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+---
+category: tool
+tool: sed
+filename: learnsed.sed
+contributors:
+     - ["Diomidis Spinellis", "https://www.spinellis.gr"]
+
+---
+
+__Sed__ is a standard tool on every POSIX-compliant UNIX system.
+It's like an editor, such as Vim, Visual Studio Code, Atom, or Sublime.
+However, rather than typing the commands interactively, you
+provide them on the command line or in a file.
+
+_Sed_'s advantages over an interactive editor is that it can be easily
+used to automate text processing tasks, and that it can process
+efficiently huge (terabyte-sized) files.
+It can perform more complex tasks than _grep_ and for many text
+processing tasks its commands are much shorter than what you would
+write in _awk_, _Perl_, or _Python_.
+
+_Sed_ works by reading a line of text (by default from its standard
+input, unless some files are specified as arguments), processing
+it with the specified commands, and then outputting the result
+on its standard output.
+You can suppress the default output by specifying the `-n` command-line
+argument.
+
+```sed
+#!/usr/bin/sed -f
+# Files that begin with the above line and are given execute permission
+# can be run as regular scripts.
+
+# Comments are like this.
+
+# Commands consist of a single letter and many can be preceded
+# by a specification of the lines to which they apply.
+
+# Delete the input's third line.
+3d
+
+# The same command specified the command line as an argument to sed:
+# sed 3d
+
+# For many commands the specification can consist of two addresses,
+# which select an inclusive range.
+# Addresses can be specified numerically ($ is the last line) or through
+# regular expressions delimited by /.
+
+# Delete lines 1-10
+1,10d
+
+# Lines can also be specified as regular expressions, delimited by /.
+
+# Delete empty lines.
+/^$/d
+
+# Delete blocks starting with SPOILER-BEGIN and ending with SPOILER-END.
+/SPOILER-BEGIN/,/SPOILER-END/d
+
+# A command without an address is applied to all lines.
+
+# List lines in in a visually unambiguous form (e.g. tab appears as \t).
+l
+
+# A command prefixed by ! will apply to non-matching lines.
+# Keep only lines starting with a #.
+/^#/!d
+
+# Below are examples of the most often-used commands.
+
+# Substitute the first occurence in a line of John with Mary.
+s/John/Mary/
+
+# Remove all underscore characters (global substitution).
+s/_//g
+
+# Remove all HTML tags.
+s/<[^>]*>//g
+
+# In the replacement string & is the regular expression matched.
+
+# Put each line inside double quotes.
+s/.*/"&"/
+
+# In the matched regular expression \(pattern\) is used to store
+# a pattern into a buffer.
+# In the replacement string \1 refers to the first pattern, \2 to the second
+# and so on. \u converts the following character to uppercase \l to lowercase.
+
+# Convert snake_case_identifiers into camelCaseIdentifiers.
+s/_\(.\)/\u\1/g
+
+
+# The p (print) command is typically used together with the -n
+# command-line option, which disables the print by default functionality.
+# Output all lines between ``` and ```.
+/```/,/```/p
+
+
+# The y command maps characters from one set to another.
+# Swap decimal and thousand separators (1,234,343.55 becomes 1.234.343,55).
+y/.,/,./
+
+# Quit after printing the line starting with END.
+/^END/q
+
+# You can stop reading here, and still get 80% of sed's benefits.
+# Below are examples of how you can specify multiple sed commands.
+
+# You can apply multiple commands by separating them with a newline or
+# a semicolon.
+
+# Delete the first and the last line.
+1d
+$d
+
+# Delete the first and the last line.
+1d;$d
+
+
+# You can group commands in { } blocks.
+
+# Convert first line to uppercase and print it.
+1 {
+  s/./\u&/g
+  p
+}
+
+# Convert first line to uppercase and print it (less readable one-liner).
+1{s/./\u&/g;p;}
+
+
+# You can also stop reading here, if you're not interested in creating
+# sed script files.
+
+# Below are more advanced commands.  You typically put these in a file
+# rather than specify them on a command line.  If you have to use
+# many of these commands in a script, consider using a general purpose
+# scripting language, such as Python or Perl.
+
+# Append a line containing "profile();" after each line ending with ";".
+/;$/a\
+profile();
+
+# Insert a line containing "profile();" before each line ending with ";".
+/;$/i\
+profile();
+
+# Change each line text inside REDACTED blocks into [REDACTED].
+/REDACTED-BEGIN/,/REDACTED-END/c\
+[REDACTED]
+
+# Replace the tag "<ourstyle>" by reading and outputting the file style.css.
+/<ourstyle>/ {
+  r style.css
+  d
+}
+
+# Change each line inside REDACTED blocks into [REDACTED].
+# Also write (append) a copy of the redacted text in the file redacted.txt.
+/REDACTED-BEGIN/,/REDACTED-END/ {
+  w redacted.txt
+  c\
+  [REDACTED]
+}
+
+# All operations described so far operate on a buffer called "pattern space".
+# In addition, sed offers another buffer called "hold space".
+# The following commands operate on the two, and can be used to keep
+# state or combine multiple lines.
+
+# Replace the contents of the pattern space with the contents of
+# the hold space.
+g
+
+# Append a newline character followed by the contents of the hold
+# space to the pattern space.
+G
+
+# Replace the contents of the hold space with the contents of the
+# pattern space.
+h
+
+# Append a newline character followed by the contents of the
+# pattern space to the hold space.
+H
+
+# Delete the initial segment of the pattern space through the first
+# newline character and start the next cycle.
+D
+
+# Replace the contents of the pattern space with the contents of
+# the hold space.
+g
+
+# Append a newline character followed by the contents of the hold
+# space to the pattern space.
+G
+
+# Replace the contents of the hold space with the contents of the
+# pattern space.
+h
+
+# Append a newline character followed by the contents of the
+# pattern space to the hold space.
+H
+
+# Write the pattern space to the standard output if the default
+# output has not been suppressed, and replace the pattern space
+# with the next line of input.
+n
+
+# Append the next line of input to the pattern space, using an
+# embedded newline character to separate the appended material from
+# the original contents.  Note that the current line number
+# changes.
+N
+
+# Write the pattern space, up to the first newline character to the
+# standard output.
+P
+
+# Swap the contents of the pattern and hold spaces.
+x
+
+# Here is a complete example of some of the buffer commands.
+# Move the file's first line to its end.
+1 {
+  h
+  d
+}
+
+$ {
+  p
+  x
+}
+
+
+# Three sed commands influence a script's control flow
+
+# Name this script position "my_label", to which the "b" and
+# "t" commands may branch.
+:my_label
+
+# Continue executing commands from the position of my_label.
+b my_label
+
+# Branch to the end of the script.
+b
+
+# Branch to my_label if any substitutions have been made since the most
+# recent reading of an input line or execution of a "t" (test) function.
+t my_label
+
+# Here is a complete example of branching
+# Join lines that end with a backspace into a single space-separated one
+
+# Name this position "loop"
+: loop
+# On lines ending with a backslash
+/\\$/ {
+  # Read the next line and append it to the pattern space
+  N
+  # Substitute backslash newline with a space
+  s/\\\n/ /
+  # Branch to the top for testing this line's ending
+  b loop
+}
+```
+
+Further Reading:
+
+* [The Open Group: sed - stream editor](https://pubs.opengroup.org/onlinepubs/9699919799/utilities/sed.html)
+  The POSIX standard regarding sed.
+  Follow this for maximum portability.
+* [FreeBSD sed -- stream editor](https://www.freebsd.org/cgi/man.cgi?query=sed&sektion=&n=1)
+  The BSD manual page.
+  This version of sed runs on BSD systems and macOS.
+* [Project GNU: sed, a stream editor](https://www.gnu.org/software/sed/manual/sed.html)
+  The GNU manual page. GNU sed is found on most Linux systems.
+* [Lee E. McMahon: SED -- A Non-interactive Text Editor](https://wolfram.schneider.org/bsd/7thEdManVol2/sed/sed.pdf)
+  The original sed documentation
+* [A collection of sed resources](http://sed.sourceforge.net/)
+* [The sed FAQ](http://sed.sourceforge.net/sedfaq.html)