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I am trying to understand how 

sed 's/\^\[/\o33/g;s/\[1G\[/\[27G\[/' /var/log/boot

worked and what the pieces mean. The man page I read just confused me more and I tried the info sai Id but had no idea how to work it! I'm pretty new to Linux. Debian is my first distro but seemed like a rather logical place to start as it is a root of many others and has been around a while so probably is doing stuff well and fairly standardized. I am running Wheezy 64 bit as fyi if needed.

hackerinheels
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Timothy Danielson
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1 Answers1

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The sed command is a stream editor, reading its file (or STDIN) for input, applying commands to the input, and presenting the results (if any) to the output (STDOUT).

The general syntax for sed is

sed [OPTIONS] COMMAND FILE

In the shell command you gave:

sed 's/\^\[/\o33/g;s/\[1G\[/\[27G\[/' /var/log/boot

the sed command is s/\^\[/\o33/g;s/\[1G\[/\[27G\[/' and /var/log/boot is the file.

The given sed command is actually two separate commands:

  1. s/\^\[/\o33/g

  2. s/\[1G\[/\[27G\[/

The intent of #1, the s (substitute) command, is to replace all occurrences of '^[' with an octal value of 033 (the ESC character). However, there is a mistake in this sed command. The proper bash syntax for an escaped octal code is \nnn, so the proper way for this sed command to have been written is:

s/\^\[/\033/g

Notice the trailing g after the replacement string? It means to perform a global replacement; without it, only the first occurrence would be changed.

The purpose of #2 is to replace all occurrences of the string \[1G\[ with \[27G\[. However, this command also has a mistake: a trailing g is needed to cause a global replacement. So, this second command needs to be written like this:

s/\[1G\[/\[27G\[/g

Finally, putting all this together, the two sed commands are applied across the contents of the /var/log/boot file, where the output has had all occurrences of ^[ converted into \033, and the strings \[1G\[ have been converted to \[27G\[.

aks
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  • Thanks for the in depth answer. To be honest It will take me some looking to understand it all. So far I think replacing the lead '^[' to the 'esc' character are what allows for the fairly easy to speed read color coding. I had read that the fact it does this is a bug that has been standing and reported. (This suggests it probably has been around a little while.) – Timothy Danielson Aug 14 '14 at 00:52
  • just wanted to say thanks for the answer and hope now you will get a notification that I sent this. – Timothy Danielson Aug 14 '14 at 01:08
  • I haven't played with this much until recently. However, I would like to point out that when I substituted 'o33' with '033' (which is correct by octal) the 'nice' color coding disappeared. I played with the other substitution of '/' vs '/g' with and without '033' and was able to prove to myself that for some reason it was the octal and NOT the global repeat that was involved. – Timothy Danielson Sep 14 '14 at 01:54