Replace a string which is present on first line in UNIX file - unix

Question

I would like to replace a string which is present on the first line though it is there on rest of the lines in the file as well. How can i do that through a shell script? Can someone help me regarding this. My code is as below. I am extracting the first line from the file and after that I am not sure how to do a replace. Any help would be appreciated. Thanks.
Guys -I would like to replace a string present in $line and write the new line into the same file at same place.
Code:
while read line
do
if [[ $v_counter == 0 ]] then
echo "$line"
v_counter=$(($v_counter + 1));
fi
done < "$v_Full_File_Nm"
Sample data:
Input
BUXT_CMPID|MEDICAL_RECORD_NUM|FACILITY_ID|PATIENT_LAST_NAME|PATIENT_FIRST_NAME|HOME_ADDRESS_LINE_1|HOME_ADDRESS_LINE_2|HOME_CITY|HOME_STATE|HOME_ZIP|MOSAIC_CODE|MOSAIC_DESC|DRIVE_TIME| buxt_pt_apnd_20140624_head_5records.txt
100106086|5000120878|7141|HARRIS|NEDRA|6246 PARALLEL PKWY||KANSAS CITY|KS|66102|S71|Tough Times|2|buxt_pt_apnd_20140624_head_5records.txt
Output
BUXT_CMPID|MEDICAL_RECORD_NUM|FACILITY_ID|PATIENT_LAST_NAME|PATIENT_FIRST_NAME|HOME_ADDRESS_LINE_1|HOME_ADDRESS_LINE_2|HOME_CITY|HOME_STATE|HOME_ZIP|MOSAIC_CODE|MOSAIC_DESC|DRIVE_TIME| SRC_FILE_NM
100106086|5000120878|7141|HARRIS|NEDRA|6246 PARALLEL PKWY||KANSAS CITY|KS|66102|S71|Tough Times|2|buxt_pt_apnd_20140624_head_5records.txt
From the above sample data I need to replace buxt_pt_apnd_20140624_head_5records.txt with SRC_FILE_NAME string.

Answer 1

Why not use sed?
sed -e '1s/fred/frog/' yourfile
will replace fred with frog on line 1.
If your 'string' is a variable, you can do this to get the variable expanded:
sed -e "1s/$varA/$varB/" yourfile
If you want to do it in place and change your file, add -i before -e.

Answer 2

awk -v old="string1" -v new="string2" '
NR==1 && (idx=index($0,old)) {
$0 = substr($0,1,idx-1) new substr($0,idx+length(old))
}
1' file > /usr/tmp/tmp$$ && mv /usr/tmp/tmp$$ file
The above will replace string1 with string2 only when it appears in the first line of file.
Any solution posted that uses awk but does not use index will not work in general. Same for any solution posted that uses sed. The reason is that those would work on REs, not strings and so behave undesirably for string replacement depending what characters are present in string1.
Looks like the OPs going with a sed RE-replacement solution so this is just for anyone else looking to replace a string: Here's what a string replacement function would look like if youd rather not have it inline:
awk -v old="string1" -v new="string2" '
function strsub(old,new,tgt, idx) {
if ( idx = index(tgt,old) ) {
tgt = substr(tgt,1,idx-1) new substr(tgt,idx+length(old))
}
return tgt
}
NR==1 { $0 = strsub(old,new,$0) }
1' file

Answer 3

A bash solution:
file="afile.txt"
str="hello"
repl="goodbye"
IFS= read -r line < "$file"
line=${line/$str/$repl}
tmpfile="/usr/tmp/$file.$$.tmp"
{
echo "$line"
tail -n+2 "$file"
} > "$tmpfile" && mv "$tmpfile" "$file"
Note that $str above will be interpreted as a "pattern" (a simple kind of regex) where * matches any number of any characters, ? matches any single character, [abc] matches any one of the characters in the brackets, and [^abc] (or [!abc]) matches any one character not in the brackets. See Pattern-Matching