grep can't get the word I need, but Linux continues to do the next job. How do I stop it?

Question

I have quite a few txt files. They contain the word 'Minimum' (it only shows once in each file), and I want to get the line that contains this word, and show the line in a new file.
To do this, I use this command:

grep 'Minimum' file1.txt > new1.txt     

I did this to each file (file1, file2, etc.), and got lots of newn.txt. But it turns out that some files don't have the word 'Minimum', so their newn.txt are empty. Well, if grep can't get the word, why doesn't Linux give me an error message? Why does it continue and make the newn.txt? Giving me some empty files is really annoying.
So is there a way that I can stop it from creating the newn.txt if it can't find the word?

Also, I'm thinking about putting it into a bash script - the grep command is the first step, and then in the second step I'll do something to the newn.txt. So if grep can't find the word, or if there's an empty file created, I want the script to stop and not to do the second step. Is there a way to do this?

Answer 1

The shell will create the output file before even running grep. The grep utility will then add contents to the file, if there is anything to add. If grep does not find anything, the file will be left empty.

Another thing that happens if grep does not find anything matching in the input file is that it will exit with a non-zero exit status, signalling "failure". This does not remove the output file, but you could act on the failure to remove the file like this:

grep 'PATTERN' infile >outfile || rm -f outfile

If grep fails to match PATTERN in infile, this would call rm -f to remove the empty outfile. Strictly speaking, this would also try to remove the file if grep failed for any reason (like e.g. not being able to read its input), or if the output file could not be created.

If you additionally want to terminate the script if no matches were found:

if ! grep 'PATTERN' infile >outfile; then
    rm -f outfile
    exit
fi

Answer 2

Try this loop. Adjust it to your needs.

i=0
for file in *; do
    line=""
    line="$(grep -hm1 'Minimum' "${file}")"
    [ -z "$line" ] || echo "$line" > new"${i}".txt
    ((i++))
done

Change * in this line: for file in *; do to your file set.

Take care to weed out any unwanted grep output that might crop up.

Answer 3

for i in file*; do grep -q Minimum "$i" && grep Minimum "$i" > "${i/file/new}"; done

• grep -q Minimum "$i" immediately exists after the first "Minimum" is found
• the second grep does the real job and is not executed if the first one fails
• "${i/file/new}" renames file in each filename to new

Answer 4

The simplest way to prevent the script from continuing when a Linux command is not successful is by adding

set -e

to the start of the file. This will make any unsuccessful command stop the script. When grep does not find any matches, it will send and unsuccessful signal. For example:

echo "starting"
echo "someword" > ex1.txt
cat ex1.txt | grep "otherword"
echo "continuing"

prints out

starting
continuing

But

set -e
echo "starting"
echo "someword" > ex1.txt
cat ex1.txt | grep "otherword"
echo "continuing"

prints out

starting