Advanced Topics in sed

1
Advanced Topics in sed
2
How Does sed Treat Files?
Input
scriptfile
Input line (Pattern Space)
Hold Space
Output
3
Working With Multi-line Pattern Space

• Up until now, we have only considered operating
  on single lines
• Although this is easy, it makes looking for
  phrases or expressions that cross line boundaries
  difficult
• sed has the ability to look at more than one line
  at a time, which allows you to extend your
  pattern search space over multiple lines
• The commands n, d, and p all have multi-line
  versions

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Relevant Commands

• n - Copy the pattern space to standard output.
  Replace the pattern space with the next line of
  input
• N - Append the next line of input to the pattern
  space with an embedded NEWLINE. (The current
  line number changes.)
• d - Delete the pattern space. Start the next
  cycle.
• D - Delete the initial segment of the pattern
  space through the first NEWLINE. Start the next
  cycle.
• p - Print. Copy the pattern space to standard
  output.
• P - Copy the initial segment of the pattern space
  up through the first NEWLINE to standard output.

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• For example, suppose I want to find all
  occurrences of "the BSD Unix system" and replace
  it with "the Linux system"
• I could start with
• s/BSD Unix/Linux/
• but this would substitute Linux EVERYWHERE "BSD
  Unix" occurred
• So, let's use
• s/the BSD Unix system/the Linux system/
• This is fine but what if my input text looks like
  this
• .the BSD
• Unix system ...

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Bummer! No Match!

• So, how about if I use the multi-line commands?
• /the/
• N
• s/the \nBSD \nUnix \nsystem/the Linux
  system/
• This will work no matter where the newline is but
  it seems awfully hard, and it is
• What if I replace the newline BEFORE I do the
  pattern match?

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• s/the BSD Unix system/the Linux system/
• /the/
• N
• s/ \n/ /
• s/the BSD Unix system /the Linux system\
• /
• This first looks for a line where the search
  pattern phrase is contained on the same line
• If not, then we look for the case where it is
  split across lines by appending lines, replacing
  the embedded newline and then doing the
  substitution, followed by an escaped newline

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The hold space

• The pattern space is a buffer that holds the
  current input line
• There is another buffer that is under programmer
  control called the hold space
• The contents of pattern space can be copied to
  and from hold space
• The hold space is used for temporary storage
• Commands cannot access it except for the ones
  that copy between it and pattern space

9
Copy Pattern Space to Hold Space

• The h and H commands move the contents of pattern
  space to hold space
• h copies pattern space to hold space, replacing
  anything that was previously there
• H appends an embedded NEWLINE ("\n") to whatever
  is currently in hold space followed by the
  contents of pattern space
• Even if the hold space is empty, the embedded
  NEWLINE is appended to hold space first

10
Get Contents of Hold Space

• g and G get the contents of hold space and place
  it in pattern space
• g copies the contents of hold space into pattern
  space, replacing whatever was there
• G appends an embedded NEWLINE character ("\n")
  followed by the contents of hold space to pattern
  space
• Even if pattern space is empty, the NEWLINE is
  still appended to pattern space before the
  contents of the hold space

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• Now, suppose that I want to capitalize a specific
  word in a file, specifically, every time I see a
  "the abc statement" I want to change it to the
  ABC statement
• A script to do this looks like this
• /the . statement/
• h
• s/.the \(.\) statement./\1/
• y/abcdefghijklmnopqrstuvwxyz/ABCDEFGHIJKLMNOPQRSTU
  VWXYZ/
• G
• s/\(.\)\n(.the \).\( statement.\)/\2\1\3/

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So How Does It Work?

• The address limits the procedure to lines that
  match "the . statement"
• h copies the current line into hold space,
  replacing whatever was there
• After the h, pattern space and hold space are
  identical
• pattern space "find the print statement"
• hold space "find the print statement"
• s/.the \(.\) statement. /\1/ extracts the name
  of the statement (\1) and replaces the entire
  line with it
• pattern space "print"
• hold space "find the print statement"

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• y/abc./ABC/ changes each lowercase letter to
  uppercase
• pattern space "PRINT"
• hold space - "find the print statement"
• The G command appends a NEWLINE ("\n") to pattern
  space followed by the line saved in hold space
• s/\(.\)\n(.the \).\( statement.\)/\2\1\3/
  matches three different parts of the pattern
  space and rearranges them