Append heredocument content to a file, only if this content doesn't already exist in that file

Question

I have a file to which I want to append some content (including the first, second and third empty spaces visible in my code below):

### I am text 1

### I am text 2

_{(The actual text I append is way longer than these 5 lines).}

To simply append content to a file, I do:

cat <<-"EOF" >> myPath/myFile
    content...
EOF

But how could I ensure that the heredocument would append the content only if the content doesn't already exist in a file, and that otherwise the operation would be aborterd?

If only part of the block content already exists in the file, the entire operation should also be aborted.

Answer 1

If the here document should only be added if none of it is present, you can use grep:

cat <<-"EOF1" > myPath/myFile.append
    content...
EOF1
if ! grep -F -q -f myPath/myFile{.append,}; then
    cat myPath/myFile.append >> myPath/myFile
fi

To understand this, consider the following.

• grep -F -q -f myPath/myFile{.append,} is expanded by the shell to grep -F -q -f myPath/myFile.append myPath/myFile.

• The grep command searches myPath/myFile (the file to which the text should be added if necessary) for any fixed string (-F) contained in myPath/myFile.append (the file containing the text to add), reading one pattern per line (-f), and indicates whether it finds any only by its exit code, with no output (-q).

• The result is then negated !, so that the if block’s then part is only run if grep doesn’t find anything.

Answer 2

_{Please don't thumb up this answer but Stephen's one which I marked correct. This is but a variation easier for me to read.}

---

If the content of the template file should only be appended to the end file if none of it is already present in the end file, you can use grep:

cat <<-"EOF1" > myPath/myTemplate
    content...
EOF1

if ! grep -F -q -f myPath/myFile myPath/myTemplate; then
    cat myPath/myTemplate >> myPath/myFile
fi

The grep command searches myPath/endFile.sh for any fixed string (-F) contained in myPath/myTemplate, reading one pattern per line (-f), and indicates whether it finds any only by its exit code, with no output (-q).

The result is then negated !, so that the then block of the if-then block will only be executed if grep doesn’t match anything in the end file.

Appendix - exit codes:

When a process exits, it returns an exit code, which is a small integer. By convention, an exit code of 0 indicates success, non-zero indicates failure or an error:

• grep returns 0 when it finds a match, and 1 if it doesn’t.

• if uses its argument’s exit code to decide what to do; if 0, process the then block, otherwise the else block (if any).

• A punctuation (!) flips that; The then block only runs if grep doesn’t match anything (or encounters an error).