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+---
+date: 2013-02-05
+title: In praise of old school UNIX
+category:   linux
+---
+
+What am I doing today? Documentation that is what. I am writing a
+document on how to do
+[this](https://www.chriscowley.me.uk/blog/2012/11/19/sftp-chroot-on-centos/).
+To any Linux user it is a very simple process and I could just give them
+a link to my own website.
+
+I am not writing this for a technical audience though. The people who
+are going to perform this work will be the 'Level 1 operatives'. This
+translates roughly to "anyone we can find on the street corners of some
+Far East city". If I tell them to press the red button labelled "press
+me" and it turns out to be orange, they will stop. I cannot assume the
+ability to edit a file in Vi. How can you work around this, well you
+need to make everything a copy and paste operation. This is easily done
+in Bash thanks to IO redirection and of course Sed.
+
+Now, a brief recap may be in order, as there are some perfectly
+knowledgable Linux users who do not know what Sed is. Really, one of
+them sits behind me. Sed stands for Stream EDitor, and it parses text
+and applies transformations to it. It was one of the first UNIX
+utilities. It kind of sits between
+[Grep](https://en.wikipedia.org/wiki/Grep) and
+[Awk](https://en.wikipedia.org/wiki/AWK_programming_language) and is
+[surprisingly powerful](https://uuner.doslash.org/forfun/).
+
+Anyway, I need to edit a line in a file then add a block of code at the
+end.
+
+    cp -v /etc/ssh/sshd_config{,.dist}
+    sed -i ''/^Subsystem/s#/usr/libexec/openssh/sftp-server#internal-sftp#g' \ 
+        /etc/ssh/sshd_config
+
+First line obviously is a contracted cp line which puts the suffix
+*.dist* on the copy.
+
+The basic idea is that it runs through the file (/etc/ssh/sshd\_config)
+and looks for any line that starts with "Subsystem" (`/^Subsystem/`).
+If it finds a line that matches it then will perform a "substituion"
+(`/s#`). The next 2 blocks tell it what the substitution will be in the
+order "\#From\#To\#". The reason for the change from `/` to `#` is
+because of the / in the path name (thanks to
+[Z0nk](https://www.reddit.com/user/z0nk) for reminding me that you can
+use arbitary seperators). The "\#g" tells Sed to perform the
+substituion on every instance it finds on the line, rather than just the
+first one. It is completely superfluous in this example, but I tend to
+put it in from force of habit. Finally the "-i" tells Sed to perform
+the edit in place, rather than outputing to Stdout.
+
+The next bit is a bit cleverer. With a single command I want to add a
+block of text to the file.
+
+    cat <<EOF | while read inrec; do echo $inrec >> /etc/ssh/sshd_config; done
+    Match Group transfer
+    ChrootDirectory /var/local/
+    ForceCommmand internal-sftp
+    X11Forwarding no
+    AllowTcpForwarding no
+
+    EOF
+
+Here `cat <<EOF` tells it send everything you type to Stdout until it
+sees the string EOF. This then gets piped to a `while` loop that appends
+each line of that Stdout to the file we want to extend
+(*/etc/ssh/sshd*config\_ in this case).
+
+Using these old tools and a bit of knowledge of how redirection works
+has enabled me to make a document that anyone who can copy/paste can
+follow. It is very easy for technical people to forget that not everyone
+has the knowledge we have. To us opening Vi is perfectly obvious, but to
+others maybe it isn't and they are not being paid enough to know. They
+are just being paid to follow a script. I may not like it, but it is the
+case - it also helped turn a boring documentation session into something
+a little more interesting. Which is nice!