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How to append line numbers in front of each line in a file?

Just use cat. No kidding, cat has the -n option to number the output lines, starting at 1:

cat -n file.txt

This works well both on Linux and Mac. Use the shell redirection to keep the output in a new file (don’t override the original file with the redirection though).

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How to delete lines containing a given string?

Just like last week, where we wanted to replace a string, we can use sed for this task:

sed '/pouet/d' file.txt

This will output file.txt on stdout without the lines containing pouet.

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How to replace a string in a file?

The simplest way to proceed is to use sed. It‘s available both on Mac and Linux. Here is the gist of it:

sed -i 's/old/new/g' file.txt

Here we’re replacing old with new.

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How to remove the first X lines of a file?

Easy, with GNU tail to the rescue:

tail -n +X file.txt

Where -n prints the last lines, but the + inverts this and starts from the top of the file.

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How to batch convert JPG images to progressive JPG images?

Progressive JPG images, as opposed to baseline JPG, will display right away in the browser, and will load bits of it in cycle, rendering it from blur to clear.

Progressive is known to provide a better user experience, preventing the ”fax loading” effect. Where the image is displayed in full, but sequentially from top to bottom.

The imagemagick package will install the convert command that you can run to convert JPG to progressive:

convert -strip -interlace Plane -quality 80 input-file.jpg output-file.jpg

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Python: How to print the full traceback without exiting the program?

The exception handling block except Exception as ex: print(ex) will only print the exception message and not its traceback.

That’s good to know, but we need more info than this to debug properly. Namely the line that raised the exception, together with its stack trace.

The traceback module, part of the stdlib, will help us with this:

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How to find the PID of a process using a given port?

Sometimes you have a process you lost the PID, and would like to kill it.

You can use top (or better htop) to search for your process name and press k to kill it. But this isn’t optimal when you aren’t sure of its name. A better alternative is to search by the port opened, for example port 80:

sudo lsof -i :80

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How to copy SSH keys to another machine?

Public key authentication is generally safer than password-based and is way more convenient.

SSH offers a command to set it up, ssh-copy-id (part of the openssh client package) will copy your public key to the remote machine. Use it like this:

ssh-copy-id -i ~/.ssh/my_key.pub remote-machine

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Where to find changes due to git fetch

fetch is useful to pull changes from an upstream repo to merge some changes. By default git fetch upstream will update or create all the remote branches locally.

Say you have upstream/master tracked by your local master branch. Here is how to see the changes between both:

git log master..upstream/master

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Email address syntax validation? Let the browser do it (without regexp)

Here is a simple and robust way to check for the validity of an email address syntax directly in the browser. No need for crazy regular expressions.

e = document.createElement('input')
e.type = 'email'
// check some email addresses
e.value = 'hi@'