Input file:
ID,Name,Values
1,A,vA|A2
2,B,VB
Expected output:
1,A,vA|VA2|vA3
2,B,VB
Search file for a given ID and then append a given value in the values {field}
use case : append 'testvalue' to the values filed of ID = 1
Problem is : How tho cache the line found ?
sed's s can be used to substitution, I used sed's p {print but of no use }.
Just set n to ID of the row you want to update and x to the value:
# vA3 to entry with ID==1
$ awk -F, '$1==n{$0=$0"|"x}1' n=1 x="vA3" file
ID,Name,Values
1,A,vA|A2|vA3
2,B,VB
# TEST_VALUE to entry with ID==2
$ awk -F, '$1==x{$0=$0"|"v}1' x=2 v="TEST_VALUE" file
ID,Name,Values
1,A,vA|A2
2,B,VB|TEST_VALUE
Explanation:
-F, sets the field separator to be a comma. $1==x checks if the line we are looking at contains the ID we want to change. Where $1 is the first field on each line and x is the variable we define.{$0=$0"|"v} where $0 is the variable containing the whole line so we are just appending the string "|" and value of the variable v to end of the line. 1 is just a shortcut in awk to say print the line. The 1 is the condition for the block which is evaluated to true and since no block is provide awk executes the default block {print $0}. Explicitly the script would be awk -F, '$1==n{$0=$0"|"x}{print $0}' n=1 x="vA3" file. 
The following script is doing something similar to Your need. It is in pure bash.
#!/usr/bin/bash
[ $# -ne 2 ] && echo "Arg missing" && exit 1;
while read l; do
[ ${l%%,*} == "$1" ] && l="$l|$2"
echo $l
done <infile
You can use as script <ID> <VALUE>. Example:
$ ./script 1 va3
ID,Name,Values
1,A,vA|A2|va3
2,B,VB
$ cat infile
ID,Name,Values
1,A,vA|A2
2,B,VB
or may be this?
awk '/vA/ { $NF=$NF"|VA2" } 1' FS=, OFS=,
$ echo "1,A,vA
2,B,VB" | awk '/vA/ { $NF=$NF"|VA2" } 1' FS=, OFS=,
1,A,vA|VA2
2,B,VB
awk started supporting in-file substitution recently. But with your requirement it is best to go with sed solution that Kent has posted above.$ cat file
ID,Name,Values
1,A,vA|A2
2,B,VB
$ awk '$1==1 { $NF=$NF"|vA3" } 1' FS=, OFS=, file
ID,Name,Values
1,A,vA|A2|vA3
2,B,VB
are your looking for this?
kent$ echo "1,A,vA
2,B,VB"|sed '/vA/s/$/|VA2/'
1,A,vA|VA2
2,B,VB
EDIT check the ID, then replace
kent$ echo "ID,Name,Values
1,A,vA|A2
2,B,VB"|sed 's/^1,.*/&|vA3/'
ID,Name,Values
1,A,vA|A2|vA3
2,B,VB
& means the matched part. that would be what you meant "cache"
sed ' 1 a\ |VA2|vA3 ' file1.txt
