A simple way to fix file headers in your Objective-C (an not only) files.

Might be worth to clean up confusions, if any. By “file header” in this case I mean the C-style header comment like this

//
// Copyright (c) 2015 NSBogan. All rights reserved.
//

Problem

For some people it doesn’t really matter what sits at the top of the file. In big projects it may become more important. I have one particular case in mind where information in file headers needs to be anonymous and should contain only a company name and copyright notice, just like the code sample at the top. There are pros and cons to this approach. You may argue that it is important to know who created the file to point a finger, but when the project is large enough, dozens of people end up touching the same file, the project outlives developers (they just come and go, but the project stays), there’s no point to point a finger any more (apologies for the tautology here).

Another note is that we are here talking about existing files. When it comes to creating new files, it is possible to modify default Xcode templates, and even manage to preserve the changes on Xcode upgrade. There’s even a tool to automate the process to a certain level.

However, if you have a script that can fix header comments in existing files, you can reuse the very same script in your git commit hooks to fix the header comments of new files. Kill two birds with one stone (though in my native language we choose hares as objects of violence for this proverb).

Solution

So, at the high level, the way to approach this problem is

• Remove all blank lines at the start of the file
• Remove all lines that start with // from the start of the file
• Insert you custom header comment at the start of the file

Before proceeding further, let’s agree we are working with a source file named Source.m and have an environment variable defined as FILE_PATH=Source.m.

Remove Blank Lines

The blank lines at the start of the file are a rare exception, but sometimes they do occur. Fixing this is possible with sed is easy

sed -i . '/./,$!d' "${FILE_PATH}"

This is one of those they call read-only language. A one-liner that takes hours to decipher, unless you are fluent with sed already.

• The -i . option means inplace edit and weird looking . with a must-have space after -i option is OS X specific way to tell sed not to create backup file.
• The /./,$ construction is a range and means from /./ to $. /./ is a regex that matches any non-blank line, $ is a special address and means last line of the input. So /./,$ in English means from first non-blank line to the end of file.
• ! means negation. So if you negate /./,$ you get the following human language description from first line of the input to last blank line inclusive. In other words, all consequent leading blank lines in the file.
• Finally, d command means delete, so you tell sed to delete all lines in the given range.

Whoa, that was a lot of words to explain one 7-characters sed command…

Remove Header

Next thing to do is to remove existing header comment. This is a matter of removing consequent lines starting with // from the start of the file. You may think this sounds just like removing consequent blank lines. It does! But I failed big time to do it with sed. I originally assumed this is just a matter of replacing /./ regex with something like /^\/\//, but no, it didn’t work for me. I spent a fair amount of time googling and trying my best with sed and awk, before I gave up and came up with the solution you will see below. However, it would be fair to say that my seding and awking skills are nowhere near good enough, I am sure there’s a better way to do it and I would welcome any comments.

# Count number of consequent lines starting with // at the beginning of the file
N=$(cat "${FILE_PATH}" | awk '{ if(/^\/\//) print; else exit; }' | wc -l | xargs)

# Remove first N lines from the file
[[ ${N} -gt 0 ]] && sed -i . "1,${N}d" "${FILE_PATH}"

You can see here that the very same /^\/\// regex I tried to use with sed works perfectly with awk. The awk command will execute print until file lines match the pattern and will stop (using exit command) as soon as the first non-matching line occurs.

Word count (wc) command is used to count lines then (-l option) and xargs is handy in this case because it strips leading and trailing spaces from the input.

Next if N is greater than zero, use good old sed to remove first N lines from the file. This is done by using 1,${N} address range and delete command. Note the use of “weak” quotation marks in sed regex, this is to resolve reference to N inside the regex string.

Insert New Header

Finally, you have the file stripped of its old header comment, time to insert a new header.

# Insert new header using current year
HEADER="//\\
// Copyright (c) $(date +'%Y') NSBogan All rights reserved.\\
//\\
\\
"

sed -i . "1s|^|${HEADER}|g" "${FILE_PATH}"

The header here is using date command to insert current year in YYYY format, e.g. 2015.

sed matches the beginning of the line (^ anchor) and inserts the header there. The 1 before replacement pattern is there to make sure replacement is made only once.

Edge Cases

The described approach will not work for header comments that use block style, e.g.

/**
    File header comment.
*/

Summary

As usual I link a version of slightly improved script. As a bonus this script can fix header comments in xcconfig files as well. I still say slightly, because it can be improved more. One possible step up is to create a text file with you custom header comment and pass this file path as an input to a header fixing script.

You can bundle this script up with imports fixing script and use as part of git commit hooks to eliminate some of the code style debates during code reviews.