pass line to function - overwrite using sed line by line - BASH - linux

Question

why this code not work? What's the problem of passing $line to a function?
function a {
echo $1 | grep $2
}
while read -r line; do
a $line "LAN"
done < database.txt
Another question, i have to overwrite line by line a txt file possibly using sed command.But not all the line, only the part to change. Something like this:
while read -r line; do
echo $line | sed "s/STRING1/STRING2/"
done < namefile
EDIT
I give you an example for my second question.
input file:
LAN 1:
[text]11111[text]
[text]22222[text]
[text]33333[text]
LAN 2:
[text]11111[text]
[text]22222[text]
[text]33333[text]
output file:
LAN 1:
[text]44444[text]
[text]22222[text]
[text]33333[text]
LAN 2:
[text]11111[text]
[text]22222[text]
[text]33333[text]
I have to overwrite database.txt so i think to do this line by line using a counter for LAN. This is my code:
while read -r line; do
echo "$line" | grep -q LAN
if [ $? = "0" ]; then
net_count=$((net_count+1))
fi
if [ $net_count = <lan choose before> ]; then # variable that contains lan number chosen by user
echo "$line" | fgrep -q "11111"
if [ "$?" = "0" ]; then
echo $line | sed "s/11111/44444/" > database.txt
break
fi
fi
done < database.txt
Thank you all

Answer 1

Running sed once for every line is typically over a thousand times slower than running just one sed instance processing your whole file of input. Don't do it.
If you want to do string manipulation on a line-by-line basis, use native bash primitives for the purpose, as documented in BashFAQ #100:
a() {
local line=$1 regex=$2
if [[ $line =~ $regex ]]; then
printf '%s\n' "$line"
fi
}
while IFS= read -r line; do
a "$line" LAN
done <database.txt
Likewise, for substring replacements, an appropriate parameter expansion primitive exists:
while read -r line; do
printf '%s\n' "${line//STRING1/STRING2}"
done < namefile
That said, those approaches are only appropriate if you need to iterate line-by-line. Typically, it makes more sense to use a single grep or sed operation, and iterate over the results of those calls if you need to do native bash operations. For instance, the following iterates over the output from grep, as emitted by a process substitution:
regex=LAN
while IFS= read -r line; do
echo "Read line from grep: $line"
done < <(grep -e "$regex" <database.txt)

Answer 2

You need to put $line in quotes for the whole line to be considered as a single parameter. Else, every space character splits the strings as multiple parameter to the function:
#!/bin/bash
function a {
echo "$1" | grep "$2"
}
while read -r line; do
a "$line" "LAN"
done < database.txt
And for the second question, if you want to print only the lines that you modify, you can use the following code:
while read -r line; do
echo "$line" | sed -n "s/STRING1/STRING2/p"
done < namefile
Here, -n will omit the lines that do not match the string; and the flag p makes sure to print the lines that match.