Keep Only Last Line Bash

This command is considered a failed experiment and unless there is enough request will be removed in future versions.Q [exit-code]This command only accepts a single address. The tail command is a simple command that by default prints the last 10 lines of a file to standard output . The most common use for tail is to follow, or continually read a log file on the command line. It is so common that you will often here Linux admins say things like "tail the logs", meaning watch the lines being written to a log file in real time. In this article we will discuss using the tail command.

We will start with the basics and move on to more advanced options like dealing with rotating log files and watching process id's. Normally, sed reads a line by reading a string of characters up to the end-of-line character . See the -b Binary command line argumentThe GNU version of sed added a feature in version 4.2.2 to use the "NULL" character instead. This can be useful if you have files that use the NULL as a record separator.

Some GNU utilities can generate output that uses a NULL instead a new line, such as "find . -print0" or "grep -lZ". This feature is useful if you are operating on filenames that might contain spaces or binary characters. This is called starting a new "cycle." The "D" command deletes the first portion of the pattern space, up to the new line character, leaving the rest of the pattern alone.

Like "d," it stops the current command and starts the command cycle over again. However, it will not print the current pattern space. If the "D" command is executed with a group of other commands in a curly brace, commands after the "D" command are ignored. The next group ofsed commands is executed, unless the pattern space is emptied. If this happens, the cycle is started from the top and a new line is read.

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. As in all GNU programs that use POSIX basic regular expressions, sed interprets these escape sequences as special characters. So, x\+ matches one or more occurrences of 'x'.abc\|def matches either 'abc' or 'def'. Note that the current pattern space is printed if auto-print is not disabled with the -n options.

The ability to return an exit code from the sed script is a GNU sed extension. If is specified, the command X will be executed only on the matched lines. Can be a single line number, a regular expression, or a range of lines . -regex regex File name matches regular expression.

This is a match on the whole pathname not just a filename.The default regular expressions understood by find are Emacs Regular Expressions. This can be changed with the -regextype option. Currently implemented types are emacs , posix-awk, posix-basic, posix-egrep and posix-extended. The "l" command prints the current pattern space. It is therefore useful in debuggingsed scripts. It also converts unprintable characters into printing characters by outputting the value in octal preceded by a "\" character.

I found it useful to print out the current pattern space, while probing the subtleties ofsed. 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. 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. Regex addresses operate on the content of the current pattern space.

If the pattern space is changed (for example with s///command) the regular expression matching will operate on the changed text. The reason is that the oldest versions of UNIX grep did not support these. The developers of grep wanted to keep it compatible with existing regular expressions, which may use these characters as literal characters. The BRE a matches a literally, while a\ matches a or aa. And \+ as an alternative syntax to \ and \, but \? E.g. \(ab\)\1 matches abab, while \1 is invalid since there's no capturing group corresponding to the backreference \1.

Options in ksh and bash can also be set using long names (e.g. -o noglob instead of -f). Scripts are not very useful if all the commands and options and filenames are explicitly coded. By using variables, you can make a script generic and apply it to different situations. As I promised earlier, here is a table that summarizes the different commands. The second column specifies if the command can have a range or pair of addresses or a single address or pattern. The next four columns specifies which of the four buffers or streams are modified by the command.

Some commands only affect the output stream, others only affect the hold buffer. If you remember that the pattern space is output (unless a "-n" was given tosed), this table should help you keep track of the various commands. In this example, the output file isn't needed, as the input was not modified. You must have exactly one space between thew and the filename.

You can also have ten files open with one instance ofsed. This allows you to split up a stream of data into separate files. Using the previous example combined with multiple substitution commands described later, you could split a file into ten pieces depending on the last digit of the first number.

You could also use this method to log error or debugging information to a special file. The "x" command exchanges the paragraph stored in hold buffer with the pattern space. Then it tests if the pattern space 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 none of these patterns were found, sed executes the "d" command that deletes everything and restarts this one-liner. This documentation frequently refers to "the" sed script; this should be understood to mean the in-order catenation of all of the scripts and script-files passed in.

This document will refer to "the" sed script; this is understood to mean the in-order concatenation of all of the scripts and script-files passed in. Given a huge file having dynamic data, write a program to read last n lines from the file at any point without reading the entire file. The problem is similar to tail command in linux which displays the last few lines of a file. It is mostly used for viewing log file updates as these updates are appended to the log files. 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. 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. 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. Then the next cycle starts for the next input line. The regular expression matches, the entire pattern space is printed with p.

No lines are printed by default due to the -n option. By default, sed reads an input line into the pattern buffer, then continues to processes all commands in order. Commands with addresses affect only matching lines. Basic and extended regular expressions are two variations on the syntax of the specified pattern. Basic Regular Expression syntax is the default in sed .

Use the POSIX-specified -E option (-r,--regexp-extended) to enable Extended Regular Expression syntax. The power of regular expressions comes from the ability to include alternatives and repetitions in the pattern. These are encoded in the pattern by the use of special characters, which do not stand for themselves but instead are interpreted in some special way. Less can be invoked with options to change its behaviour, for example, the number of lines to display on the screen. A few options vary depending on the operating system.

While less is displaying the file, various commands can be used to navigate through the file. These commands are based on those used by both more and vi. It is also possible to search for character patterns in the file. As far as I am concerned, the only time the semicolon is useful is when you want to type thesed script on the command line. If you are going to place it in a script, format it so it is readable.

I have mentioned earlier that many versions ofsed do not support comments except on the first line. You may want to write your scripts with comments in them, and install them in "binary" form without comments. I won't even tell you how to write a script to strip out comments.

Also - some operating systems do NOT let you use semicolons. So if you see a script with semicolons, and it does not work on a non-Linux system, replace the semicolon with a new line character. (As long as you are not using csh/tcsh, but that's another topic. Most commands operate on the pattern space, and subsequent commands may act on the results of the last modification. The three previous commands, like the read file command, add the new lines to the output stream, bypassing the pattern space.

You can use this to exclude part of the characters matched by the regular expression. The "\1" is the first remembered pattern, and the "\2" is the second remembered pattern. 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. The first part has an interesting pattern match "/./". 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.

Character to the end of an address specification negates the sense of the match. Character follows an address range, then only lines which do not match the address range will be selected. This also works for singleton addresses, and, perhaps perversely, for the null address. 0,/regexp/A line number of 0 can be used in an address specification like 0,/regexp/ so that sed will try to match regexp in the first input line too. Multiple lines can be processed as one buffer using theD,G,H,N,P. Unless special commands (like 'D') are used, the pattern space is deleted between two cycles.

The hold space, on the other hand, keeps its data between cycles (see commands 'h', 'H', 'x', 'g', 'G' to move data between both buffers). Regexp1\|regexp2Matches either regexp1 or regexp2. Use parentheses to use complex alternative regular expressions.

The matching process tries each alternative in turn, from left to right, and the first one that succeeds is used. 0,/regexp/A line number of 0 can be used in an address specification like0,/regexp/ so that sed will try to matchregexp in the first input line too. EThis command allows one to pipe input from a shell command into pattern space.