4. Selective Printing of Certain Lines.
44. Print the first 10 lines of a file (emulates "head -10").
sed 10q
This one-liner restricts the "q" (quit) command to line "10". It means that this command gets executed only when sed reads the 10th line. For all the other lines there is no command specified. When there is no command specified, the default action is to print the line as-is. This one-liner prints lines 1-9 unmodified and at 10th line quits. Notice something strange? It was supposed to print first 10 lines of a file, but it seems that it just printed only the first 9... Worry not! The quit command is sneaky in its nature. Upon quitting with "q" command, sed actually prints the contents of pattern space and only then quits. As a result lines 1-10 get printed!
Please see the first part of the article for explanation of "pattern space".
45. Print the first line of a file (emulates "head -1").
sed q
The explanation of this one-liner is almost the same as of the previous. Sed quits and prints the first line.
A more detailed explanation - after the first line has been placed in the pattern space, sed executes the "q" command. This command forces sed to quit; but due to strange nature of the "q" command, sed also prints the contents of pattern space. As a result, only the first line gets printed.
46. Print the last 10 lines of a file (emulates "tail -10").
sed -e :a -e '$q;N;11,$D;ba'
This one-liner is tricky to explain. It always keeps the last 10 lines in pattern space and at the very last line of input it quits and prints them.
I'll try to explain it. The first "-e :a" creates a label called "a". The second "-e" does the following: "$q" - if it is the last line, quit and print the pattern space. If it is not the last line, execute three commands "N", "11,$D" and "ba". The "N" command reads the next line of input and appends it to the pattern space. The line gets separated from the rest of the pattern space by a new line character. The "11,$D" command executes the "D" command if the current line number is greater than or equal to 11 ("11,$" means from 11th line to end of file). The "D" command deletes the portion of pattern space up to the first new line character. The last command "ba" branches to a label named "a" (beginning of script). This guarantees that the pattern space never contains more than 10 lines, because as line 11 gets appended to pattern space, line 1 gets deleted, as line 12 gets appended line 2 gets deleted, etc.
47. Print the last 2 lines of a file (emulates "tail -2").
sed '$!N;$!D'
This one-liner is also tricky. First of all, the "$!" address restricts commands "N" and "D" to all the lines except the last line.
Notice how the addresses can be negated. If "$<command>" restricts a command to the last line, then "$!<command>" restricts the command to all but the last line. This can be applied to all restriction operations.
In this one-liner the "N" command reads the next line from input and appends it to pattern space. The "D" command deletes everything in pattern space up to the first "\n" symbol. These two commands always keep only the most recently read line in pattern space. When processing the second-to-last line, "N" gets executed and appends the last line to the pattern space. The "D" does not get executed as "N" consumed the last line. At this moment sed quits and prints out the last two lines of the file.
48. Print the last line of a file (emulates "tail -1").
sed '$!d'
This one-liner discards all the lines except the last one. The "d" command deletes the current pattern space, reads in the next line, and restarts the execution of commands from the first. In this case it just loops over itself like "dddd...ddd" until it hits the last line. At the last line no command is executed ("$!d" restricted execution of "d" to all the lines but last) and the pattern space gets printed.
Another way to do the same:
sed -n '$p'
The "-n" parameter suppresses automatic printing of pattern space. It means that without an explicit "p" command (or other commands that act directly on the output stream), sed is dead silent. The "p" command stands for "print" and it prints the pattern space. This one-liner calls the "p" command at the very last line of input. All the other lines are silently discarded.
49. Print next-to-the-last line of a file.
Eric gives three different one-liners to do this. The first one prints a blank line if the file contains just 1 line:
sed -e '$!{h;d;}' -e x
This one-liner executes the "h;d" commands for all the lines except the last one ("$!" restricts "h;d" commands to all lines except last). The "h" command puts the current line in hold buffer and "d" deletes the current line, and starts execution at the first sed command ("h;d" gets executed again, and again, ...). At every single line, that line gets copied to hold buffer. At the very last line "h;d" does not get executed. At this moment "x" gets a chance to execute. The "x" command exchanges the contents of hold buffer with pattern space. Remember that the previous line is still in the hold buffer. The "x" command puts it back in pattern space, and sed prints it! There you go, the next-to-last line was printed!
In case there is just 1 line in the file, only the "x" command gets executed. As the hold buffer initially is empty, "x" puts emptiness in pattern space (I use word "put" here but it actually exchanges the pattern space with hold space). Now sed prints the contents of pattern space, but it's empty, so sed prints out just a blank line.
The second prints the first line if the file contains just 1 line:
sed -e '1{$q;}' -e '$!{h;d;}' -e x
This sed-one liner is divided in two parts. The first part "1{$q;}" handles the case when the file contains just a single line. The second part "$!{h;d;} x" is exactly the same as in the previous one-liner! Thus, I need to explain just the first part.
The first part says - if it is the first line "1", then execute "$q". The "$q" command means - if it is the last line, then quit. What it effectively does is it quits if the first line is the last line (i.e. file contains just one line). Remember from one-liner #44 that before quitting sed prints the contents of pattern space. As a result, if the file contains just one line, sed prints it.
The third prints nothing for 1 line files:
sed -e '1{$d;}' -e '$!{h;d;}' -e x
This one-liner is again divided in two parts. The first part is "1{$d;}" and the second is exactly the same as in the previous two one-liners. I will explain just the first part.
The first part says - if it is the first line "1", then execute "$d". The "$d" command means - if it is the last line, then delete the pattern space and start all over again. In case the first line is the last (only one line in file), there is nothing more to be done and sed quits, printing nothing.
50. Print only the lines that match a regular expression (emulates "grep").
sed -n '/regexp/p'
This one-liner suppresses automatic printing of pattern space with the "-n" switch and makes use of "p" command to print only the lines that match "/regexp/". The lines that do not match this regex get silently discarded. The ones that match get printed. That's it.
Another one-liner that does the same:
sed '/regexp/!d'
This one-liner deletes all the lines that do not match "/regexp/". The other lines get printed by default. The "!" before "d" command inverts the line matching.
51. Print only the lines that do not match a regular expression (emulates "grep -v").
sed -n '/regexp/!p'
This one-liner is the inverse of the previous.
The "-n" prevents automatic printing of pattern space. The "/regexp/" restricts the "!p" command only to lines that match "/regexp/", but the "!" switch prevents "p" from acting on these lines. What happens is "p" acts on all lines that do not match "/regexp/", and they get "p"rinted.
sed '/regexp/d'
This one-liner is the inverse of the previous (#50).
This one-liner executed the "d" (delete) command on all lines that match "/regexp/", thus leaving only the lines that do not match. They get printed automatically.
52. Print the line immediately before regexp, but not the line containing the regexp.
sed -n '/regexp/{g;1!p;};h'
This one-liner saves each line in hold buffer with "h" command. If a line matches the regexp, the hold buffer (containing the previous line) gets copied to pattern space with "g" command and the pattern space gets printed out with "p" command. The "1!" restricts "p" not to print on the first line (as there are no lines before the first).
53. Print the line immediately after regexp, but not the line containing the regexp.
sed -n '/regexp/{n;p;}'
First of all, this one-liner disables automatic printing of pattern space with "-n" command line argument. Then, for all the lines that match "/regexp/", this one-liner executes "n" and "p" commands. The "n" command is the only command that depends on "-n" flag explicitly. If "-n" is specified it will empty the current pattern space and read in the next line of input. If "-n" is not specified, it will print out the current pattern space before emptying it. As in this one-liner "-n" is specified, the "n" command empties the pattern space, reads in the next line and then the "p" command prints that line out.
54. Print one line before and after regexp. Also print the line matching regexp and its line number. (emulates "grep -A1 -B1").
sed -n -e '/regexp/{=;x;1!p;g;$!N;p;D;}' -e h
First let's look at "h" command at the end of script. It gets executed on every line and stores the current line in pattern space in hold buffer. The idea of storing the current line in hold buffer is that if the next line matches "/regexp/" then the previous line is available in hold buffer.
Now let's look at the complicated "/regexp/{=;x;1!p;g;$!N;p;D;}" command. It gets executed only if the line matches "/regexp/". The first thing it does is it prints the current line number with "=" command. Then, it exchanges the hold buffer with pattern space by using the "x" command. As I explained, the "h" command at the end of the script makes sure that the hold buffer always contains the previous line. Now we have put it in the pattern space with "x" command. Next, if it's not the first line, "1!p" prints the pattern space, effectively printing the previous line. Now the "g" command gets executed. It copies the original line that was just exchanged with hold buffer back to pattern space. Now the "$!N" executes. If it is not the last line, "N" appends the next line to the current pattern space (and separates them with "\n" char). Pattern space now contains the line that matched "/regexp/" and the next line. The "p" command prints that. "D" deletes the current line (line that matched "/regexp/") from pattern space and finally "h" gets executed again, that puts the contents of pattern space into hold buffer. As "D" deleted the current line, the next line was put in hold buffer.
55. Grep for "AAA" and "BBB" and "CCC" in any order.
sed '/AAA/!d; /BBB/!d; /CCC/!d'
This one-liner inverts the "d" command to be executed on lines that do not contain either "AAA", "BBB" or "CCC". If a line does not contain one of them, it gets deleted and sed proceeds to the next line. Only if all three of the patterns are present, does the sed print the line.
56. Grep for "AAA" and "BBB" and "CCC" in that order.
sed '/AAA.*BBB.*CCC/!d'
This one-liner deletes lines that do not match regexp "/AAA.*BBB.*CCC/". For example, a line "AAAfooBBBbarCCC" will get printed but "AAAfooCCCbarBBB" baz will not.
It can also be written as:
sed -n '/AAA.*BBB.*CCC/p'
This one-liner prints lines that contain AAA...BBB...CCC in that order.
57. Grep for "AAA" or "BBB", or "CCC".
sed -e '/AAA/b' -e '/BBB/b' -e '/CCC/b' -e d
This one-liner uses the "b" command to branch to the end of the script if the line matches "AAA" or "BBB" or "CCC". At the end of the script the line gets implicitly printed. If the line does not match "AAA" or "BBB" or "CCC", the script reaches the "d" command that deletes the line.
gsed '/AAA\|BBB\|CCC/!d'
This one-liner works with GNU sed. GNU sed allows alternation operator | to be used to match separate things. It's a more compact way of saying match "AAA" or "BBB", or "CCC".
If you are using GNU sed, then there is actually no need to escape the pipes |. You may specify the "-r" command line option to use extended regular expressions. This way this one liner becomes:
gsed -r '/AAA|BBB|CCC/!d'
or
gsed -rn '/AAA|BBB|CCC/p'
58. Print a paragraph that contains "AAA". (Paragraphs are separated by blank lines).
sed -e '/./{H;$!d;}' -e 'x;/AAA/!d;'
First notice that this one-liner is divided in two parts for clearness. The first part is "/./{H;$!d;}" and the second part is "x;/AAA/!d".
The first part has an interesting pattern match "/./". What do you think it does? Well, a line separating paragraphs would be a blank line, meaning it would not have any characters in it. This pattern matches only the lines that are not separating paragraphs. These lines get appended to hold buffer with "H" command. They also get prevented from printing with "d" command (except for the last line, when "d" does not get executed ("$!" restricts "d" to all but the last line)). Once sed sees a blank line, the "/./" pattern no longer matches and the second part of one-liner gets executed.
The second part exchanges the hold buffer with pattern space by using the "x" command. The pattern space now contains the whole paragraph of text. Next sed tests if the paragraph contains "AAA". If it does, sed does nothing which results in printing the paragraph. If the paragraph does not contain "AAA", sed executes the "d" command that deletes it without printing and restarts execution at first command.
59. Print a paragraph if it contains "AAA" and "BBB" and "CCC" in any order.
sed -e '/./{H;$!d;}' -e 'x;/AAA/!d;/BBB/!d;/CCC/!d'
This one-liner is also split in two parts for clarity. The first part is exactly the same as the first part of previous one-liner. The second part is very similar to one-liner #55 and also the previous.
The "x" command in the 2nd part does exactly the same as in previous one-liner, it exchanges the hold buffer, that contains the paragraph with pattern space. Next sed does three tests - it tests if the paragraph contains "AAA", "BBB" and "CCC". If the paragraph does not contain even one of them, the "d" command gets executed that purges the paragraph. If it contains all three patterns, sed happily prints the paragraph.
60. Print a paragraph if it contains "AAA" or "BBB" or "CCC".
sed -e '/./{H;$!d;}' -e 'x;/AAA/b' -e '/BBB/b' -e '/CCC/b' -e d
The first part is exactly the same as in previous two one-liners and does not require explanation. The second part that happens to be "-e 'x;/AAA/b' -e '/BBB/b' -e '/CCC/b' -e d" is almost exactly the same as in one-liner #57.
The "x" command exchanges the paragraph stored in hold buffer with the pattern space. Then it tests if the pattern space (paragraph) contains "AAA", if it does, sed branches to end of script with "b" command, that happily makes sed print the paragraph. If "AAA" did not match, sed does exactly the same testing for pattern "BBB". If it again did not match, it tests for "CCC". If none of these patterns were found, sed executes the "d" command that deletes everything and restarts this one-liner.
Here is another way to do the same with GNU sed:
gsed '/./{H;$!d;};x;/AAA\|BBB\|CCC/b;d'
This one-liner is exactly the same as previous one. It just compresses the three tests for "AAA", "BBB" or "CCC" into one "/AAA\|BBB\|CCC/" as explained in one-liner #57.
61. Print only the lines that are 65 characters in length or more.
sed -n '/^.\{65\}/p'
This one-liner prints lines that are 65 characters in length or more. It does it by using a regular expression "^.{65}" that matches any 65 characters at the beginning of line. If there are less than 65 characters, the regex does not match and the line does not get printed (as automatic printing was disabled with "-n" command line option).
62. Print only the lines that are less than 65 chars.
sed -n '/^.\{65\}/!p'
This one-liner inverts the previous one. If the line matches 65 characters, then it is not printed "!p". If it does not match, it gets printed.
Another way to do the same:
sed '/^.\{65\}/d'
This one-liner deletes all lines that match 65 characters. All others implicitly get printed.
63. Print section of a file from a regex to end of file.
sed -n '/regexp/,$p'
This one-liner uses a tricky range match "/regex/,$". It matches lines starting from the first line that matches "/regex/" to the end of file "$". The "p" command prints these lines. All other lines get silently discarded.
64. Print lines 8-12 (inclusive) of a file.
sed -n '8,12p'
This is another type of range match. This range matches a section of lines between two lines numbers (inclusive). In this case it's lines [8 to 12].
sed '8,12!d'
This is the same one-liner, just written differently. It deletes lines that are outside of range [8, 12] and prints those in this range.
65. Print line number 52.
sed -n '52p'
This one-liner restricts the "p" command to line "52". Only this line gets "p"rinted.
sed '52!d'
This one-liner deletes all lines except line 52. Line 52 gets printed.
sed '52q;d'
This one is the smartest. It quits at line 52 with "q" command. The previous two one-liners would loop over all the remaining lines and do nothing. Remember from one-liner #44 that quit command prints the pattern space with it. The "d" command makes sure that no other line gets printed while sed gets to line 52.
66. Beginning at line 3, print every 7th line.
gsed -n '3~7p'
This one-liner uses a line range match extension of GNU sed. A line range in format "first~step" matches every step'th line starting from first. In this one-liner it's "3~7", meaning match every 7th line starting from 3rd. The "-n" flag prevents printing any other lines, and "p" in "3~7p" prints the matched line.
For everyone else, this one-liner works:
sed -n '3,${p;n;n;n;n;n;n;}'
This one-liner executes commands "p;n;n;n;n;n;n" for lines starting the 3rd line. The "3,$" is a line range match that restricts commands by line numbers. The "$" means end of file and "3" means 3rd line.
The "p;n;n;n;n;n;n" command prints the line, then skips 6, prints the 7th, skips 6, prints the 14th, etc. As it starts executing at line 3, the effect is - print line 3, skip 6, print line 10, skip 6, print line 17, .... That is, print every 7th line beginning at 3rd.
67. Print section of lines between two regular expressions (inclusive).
sed -n '/Iowa/,/Montana/p'
This one-liner prints all the lines between the first line that matches a regular expression "Iowa" and the first line that matches a regular expression "Montana".
It uses a range match "/start/,/finish/" that matches all lines starting from a line that matches "start" and ending with the first line that matches "finish".
An Important Comment About Ranges!
I have an important comment about ranges. Ranges in form "/start/,/finish/" always match 2 lines or more. If "/finish/" is on the same line as "/start/" it will not work. Please see the Sed FAQ 3.3 for more details.
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