shell loop match a regex in the current line - shell

Question

I'm trying to create a script to fix a csv file like this:
field_one,field_two,field_three
,field_two,field_three
So I need to check inside my loop if the current line is missing field_one and replace it with sed with a new value for field_one (overwrite the line missing field_one).
For this i have a loop but i need some help with identifying if the line is missing field one or not. I should probably use grep? but how to use it in a loop and get its response?
while read -r line; do
# this is pseudocode:
# if $line matches regex then
# sed 's/,/newfieldone/'
# overwrite the corrected line in the file
# end if
done < my_file
Thanks a lot in advance for your help!!!!

Answer 1

Inside your loop you can run following sed command:
sed 's/^\s*,/newfieldone,/'

Answer 2

To see if a line begins with a , and is hence missing field one, you can use if [[ "$line" =~ ^, ]].
For example:
while read -r line; do
if [[ "$line" =~ ^, ]]
then
echo "newfieldone$line"
else
echo "$line"
fi
done < my_file

Answer 3

Just for the heck of it, here's a solution in awk:
awk '{FS=","} {if ($1 == "") print "field_one" $0;else print $0} ' < /tmp/test.txt

Answer 4

$ sed -e "/^,/s/^,\([^,]*\),\([^,]\)/new_field_one,\1,\2/" < my_file
Edit: This probably is too complicated. Take one of the other fine answers :)

Answer 5

with sed try something like that:
sed -i 's|\(^,.*\)|new_field_one\1|g' <your file>

Answer 6

This might work for you:
a=Field_one,Field_two,Field_three
sed '/^,/c\'$a'' file
field_one,field_two,field_three
Field_one,Field_two,Field_three
Or if just inserting field_one:
a=Field_one
sed '/^,/s/^/'$a'/' file
field_one,field_two,field_three
Field_one,field_two,field_three

Answer 7

Simple bash solution using case statemetn:
while read -r line; do
case "$line" in
,*) printf "%s%s\n" newfieldone "$line" ;;
*) printf "%s\n" "$line" ;;
esac
done < my_file
case uses "glob" matching, not regular expressions, so ,* matches a string beginning with a comma.

Answer 8

sed -i 's/^,/fieldone,/' YOURFILE
Will replace every line starting , with fieldone, (inplace, so the original file gets overwritten, if you need a backup, try -i.backup).
If you want a dynamic fieldone value, well it depends, how dynamic want it to be :-), e.g.:
MYDYNAMICFIELDONE="DYNAF1"
sed -i "s/^,/${MYDYNAMICFIELDONE},/" YOURFILE
Or with your while loop:
while read -r line; do
MYDYNAMICFIELDONE="SET IT"
sed -i "s/^,/${MYDYNAMICFIELDONE},/"
done < my_file > tmpfile
mv tmpfile my_file
Or with awk:
awk '{
/^,/ {
DYNAF1="SET IT HERE"
print gensub("^,",DYNAF1 ",","g",$0)
}
} INPUT > OUTPUT

Answer 9

This is a pretty short 1-liner with awk
awk '{$1="field_one"}1' FS=',' OFS=',' file.csv

Answer 10

. . . and another awk one-liner:
awk '$1==""{$1="field_one"}1' FS=',' OFS=',' file

Answer 11

What about the use of bash only
while IFS=\, read field_one field_two rest_of_line
echo "${field_one:-default_field_one_value},$field_two,$rest_of_line"
doen < my_file > my_corecct_file
where the 'default_field_one_value' is used if the 'field_one' is empty