How to edit/prepend current line in a while read loop in bash?

[Birion]

Hi, I've been trying to write a bash script that reads a file with single words delimited by newlines, looking for the first line that does not start with a specific character (e.g. #), does something based on the line, and puts # at the beginning of the line to mark it done.



$ cat ~/example.txt
#cat
#dog
cat
dog
fish





SOURCE_FILE=~/example.txt

while read line
do
ISDONE=`echo $line | sed "s/^(.).*/\$1/" -` # this line may be wrong, since I'm not at a computer that can run bash, so I can't remember exactly (and the computer that can is currently Internet-less and far away), but it's correct in the script
if [[ $ISDONE -eq "#" ]]
then
continue
else
case $line in
... # do stuff here based on the contents of the line
esac
# here's the problem
# I wanted to put something like
sed "$line s/^/#/"
# but I kept hitting some errors
exit
fi
done < $SOURCE_FILE



so that after running the script once, the file should look like



$ cat ~/example.txt
#cat
#dog
#cat
dog
fish

[mobilediesel]

Hi, I've been trying to write a bash script that reads a file with single words delimited by newlines, looking for the first line that does not start with a specific character (e.g. #), does something based on the line, and puts # at the beginning of the line to mark it done.



$ cat ~/example.txt
#cat
#dog
cat
dog
fish


so that after running the script once, the file should look like



$ cat ~/example.txt
#cat
#dog
#cat
dog
fish


This should do it:

SOURCE_FILE=~/example.txt

while read line
do
if [[ ${line:0:1} != "#" ]]
then
case $line in
... # do stuff here based on the contents of the line
esac
sed -i "0,/^${line}$/ s/^/#/"
exit
fi
done < $SOURCE_FILE
The ${line:0:1} returns only the first character of $line.
!= to test that ${line:0:1} is not equal to "#".
The -i makes sed edit the file in place.
The 0, makes sure sed only changes the first occurrence.
/^${line}$/ makes sure it matches the exact line.

[Birion]

Thanks. I didn't know about the ${line:0:1} use, so I'm glad to learn something new (and perhaps simplify some other scripts that relied on the echo|sed combination I used). Unfortunately, it'll be hours before I can try it out, but from your explanation, it seems to do exactly what I wanted.



...I really need to learn sed... :/