The 'sed' command in Linux

So I am building a new project involving PHP and S3 object storage, and as part of the solution I am looking at adjusting some of the defaults in the PHP configuration. This is not too hard to do with interactive commands, but a bit more complex when scripting the solution in a Dockerfile for automated install.

First hurdle: The docker image doesn't have a text editor. No nano, no vi, no nothing. This is actually a security measure. The general rule with containers is to make them as small as possible to keep the images small, but also improve security - the less on the server, the less can go wrong or get used the wrong way.

Second hurdle: Replacing a couple of lines of text in the php.ini file. 

Here is where 'sed' comes in.

When learning how to pipe or append data to a file I've often used | tee, or > or >> to send data to a file. 

Example:

$echo "some random text" > test.txt

That would create a file called test.txt with the 'some random text' as the content of the file. 

To append you can use the >> flag. Building on the previous example:

$echo "some more random text" >> test.txt

This adds to the file like below. 

some random text
some more random text

That is all fine, and works in the PHP containers from Docker Hub. However, if one uses the example php.ini files - php.ini-development or php.ini-production some of the values are written already. 

Here is where the 'sed' command can be employed. 

We have a very long config file with a bunch of comments and several settings that PHP can use to overwrite or adjust the defaults. In my case I want to change the default upload file size from 2M to something larger. With a text editor it is pretty easy to just search for that line and change it. To do this automatically, 'sed' is perfect. 

The syntax for substituting a line of text with sed is as below:

sed 's/<original text>/<replacement text>/' <file to change>

An example, if a file has the phrase 'some random text' we can change the sed output to 'some more text' with the below command.

sed 's/random/more/' filename

The output would be 'some more text'. 

IMPORTANT: 'sed' just changes the output, but it does not change the information in the file. To insert the changes to the file use the '-i' flag.

For example, if I wish to permanently replace the word 'random' with the word 'more', I would change the previous command to something like the following:

sed -i 's/random/more/' filename

This is a super useful command that honestly doesn't get a lot of attention, at least not in my decade plus of putzing around with Linux. Again really useful for scripting and I should have an example build in another couple of weeks.

More BASH - Auto Sorting Files

BASH SMASH REHASH

I'm sharing a recent script I wrote that might be helpful. 

I had a picture folder on my NAS that was just simply way too large for use on a relatively slow Wifi connection. Essentially hundreds of picture and video files from our family photos.

What would happen on our computers is that when you want to open one file it takes forever as the file viewer tries to cache all the images in the folder to make flipping through them faster. That's a nice feature, but not when the folder / directory is dozens to hundreds of gigabytes in size. 

This script essentially prompts for which folder to sort (must be absolute), then prompts for how many files the user wants to sort per folder. The example below prompts to sort a directory with around 200 files into directories of 20 files apiece. 

Link to the code: autosort.sh

I included a lot of comments so hopefully the script makes sense. Again just a useful tool and might be helpful for those interested in shell scripting. 

BASH! (Bourne Again SHell)

 

BASH and the Linux terminal are essential tools in Linux, and can also be be a very powerful programming language. As shown, it allows for a user to run through the operating system, create, edit, delete files and directories, and have full control over permissions. 

In regards to scripting, probably the most powerful thing about using BASH is the ability to interact and leverage existing Linux commands to grab appropriate input and output. The timeconverter.sh script demonstrates this capability by calling the default 'date' command and using it to get a point of reference of where the user is (or rather where the system is set to), and then using that to help calculate other times around the globe.

What I like about this script - some self pride showing through, sorry - is it's very fast to run and execute. So if one needs to schedule a meeting for 4 pm their local time and wants to see what that time would be in other parts of the world you can run through a slew of possible times in no time at all! 

A few corrections:
Please excuse some typos and little goofs in the video, like having the extra '/' at the end of #!/bin/bash, and finding the /usr/bin/ directory. Hopefully showing the fixes were equally helpful :). The script also doesn't necessarily need the .sh at the end. That is just a habit since many shell scripts have that, but one could simply name the file timeconverter or magictimes or whatever. I should add it is more common for users to add scripts to /usr/local/bin rather than /usr/bin, though as shown either works. 

Tip: cat /etc/environment to see where the PATH links to. PATH in Unix/BSD/Linux defines where all the commands a user can use are located so that they can be run without having to know where each command file is located.

If anyone notices any bugs - the trickiest part is the daylight savings / standard time section - just please leave a comment and I can update.

------------------------------------------

timeconverter:

Source code of the script

--------------------------------------------

Additional reference:

BASH shell

Linux Permissions

The Linux Command Line