Question about using sed (solved)

[stu90]

Hello,
I am requiring a little help with sed.

If have this file which i wish to run sed on:

Code: Select all

/usr
/usr/bin
 yad
/usr/share
/usr/share/pixmaps
 yad.png

sed -i '/\//s/^/\/root/'
This sed code seems to do what i require - which is to add /root only to the start of the lines that begin with a forward slash /

sed result:

Code: Select all

/root/usr
/root/usr/bin
 yad
/root/usr/share
/root/usr/share/pixmaps
 yad.png

I wish to move the forward slash and /root to variables and if possible mabe lose the back slash escape to make it easier to remember for future use - how would i format the sed string to accomplish this?

what i would like:

Code: Select all

FIND="/"
ADD="/root"
sed -i '/\$FIND/s/^/\$ADD/'

thanks.

[dejan555]

I've learnt that for separator with sed you can use anything you like so if you want to lose backslash use some other separator:

Code: Select all

sed -i 's:^/:/root/:'

[dejan555]

And with variables it would go something like this, someone correct me if I'm wrong but I tested it and it works:

Code: Select all

FIND="/"
ADD="/root/"
sed -i 's:^'"$FIND"':'"$ADD"':' 

[stu90]

And with variables it would go something like this, someone correct me if I'm wrong but I tested it and it works:

Code: Select all

FIND="/"
ADD="/root/"
sed -i 's:^'"$FIND"':'"$ADD"':' 

Many thanks dejan555 - that is just what i am looking for!

[technosaurus]

If you have a variable, you don't even need sed many times

echo ${VAR//$OLD/$NEW}

Will echo the string $VAR and replace substring $OLD with substring $NEW

[dejan555]

Good to know, but will this work with text files with multiple VAR instances? Can you use it with cat or something?
sed also has 'g' switch to replace globaly even if there are multiple instances on same line

[seaside]

dejan555,

You can use bash like this-

Replace only first match-
${string/pattern/replacement}

Replace all matches-
${string//pattern/replacement}

Replace beginning-
${string/#pattern/replacement}

Replace end-
${string/%pattern/replacement}

Example:replace all patterns in file-
cat file | echo "${string//pattern/replacement}"

Sed has more "line power".

Regards,
s

[dejan555]

Thanks seaside