of these tools and others, and learning them is one of the best bang-for-the-buck uses you can put your study time to.

Here are a few regular expressions tutorials:
http://mywiki.wooledge.org/RegularExpression
http://www.grymoire.com/Unix/Regular.html
http://www.regular-expressions.info/

Finally, you might consider using ed for some of this. As a real, and very light, universally available, text editor, it's a good choice for scripting out simple edits, such as deleting lines or moving them around.

How to use ed:
http://wiki.bash-hackers.org/howto/edit-ed
http://snap.nlc.dcccd.edu/learn/nlc/ed.html
(also read the info page)

xerox · 07-11-2012, 12:16 AM

Hi David,

Thanks for your strong review comments.
Im pretty new to shell scripting, i will consider all the points you have given me.

Xerox.

David the H. · 07-11-2012, 10:55 AM

Not to worry. You're doing fine.
I'm just here to help people improve their scripting (and continue to improve my own).

Keep up the good work!

shiftshifter · 07-13-2012, 12:16 AM

Thanks xerox and David the H. Your inputs were really helpful. I've been busy these couple of days so I wasn't able to check this forum until now. I do have another question. Say for example I have a file named names.txt with the following contents.

#names.txt
Name 1: John Doe
Name 2: Jane Doe

I've managed to create a script using your suggestions to change either the first name or the last name. However, my problem now is how to change both (e.g. change name 2 to Mary Wayne). Can't I assign all strings after "Name 2:" into a variable to change it in one go?

Thanks again for your help!

David the H. · 07-13-2012, 01:18 PM

I'm not quite sure I understand what you're asking at the end there, but again, sed or ed, and an appropriate regex can handle it.

Code:

sed -i '/^Name 2/ s/:.*/: Foo Bar/' file.txt
printf '%s\n' '/^Name 2/ s/:.*/: Foo Bar/' 'w' | ed -s file.txt

Another option is to not worry about matching substrings and simply replace the entire line.

Code:

sed '/^Name 2/c Name 2: Foo Bar' file.txt
printf '%s\n' '/^Name 2/c' 'Name 2: Foo Bar' '.' 'w' | ed -s file.txt

I learned how to do all this from the links I gave you above. Check them out!