sed -- the superman of unix stream editingThis is the third and final part of an article on the sed one-liners. This part will explain sed one-liners for selective deletion of certain lines and special applications of sed. See part one for introduction of the series.

Similarly to Awk one-liners, sed one-liners are short and concise sed programs that span less than one terminal line. They were written by Eric Pement and are floating around on the Internet as 'sed1line.txt' file.

If you are intrigued by this article series, I suggest that you subscribe to my posts!

Update: Spanish translation of part three is available!

Eric's sed one-liners are divided into several sections:

Grab my "sed cheat sheet" and a local copy of sed1line.txt file, and let the explanation begin!

Awesome news: I have written an e-book based on this article series. Check it out:

sed book

5. Selective Deletion of Certain Lines

68. Print all lines in the file except a section between two regular expressions.

sed '/Iowa/,/Montana/d'

This one-liner continues where the previous left off. One-liner #67 used the range match "/start/,/finish/" to print lines between two regular expressions (inclusive). This one-liner, on the other hand, deletes lines between two regular expressions and prints all the lines outside this range. Just to remind you, a range "/start/,/finish/" matches all lines starting from the first line that matches a regular expression "/start/" to the first line that matches a regular expression "/finish/". In this particular one-liner the "d", delete, command is applied to these lines. The delete command prevents the matching lines from ever seeing the light.

For example, suppose your input to this one-liner was:

Florida
<strong>Iowa
New York
San Jose
Montana</strong>
Texas
Fairbanks

Then after the sed program has finished running, the output is:

Florida
Texas
Fairbanks

We see this output because the lines from Iowa to Montana matched the "/Iowa/,/Montana/" range match (i put the matched lines in bold) and were deleted.

69. Delete duplicate, consecutive lines from a file (emulates "uniq").

sed '$!N; /^\(.*\)\n\1$/!P; D'

This one-liner acts as the "uniq" Unix utility. So how does it work? First of all, for every line that is not the very last line of input, sed appends the next line to the pattern space by the "N" command. The "N" command is restricted to all but the last line by "$!" restriction pattern. The newly appended line is separated from the previous line by the "\n" character. Next, the pattern space is matched against "/^(.*)\n\1$/" regular expression. This regular expression captures the previous line up to "\n" character and saves it in the match group "\1". Then it tests if the newly appended line is the same as the previous one. If it is not, the "P" gets executed. If it is, the "P" command does not get executed. The "P" command prints everything in the pattern space up to the first "\n" character. Next the "D" command executes and deletes everything up to the first "\n" char, leaving only the newly read line in pattern space. It also forces the sed script to begin from the first command.

This way it loops over all lines, comparing two consecutive lines. If they are equal, the first line gets deleted, and a new line gets appended to what's left. If they are not equal, the first one gets deleted, and deleted.

I think it's hard to understand what is going on from this description. I'll illustrate it with an example. Suppose this is the input:

foo
foo
foo
bar
bar
baz

The first thing sed does is it reads the first line of input in pattern space. The pattern space now contains "foo". Now the "N" command executed. The pattern space now contains "foo\nfoo". Next the pattern space is tested against "/^(.)\n\1$/" regular expression. This regular expression matches because "(.)" is "foo" and "/^(.)\n\1$/" is "foo\nfoo", exactly what we have in the pattern space. As it matched, the "P" command does not get executed. Now the "D" command executes, deleting the everything up to first "\n" from pattern space. The pattern space now contains just "foo". The "D" command forces sed to start from the first command. Now the "N" is executed again, the pattern space now contains "foo\nfoo" again and the same thing happens, "P" does not get executed and "D" deletes the first "foo", leaving the pattern space with just "foo" in it. Now the "N" gets executed once again, this time "bar" gets appended to pattern space. It contains "foo\nbar" now. The regular expression "/^(.)\n\1$/" does not match and "P" gets executed, printing "foo". After that "D" gets executed wiping "foo" from pattern space. The pattern space now contains "bar". The commands restart and "N" gets executed, it appends the next "bar" to current pattern space. Now it contains "bar\nbar". Just like with "foo\nfoo", nothing gets printed, and "D" deletes the first "bar", leaving pattern space with "bar". The one-liner restarts its execution. Now "N" reads in the final line "baz". The pattern space contains "bar\nbaz" which does not match the regular expression. The "P" prints out the "bar" and "D" deletes "bar". Now "N" does not get executed because we are at the last line of input. The "$!N" restricts "N" to all lines but last. At this moment pattern space contains only the last "baz", the regular expression does not match, so "baz" gets printed. The "D" command executes, emptying the pattern space. There is no more input and sed quits.

The output for this example is:

foo
bar
baz

I think this is one of the most detailed explanations I have written about a single one liner. :)

70. Delete duplicate, nonconsecutive lines from a file.

sed -n 'G; s/\n/&&/; /^\([ -~]*\n\).*\n\1/d; s/\n//; h; P'

This is a very tricky one-liner. It stores the unique lines in hold buffer and at each newly read line, tests if the new line already is in the hold buffer. If it is, then the new line is purged. If it's not, then it's saved in hold buffer for future tests and printed.

A more detailed description - at each line this one-liner appends the contents of hold buffer to pattern space with "G" command. The appended string gets separated from the existing contents of pattern space by "\n" character. Next, a substitution is made to that substitutes the "\n" character with two "\n\n". The substitute command "s/\n/&&/" does that. The "&" means the matched string. As the matched string was "\n", then "&&" is two copies of it "\n\n". Next, a test "/^([ -~]\n).\n\1/" is done to see if the contents of group capture group 1 is repeated. The capture group 1 is all the characters from space " " to "~" (which include all printable chars). The "[ -~]" matches that. Replacing one "\n" with two was the key idea here. As "([ -~]\n)" is greedy (matches as much as possible), the double newline makes sure that it matches as little text as possible. If the test is successful, the current input line was already seen and "d" purges the whole pattern space and starts script execution from the beginning. If the test was not successful, the doubled "\n\n" gets replaced with a single "\n" by "s/\n//" command. Then "h" copies the whole string to hold buffer, and "P" prints the new line.

71. Delete all lines except duplicate consecutive lines (emulates "uniq -d").

sed '$!N; s/^\(.*\)\n\1$/\1/; t; D'

This sed one-liner prints only the duplicate lines. This sed one-liner starts with reading in the next line from input with the "N" command. As I already mentioned, the current line and the next get separated by "\n" character after "N" executes. This one-liner also restrics "N" to all lines but last with "$!" restriction. Now a substitution "s/^(.*)\n\1$/\1/" is tried. Similarly to one-liner #69, this substitution replaces two repeating strings with one. For example, a string "foo\nfoo" gets replaced with just "foo". Now, if this substitution was successful (there was a repeated string), the "t" command takes the script to the end where the current pattern space gets printed automatically. If the substitution was not successful, "D" executes, deleting the non-repeated string. The cycle continues and this way only the duplicate lines get printed once.

Let's take a look at an example. Suppose the input is:

foo
foo
bar
baz

This one-liner reads the first line and immediately executes the "N" command. The pattern space now is "foo\nfoo". The substitution "s/^(.*)\n\1$/\1/" is tried and it's successful, because "foo" is repeated twice. The pattern space now contains just a single "foo". As the substitution was successful, "t" command branches to the end of the script. At this moment "foo" gets printed. Now the cycle repeats. Sed reads in "bar", the "N" command appends "baz" to "bar". The pattern space now is "bar\nbaz". The substitution is tried, but it's not successful, as "bar" is not repeated. As the substitution failed, "t" does nothing and "D" executes, deleting "bar" from pattern space. The pattern space is left with single "baz". Command "N" no longer executes as we reached end of file, substitution fails, "t" fails, and "D" deletes the "baz".

The end result is:

foo

Just as we expected - only the duplicate line got printed.

72. Delete the first 10 lines of a file.

sed '1,10d'

This one-liner restricts the "d" command to a range of lines by number. The "1,10" means a range matching lines 1 to 10 inclusive. On each of the lines the "d" command gets executed. It deletes the current pattern space, and restarts the commands from beginning. The default action for lines > 10 is to print the line.

73. Delete the last line of a file.

sed '$d'

This one-liner restricts the "d" command to the last line of file. It's done by specifying the special char "$" as the line to match. It matches only the last line. The last line gets deleted, but the others get printed implicitly.

74. Delete the last 2 lines of a file.

sed 'N;$!P;$!D;$d'

This one-liner always keeps two lines in the pattern space. At the very last line, it just does not output these last two. All the others before last two get output implicitly. Let's see how it does it. As soon as sed reads the first line of input in pattern space, it executes the first command "N". It places the 2nd line of input in pattern space. The next two commands "$!P" and "$!D" print the first part of pattern space up to newline character, and delete this part from pattern space. They keep doing it until the very last line gets appended to pattern space by "N" command. At this moment the last two lines are in pattern space and "$d" executes, deleting them both. That's it. Last two lines got deleted.

If there is just one line of data, then it outputs it.

75. Delete the last 10 lines of a file.

sed -e :a -e '$d;N;2,10ba' -e 'P;D'

This is really straight forward one-liner. It always keeps 10 lines in pattern-space, by appending each new input line with "N", and deleting the 11th excessive line with "D". Once the end of file is reached, it "d" the whole pattern space, deleting the last 10 lines.

sed -n -e :a -e '1,10!{P;N;D;};N;ba'

This is also a straight forward one-liner. For the lines that are not 1-10, it appends them to pattern space with "N". For lines > 10, it prints the first line in pattern space with "P", appends another line with "N" and deletes the printed line with "D". The "D" command causes sed to branch to the beginning of script! The "N;ba" at the end never, ever gets executed again for lines > 10. It keeps looping this way "P", "N", "D", always keeping 10 lines in pattern space and printing line-10 on each cycle. The "N" command causes script to quit if it tries to read past end of file.

76. Delete every 8th line.

gsed '0~8d'

This one-liner only works with GNU Sed only. It uses a special address range match "first~step" that matches every step'th line starting with the first. In this one-liner first is 0 and step is 8. Zero is not a valid physical line number, so the very first line of input does not match. The first line to match is 8th, then 16th, then 24th, etc. Each line that matches is deleted by "d" command.

sed 'n;n;n;n;n;n;n;d;'

This is a portable version. The "n" command prints the current pattern space, empties it, and reads in the next line. It does so for every 7 lines, and 8th line gets deleted with "d". This process continues until all input has been processed.

77. Delete lines that match regular expression pattern.

sed '/pattern/d'

This one-liner executes the "d" command on all lines that match "/pattern/". The "d" command deletes the line and skips to the next line.

78. Delete all blank lines in a file (emulates "grep '.'".

sed '/^$/d'

The regular expression "/^$/" in this one-liner tests if the beginning of line matches the end of the line. Only the empty lines have this property and sed deletes them.

Another way to do the same is:

sed '/./!d'

This one-liner tests if the line matches at least one character. The dot "." in the regular expression matches any character. An empty line does not have any characters and it does not match this regular expression. Sed deletes all the lines that do not match this regular expression.

79. Delete all consecutive blank lines from a file (emulates "cat -s").

sed '/./,/^$/!d'

This one-liner leaves one blank line at the end of the file, if there are multiple blanks at the end. Other than that, all consecutive blanks are stripped.

It uses an inverse range match "/start/,/finish/!" to "d" delete lines from first blank line, to first non-blank, non-inclusive.

sed '/^$/N;/\n$/D'

This one-liner leaves one blank line at the beginning and end of the file, if there are multiple blanks at both sides. Other than that, all consecutive blanks are stripped.

The consecutive empty lines get appended in pattern space by "/^$/N" command. The "/\n$/D" command matches and deletes blanks until only 1 is left. At that moment it no longer matches, and the line is output.

80. Delete all consecutive blank lines from a file except the first two.

sed '/^$/N;/\n$/N;//D'

In case of > 2 blank lines, this one-liner trims them down to two. There is a catch to this one-liner. Let me explain it first. See the last command "//D"? It's a shortcut for "/previous-match/D". In this case it's shortcut for "/\n$/D". Alright, now the one-liner itself. On every empty line, it appends the next to current pattern space with "/^$/N" command. Next it tests if the line just read in was actually a blank line with "/\n$/", if it is, it reads another line in with "N". At this moment it repeats the same test "/\n$/". If the line was a blank one again, it deletes the first blank line and restarts sed script from the beginning. Notice that at all times only 2 consecutive blank lines are in pattern space. This way any number of blank lines get deleted and only two are left.

81. Delete all leading blank lines at the top of a file.

sed '/./,$!d'

This one-liner inverts a match "match from the first non-blank line to end of file". It becomes "match from the beginning of file to last blank line".

82. Delete all trailing blank lines at the end of a file.

sed -e :a -e '/^\n*$/{$d;N;ba' -e '}'

This one-liner accumulates blank lines in pattern space until it either hits end or hits a non-blank line. If it hits end, "$d" deletes the whole pattern space (which contained just the trailing blank lines) and quits. If however, it hits non-blank line, the whole pattern space gets printed implicitly and script continues as if nothing had happened.

This one is a portable version.

gsed -e :a -e '/^\n*$/N;/\n$/ba'

This is the same script, except a shorter version, made to work with Gnu Sed.

83. Delete the last line of each paragraph.

sed -n '/^$/{p;h;};/./{x;/./p;}'

This one-liner always keeps the previous line in hold buffer. It's accomplished by 2nd block of commands "/./{x;/./p;}". In this block, the pattern space (1 line) gets exchanged with hold buffer (1 line) by "x" command and if the hold buffer was not empty, it gets printed by "p". The next moment to note is what happens on the first empty line. That is the line after the paragraph. At this moment "/^$/{p;h;}" gets executed, that prints the blank line (but does not print the last line of paragraph!), and puts the blank line in hold buffer. Once a new paragraph is reached, the script executed just like it was the very first paragraph of the input.

6. Special Sed Applications

84. Remove nroff overstrikes.

Nroff overstrikes are chars that are formatted to stand out in bold. They are achieved like in old typewriters, where you would do backspace and hit the same key again. In nroff it's key CHAR, CTRL+H, CHAR. This one-liner deletes the CHAR, CTRL+H, leaving just plain CHAR.

sed 's/.^H//g'

Press Ctrl+V and then Ctrl+H to insert ^H literally in sed one-liner. It then uses the substitute command to delete any char "." followed by CTRL+H "^H".

Another way to do the same is use a hex escape expression that works in most recent seds:

sed 's/.\x08//g'

Yet another way is to use "echo" and enable interpretation of backslashed characters:

sed 's/.'`echo -e "\b"`'//g'

85. Print Usenet/HTTP/Email message header.

gsed -r '/^\r?$/q'

Usenet, HTTP and Email headers are similar. They are a bunch of text lines, separated from the body of the message with two new lines "\r\n\r\n". Some implementations might even go with just "\n\n". This one-liner quits on the first line that is either empty or contains "\r". In other words, it prints the message header and quits.

86. Print Usenet/HTTP/Email message body.

sed '1,/^$/d'

This one-liner uses a range match "1,/^$/" to delete lines starting from 1st, and ending with the first blank line (inclusive). As I explained in the previous one-liner #78 above, "/^$/" matches empty lines. All the lines before first blank line in a Usenet/Email message or a HTTP header are message headers. They get deleted.

87. Extract subject from an email message.

sed '/^Subject: */!d; s///; q'

This one-liner deletes all lines that do not match "^Subject: ". Then it re-uses the match in "s///" to delete "Subject: " part from the line, leaving just the real subject. Please notice how "s///" is equivalent to "s/previous-match//", where "previous-match" is "^Subject: *" in this one-liner.

88. Extract sender information from an email message.

sed '/^From: */!d; s///; q'

This one liner is equivalent to the previous one, except it prints sender information from email.

89. Extract email address from a "Name Surname <email@domain.com>" string.

sed 's/.*< *//;s/ *>.*//;

This one-liner strips all symbols before < symbol (and any whitespace after it), and stips all symbols after > symbol (including whitespace before it). That's it. What's left is email@domain.com.

90. Add a leading angle bracket and space to each line (quote an email message).

sed 's/^/> /'

This one-liner substitutes zero-width anchor "^" that matches beginning of line with "> ". As it's a zero-width anchor, the result is that "> " gets added to beginning of each line.

91. Delete leading angle bracket from each line (unquote an email message).

sed 's/^> //'

It does what it says, deletes two characters ">" and a space " " from the beginning of each line.

92. Strip HTML tags.

sed -e :a -e 's/&lt;[^>]*>//g;/&lt;/N;//ba'

Sed is not made for parsing HTML. This is a very crude version of HTML tag eraser. It starts by creating a branch label named "a". Then on each line it substitutes "<[^>]>" with nothing as many times as possible ("g" flag for s/// command). The "<[^>]>" expression means match match symbol "<" followed by any other symbols that are not ">", and that ends with ">". This is a common pattern in regular expressions for non-greediness. Next, the one-liner tests if there are any open tags left on the line, if there are "N" reads the next line of input to make it work across multiple lines. "//ba" finally branches to the beginning of the script (it's short for "/previous-expression/ba" which in this case is "/</ba").

Sed One-Liners Explained E-Book

I have written an e-book called "Sed One-Liners Explained". I improved the explanations of the one-liners in this article series, added new one-liners and added three new chapters - an introduction to sed, a summary of sed addresses and ranges, and debugging sed scripts with sed-sed. Please take a look:

sed book

Have Fun!

This post completes my three part article on sed Unix stream editor. I hope you learned a bunch of new things and it was my pleasure explaining them.

If you liked this article, then you'll also like my three part article about Awk Unix utility called Awk One-Liners Explained.

Next, my plan is to create a sed1line-explained.txt file that will supplement 'sed1line.txt' and explain each one-liner in one sentence.

Have fun and see you next time!