Regular Expressions --正则表达式官方教程（正则表达式详解）

25-03-09 10

对于想了解RegularExpressionsinGrepCommandwith10Examples--reference的读者，本文将是一篇不可错过的文章，并且为您提供关于10LinuxDIGCom

对于想了解Regular Expressions in Grep Command with 10 Examples --reference的读者，本文将是一篇不可错过的文章，并且为您提供关于10 Linux DIG Command Examples for DNS Lookup--reference、10 Questions To Make Programming Interviews Less Expensive--reference、15 Advanced PostgreSQL Commands with Examples、15 Linux Split and Join Command Examples to Manage Large Files--reference的有价值信息。

本文目录一览：

Regular Expressions in Grep Command with 10 Examples --reference
10 Linux DIG Command Examples for DNS Lookup--reference
10 Questions To Make Programming Interviews Less Expensive--reference
15 Advanced PostgreSQL Commands with Examples
15 Linux Split and Join Command Examples to Manage Large Files--reference

Regular Expressions in Grep Command with 10 Examples --reference

Regular expressions are used to search and manipulate the text,based on the patterns. Most of the Linux commands and programming languages use regular expression.

Grep command is used to search for a specific string in a file. Please refer our earlier article for .

You can also use regular expressions with grep command when you want to search for a text containing a particular pattern. Regular expressions search for the patterns on each line of the file. It simplifies our search operation.This articles is part of a 2 article series.

This part 1 article covers grep examples for simple regular expressions. The future part 2 article will cover advanced regular expression examples in grep.

Let us take the file /var/log/messages file which will be used in our examples.

Example 1. Beginning of line ( ^ )

In grep command,caret Symbol ^ matches the expression at the start of a line. In the following example,it displays all the line which starts with the Nov 10. i.e All the messages logged on November 10.

$ grep "^Nov 10" messages.1
Nov 10 01:12:55 gs123 ntpd[2241]: time reset +0.177479 s
Nov 10 01:17:17 gs123 ntpd[2241]: synchronized to LOCAL(0),stratum 10
Nov 10 01:18:49 gs123 ntpd[2241]: synchronized to 15.1.13.13,stratum 3
Nov 10 13:21:26 gs123 ntpd[2241]: time reset +0.146664 s
Nov 10 13:25:46 gs123 ntpd[2241]: synchronized to LOCAL(0),stratum 10
Nov 10 13:26:27 gs123 ntpd[2241]: synchronized to 15.1.13.13,stratum 3

The ^ matches the expression in the beginning of a line,only if it is the first character in a regular expression. ^N matches line beginning with N.

Example 2. End of the line ( $)

Character $ matches the expression at the end of a line. The following command will help you to get all the lines which ends with the word “terminating”.

$ grep "terminating.$" messages
Jul 12 17:01:09 cloneme kernel: Kernel log daemon terminating.
Oct 28 06:29:54 cloneme kernel: Kernel log daemon terminating.

From the above output you can come to kNow when all the kernel log has got terminated. Just like ^ matches the beginning of the line only if it is the first character,$ matches the end of the line only if it is the last character in a regular expression.

Example 3. Count of empty lines ( ^$ )

Using ^ and $ character you can find out the empty lines available in a file. “^$” specifies empty line.

$ grep -c  "^$" messages anaconda.log
messages:0
anaconda.log:3

The above commands displays the count of the empty lines available in the messages and anaconda.log files.

Example 4. Single Character (.)

The special Meta-character “.” (dot) matches any character except the end of the line character. Let us take the input file which has the content as follows.

$ cat input
1. first line
2. hi hello
3. hi zello how are you
4. cello
5. aello
6. eello
7. last line

Now let us search for a word which has any single character followed by ello. i.e hello,cello etc.,

$ grep ".ello" input
2. hi hello
3. hi zello how are you
4. cello
5. aello
6. eello

In case if you want to search for a word which has only 4 character you can give grep -w “….” where single dot represents any single character.

Example 5. Zero or more occurrence (*)

The special character “*” matches zero or more occurrence of the prevIoUs character. For example,the pattern ’1*’ matches zero or more ’1′.

The following example searches for a pattern “kernel: *” i.e kernel: and zero or more occurrence of space character.

$ grep "kernel: *." *
messages.4:Jul 12 17:01:02 cloneme kernel: ACPI: PCI interrupt for device 0000:00:11.0 disabled
messages.4:Oct 28 06:29:49 cloneme kernel: ACPI: PM-Timer IO Port: 0x1008
messages.4:Oct 28 06:31:06 btovm871 kernel:  sda: sda1 sda2 sda3
messages.4:Oct 28 06:31:06 btovm871 kernel: sd 0:0:0:0: Attached scsi disk sda
.
.

In the above example it matches for kernel and colon symbol followed by any number of spaces/no space and “.” matches any single character.

Example 6. One or more occurrence (\+)

The special character “\+” matches one or more occurrence of the prevIoUs character. ” \+” matches at least one or more space character.

If there is no space then it will not match. The character “+” comes under extended regular expression. So you have to escape when you want to use it with the grep command.

$ cat input hi hello hi hello how are you hihello

$ grep "hi +hello" input
hi hello
hi hello how are you

In the above example,the grep pattern matches for the pattern ‘hi’,followed by one or more space character,followed by “hello”.

If there is no space between hi and hello it wont match that. However,* character matches zero or more occurrence.

“hihello” will be matched by * as shown below.

$ grep "hi *hello" input
hi hello
hi    hello how are you
hihello
$

Example 7. Zero or one occurrence (\?)

The special character “?” matches zero or one occurrence of the prevIoUs character. “0?” matches single zero or nothing.

$ grep "hi \?hello" input
hi hello
hihello

“hi \?hello” matches hi and hello with single space (hi hello) and no space (hihello).

The line which has more than one space between hi and hello did not get matched in the above command.

Example 8.Escaping the special character (\)

If you want to search for special characters (for example: *,dot) in the content you have to escape the special character in the regular expression.

$ grep "127\.0\.0\.1"  /var/log/messages.4
Oct 28 06:31:10 btovm871 ntpd[2241]: Listening on interface lo,127.0.0.1#123 Enabled

Example 9. Character Class ([0-9])

The character class is nothing but list of characters mentioned with in the square bracket which is used to match only one out of several characters.

$ grep -B 1 "[0123456789]\+ times" /var/log/messages.4
Oct 28 06:38:35 btovm871 init: open(/dev/pts/0): No such file or directory
Oct 28 06:38:35 btovm871 last message repeated 2 times
Oct 28 06:38:38 btovm871 pcscd: winscard.c:304:SCardConnect() Reader E-Gate 0 0 Not Found
Oct 28 06:38:38 btovm871 last message repeated 3 times

Repeated messages will be logged in messages logfile as “last message repeated n times”. The above example searches for the line which has any number (0to9) followed by the word “times”. If it matches it displays the line before the matched line and matched line also.

With in the square bracket,using hyphen you can specify the range of characters. Like [0123456789] can be represented by [0-9]. Alphabets range also can be specified such as [a-z],[A-Z] etc. So the above command can also be written as

$ grep -B 1 "[0-9]\+ times" /var/log/messages.4

Example 10. Exception in the character class

If you want to search for all the characters except those in the square bracket,then use ^ (Caret) symbol as the first character after open square bracket. The following example searches for a line which does not start with the vowel letter from dictionary word file in linux.

$ grep -i  "^[^aeIoU]" /usr/share/dict/linux.words
1080
10-point
10th
11-point
12-point
16-point
18-point
1st
2

First caret symbol in regular expression represents beginning of the line. However,caret symbol inside the square bracket represents “except” — i.e match except everything in the square bracket.

http://www.thegeekstuff.com/2011/01/regular-expressions-in-grep-command/

字符 \

将下一字符标记为特殊字符、文本、反向引用或八进制转义符。例如，“n”匹配字符“n”。“\n”匹配换行符。序列“\\”匹配“\”，“\(”匹配“(”。

匹配输入字符串开始的位置。如果设置了 RegExp 对象的 Multiline 属性，^ 还会与“\n”或“\r”之后的位置匹配。

匹配输入字符串结尾的位置。如果设置了 RegExp 对象的 Multiline 属性，$ 还会与“\n”或“\r”之前的位置匹配。

零次或多次匹配前面的字符或子表达式。例如，zo* 匹配“z”和“zoo”。* 等效于 {0,}。

一次或多次匹配前面的字符或子表达式。例如，“zo+”与“zo”和“zoo”匹配，但与“z”不匹配。+ 等效于 {1,}。

零次或一次匹配前面的字符或子表达式。例如，“do(es)?”匹配“do”或“does”中的“do”。? 等效于 {0,1}。

{n}

n 是非负整数。正好匹配 n 次。例如，“o{2}”与“Bob”中的“o”不匹配，但与“food”中的两个“o”匹配。

{n,}

n 是非负整数。至少匹配 n 次。例如，“o{2,}”不匹配“Bob”中的“o”，而匹配“foooood”中的所有 o。“o{1,}”等效于“o+”。“o{0,}”等效于“o*”。

{n,m}

M 和 n 是非负整数，其中 n <= m。匹配至少 n 次，至多 m 次。例如，“o{1,3}”匹配“fooooood”中的头三个 o。'o{0,1}' 等效于 'o?'。注意：您不能将空格插入逗号和数字之间。

当此字符紧随任何其他限定符（*、+、?、{n}、{n,}、{n,m}）之后时，匹配模式是“非贪心的”。“非贪心的”模式匹配搜索到的、尽可能短的字符串，而默认的“贪心的”模式匹配搜索到的、尽可能长的字符串。例如，在字符串“oooo”中，“o+?”只匹配单个“o”，而“o+”匹配所有“o”。

匹配除“\n”之外的任何单个字符。若要匹配包括“\n”在内的任意字符，请使用诸如“[\s\S]”之类的模式。

(pattern)

匹配 pattern 并捕获该匹配的子表达式。可以使用 $0…$9 属性从结果“匹配”集合中检索捕获的匹配。若要匹配括号字符 ( )，请使用“$”或者“$”。

(?:pattern)

匹配 pattern 但不捕获该匹配的子表达式，即它是一个非捕获匹配，不存储供以后使用的匹配。这对于用“or”字符 (|) 组合模式部件的情况很有用。例如，'industr(?:y|ies) 是比 'industry|industries' 更经济的表达式。

(?=pattern)

执行正向预测先行搜索的子表达式，该表达式匹配处于匹配 pattern 的字符串的起始点的字符串。它是一个非捕获匹配，即不能捕获供以后使用的匹配。例如，'Windows (?=95|98|NT|2000)' 匹配“Windows 2000”中的“Windows”，但不匹配“Windows 3.1”中的“Windows”。预测先行不占用字符，即发生匹配后，下一匹配的搜索紧随上一匹配之后，而不是在组成预测先行的字符后。

(?!pattern)

执行反向预测先行搜索的子表达式，该表达式匹配不处于匹配 pattern 的字符串的起始点的搜索字符串。它是一个非捕获匹配，即不能捕获供以后使用的匹配。例如，'Windows (?!95|98|NT|2000)' 匹配“Windows 3.1”中的 “Windows”，但不匹配“Windows 2000”中的“Windows”。预测先行不占用字符，即发生匹配后，下一匹配的搜索紧随上一匹配之后，而不是在组成预测先行的字符后。

x|y

匹配 x 或 y。例如，'z|food' 匹配“z”或“food”。'(z|f)ood' 匹配“zood”或“food”。

[xyz]

字符集。匹配包含的任一字符。例如，“[abc]”匹配“plain”中的“a”。

[^xyz]

反向字符集。匹配未包含的任何字符。例如，“[^abc]”匹配“plain”中的“p”。

[a-z]

字符范围。匹配指定范围内的任何字符。例如，“[a-z]”匹配“a”到“z”范围内的任何小写字母。

[^a-z]

反向范围字符。匹配不在指定的范围内的任何字符。例如，“[^a-z]”匹配任何不在“a”到“z”范围内的任何字符。

匹配一个字边界，即字与空格间的位置。例如，“er\b”匹配“never”中的“er”，但不匹配“verb”中的“er”。

非字边界匹配。“er\B”匹配“verb”中的“er”，但不匹配“never”中的“er”。

\cx

匹配 x 指示的控制字符。例如，\cM 匹配 Control-M 或回车符。x 的值必须在 A-Z 或 a-z 之间。如果不是这样，则假定 c 就是“c”字符本身。

数字字符匹配。等效于 [0-9]。

非数字字符匹配。等效于 [^0-9]。

换页符匹配。等效于 \x0c 和 \cL。

换行符匹配。等效于 \x0a 和 \cJ。

匹配一个回车符。等效于 \x0d 和 \cM。

匹配任何空白字符，包括空格、制表符、换页符等。与 [ \f\n\r\t\v] 等效。

匹配任何非空白字符。与 [^ \f\n\r\t\v] 等效。

制表符匹配。与 \x09 和 \cI 等效。

垂直制表符匹配。与 \x0b 和 \cK 等效。

匹配任何字类字符，包括下划线。与“[A-Za-z0-9_]”等效。

与任何非单词字符匹配。与“[^A-Za-z0-9_]”等效。

\xn

匹配 n，此处的 n 是一个十六进制转义码。十六进制转义码必须正好是两位数长。例如，“\x41”匹配“A”。“\x041”与“\x04”&“1”等效。允许在正则表达式中使用 ASCII 代码。

\num

匹配 num，此处的 num 是一个正整数。到捕获匹配的反向引用。例如，“(.)\1”匹配两个连续的相同字符。

标识一个八进制转义码或反向引用。如果 \n 前面至少有 n 个捕获子表达式，那么 n 是反向引用。否则，如果 n 是八进制数 (0-7)，那么 n 是八进制转义码。

\nm

标识一个八进制转义码或反向引用。如果 \nm 前面至少有 nm 个捕获子表达式，那么 nm 是反向引用。如果 \nm 前面至少有 n 个捕获，则 n 是反向引用，后面跟有字符 m。如果两种前面的情况都不存在，则 \nm 匹配八进制值 nm，其中 n 和 m 是八进制数字 (0-7)。

\arameter">nml

当 n 是八进制数 (0-3)，m 和 l 是八进制数 (0-7) 时，匹配八进制转义码 nml。

\un

匹配 n，其中 n 是以四位十六进制数表示的 Unicode 字符。例如，\u00A9 匹配版权符号 (©)。

http://msdn.microsoft.com/zh-cn/library/ae5bf541(v=vs.80).aspx

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1. Simple dig Command Usage (Understand dig Output)

When you pass a domain name to the dig command,by default it displays the A record (the ip-address of the site that is queried) as shown below.In this example,it displays the A record of redhat.com in the “ANSWER SECTION” of the dig command output.

$ dig redhat.com
; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> redhat.com

;; global options: +cmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY,status: NOERROR,id: 62863

;; flags: qr rd ra; QUERY: 1,ANSWER: 1,AUTHORITY: 4,ADDITIONAL: 3
;; QUESTION SECTION:

;redhat.com.                    IN      A
;; ANSWER SECTION:

redhat.com.             37      IN      A       209.132.183.81
;; AUTHORITY SECTION:

redhat.com.             73      IN      NS      ns4.redhat.com.

redhat.com.             73      IN      NS      ns3.redhat.com.

redhat.com.             73      IN      NS      ns2.redhat.com.

redhat.com.             73      IN      NS      ns1.redhat.com.
;; ADDITIONAL SECTION:

ns1.redhat.com.         73      IN      A       209.132.186.218

ns2.redhat.com.         73      IN      A       209.132.183.2

ns3.redhat.com.         73      IN      A       209.132.176.100
;; Query time: 13 msec

;; SERVER: 209.144.50.138#53(209.144.50.138)

;; WHEN: Thu Jan 12 10:09:49 2012

;; MSG SIZE  rcvd: 164

The dig command output has the following sections:

Header: This displays the dig command version number,the global options used by the dig command,and few additional header information.
QUESTION SECTION: This displays the question it asked the DNS. i.e This is your input. Since we said ‘dig redhat.com’,and the default type dig command uses is A record,it indicates in this section that we asked for the A record of the redhat.com website
ANSWER SECTION: This displays the answer it receives from the DNS. i.e This is your output. This displays the A record of redhat.com
AUTHORITY SECTION: This displays the DNS name server that has the authority to respond to this query. Basically this displays available name servers of redhat.com
ADDITIONAL SECTION: This displays the ip address of the name servers listed in the AUTHORITY SECTION.
Stats section at the bottom displays few dig command statistics including how much time it took to execute this query

2. display Only the ANSWER SECTION of the Dig command Output

For most part,all you need to look at is the “ANSWER SECTION” of the dig command. So,we can turn off all other sections as shown below.

+nocomments – Turn off the comment lines
+noauthority – Turn off the authority section
+noadditional – Turn off the additional section
+nostats – Turn off the stats section
+noanswer – Turn off the answer section (Of course,you wouldn’t want to turn off the answer section)

The following dig command displays only the ANSWER SECTION.

$ dig redhat.com +nocomments +noquestion +noauthority +noadditional +nostats
; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> redhat.com +nocomments +noquestion +noauthority +noadditional +nostats

;; global options: +cmd

redhat.com.             9       IN      A       209.132.183.81

Instead of disabling all the sections that we don’t want one by one,we can disable all sections using +noall (this turns off answer section also),and add the +answer which will show only the answer section.

The above command can also be written in a short form as shown below,which displays only the ANSWER SECTION.

$ dig redhat.com +noall +answer
; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> redhat.com +noall +answer

;; global options: +cmd

redhat.com.             60      IN      A       209.132.183.81

3. Query MX Records Using dig -t MX

To query MX records,pass MX as an argument to the dig command as shown below.

$ dig redhat.com  MX +noall +answer
; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> redhat.com MX +noall +answer

;; global options: +cmd

redhat.com.             513     IN      MX      5 mx1.redhat.com.

redhat.com.             513     IN      MX      10 mx2.redhat.com.

You can also use option -t to pass the query type (for example: MX) as shown below.

$ dig -t MX redhat.com +noall +answer
; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> -t MX redhat.com +noall +answer

;; global options: +cmd

redhat.com.             489     IN      MX      10 mx2.redhat.com.

redhat.com.             489     IN      MX      5 mx1.redhat.com.

4. Query NS Records Using dig -t NS

To query the NS record use the type NS as shown below.

$ dig redhat.com NS +noall +answer
; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> redhat.com NS +noall +answer

;; global options: +cmd

redhat.com.             558     IN      NS      ns2.redhat.com.

redhat.com.             558     IN      NS      ns1.redhat.com.

redhat.com.             558     IN      NS      ns3.redhat.com.

redhat.com.             558     IN      NS      ns4.redhat.com.

You can also use option -t to pass the query type (for example: NS) as shown below.

$ dig -t NS redhat.com +noall +answer
; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> -t NS redhat.com +noall +answer

;; global options: +cmd

redhat.com.             543     IN      NS      ns4.redhat.com.

redhat.com.             543     IN      NS      ns1.redhat.com.

redhat.com.             543     IN      NS      ns3.redhat.com.

redhat.com.             543     IN      NS      ns2.redhat.com.

5. View ALL DNS Records Types Using dig -t ANY

To view all the record types (A,MX,NS,etc.),use ANY as the record type as shown below.

$ dig redhat.com ANY +noall +answer

; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> redhat.com ANY +noall +answer
;; global options: +cmd
redhat.com. 430 IN MX 5 mx1.redhat.com.
redhat.com. 430 IN MX 10 mx2.redhat.com.
redhat.com. 521 IN NS ns3.redhat.com.
redhat.com. 521 IN NS ns1.redhat.com.
redhat.com. 521 IN NS ns4.redhat.com.
redhat.com. 521 IN NS ns2.redhat.com.

(or) Use -t ANY

$ dig -t ANY redhat.com +noall +answer

; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> -t ANY redhat.com +noall +answer
;; global options: +cmd
redhat.com. 367 IN MX 10 mx2.redhat.com.
redhat.com. 367 IN MX 5 mx1.redhat.com.
redhat.com. 458 IN NS ns4.redhat.com.
redhat.com. 458 IN NS ns1.redhat.com.
redhat.com. 458 IN NS ns2.redhat.com.
redhat.com. 458 IN NS ns3.redhat.com.

6. View Short Output Using dig +short

To view just the ip-address of a web site (i.e the A record),use the short form option as shown below.

$ dig redhat.com +short
209.132.183.81

You can also specify a record type that you want to view with the +short option.

$ dig redhat.com ns +short
ns2.redhat.com.
ns3.redhat.com.
ns1.redhat.com.
ns4.redhat.com.

7. DNS Reverse Look-up Using dig -x

To perform a DNS reverse look up using the ip-address using dig -x as shown below

For example,if you just have an external ip-address and would like to kNow the website that belongs to it,do the following.

$ dig -x 209.132.183.81 +short
www.redhat.com.

To view the full details of the DNS reverse look-up,remove the +short option.

$ dig -x 209.132.183.81
; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> -x 209.132.183.81

;; global options: +cmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY,id: 62435

;; flags: qr rd ra; QUERY: 1,ADDITIONAL: 3
;; QUESTION SECTION:

;81.183.132.209.in-addr.arpa.   IN      PTR
;; ANSWER SECTION:

81.183.132.209.in-addr.arpa. 600 IN     PTR     www.redhat.com.
;; AUTHORITY SECTION:

183.132.209.in-addr.arpa. 248   IN      NS      ns2.redhat.com.

183.132.209.in-addr.arpa. 248   IN      NS      ns1.redhat.com.

183.132.209.in-addr.arpa. 248   IN      NS      ns3.redhat.com.

183.132.209.in-addr.arpa. 248   IN      NS      ns4.redhat.com.
;; ADDITIONAL SECTION:

ns1.redhat.com.         363     IN      A       209.132.186.218

ns2.redhat.com.         363     IN      A       209.132.183.2

ns3.redhat.com.         363     IN      A       209.132.176.100
;; Query time: 35 msec

;; SERVER: 209.144.50.138#53(209.144.50.138)

;; WHEN: Thu Jan 12 10:15:00 2012

;; MSG SIZE  rcvd: 193

8. Use a Specific DNS server Using dig @dnsserver

By default dig uses the DNS servers defined in your /etc/resolv.conf file.

If you like to use a different DNS server to perform the query,specify it in the command line as @dnsserver.

The following example uses ns1.redhat.com as the DNS server to get the answer (instead of using the DNS servers from the /etc/resolv.conf file).

$ dig @ns1.redhat.com redhat.com
; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> @ns1.redhat.com redhat.com

; (1 server found)

;; global options: +cmd

;; Got answer:

;; ->>HEADER<<- opcode: QUERY,id: 20963

;; flags: qr aa rd; QUERY: 1,ADDITIONAL: 4

;; WARNING: recursion requested but not available
;; QUESTION SECTION:

;redhat.com.                    IN      A
;; ANSWER SECTION:

redhat.com.             60      IN      A       209.132.183.81
;; AUTHORITY SECTION:

redhat.com.             600     IN      NS      ns1.redhat.com.

redhat.com.             600     IN      NS      ns4.redhat.com.

redhat.com.             600     IN      NS      ns3.redhat.com.

redhat.com.             600     IN      NS      ns2.redhat.com.
;; ADDITIONAL SECTION:

ns1.redhat.com.         600     IN      A       209.132.186.218

ns2.redhat.com.         600     IN      A       209.132.183.2

ns3.redhat.com.         600     IN      A       209.132.176.100

ns4.redhat.com.         600     IN      A       209.132.188.218
;; Query time: 160 msec

;; SERVER: 209.132.186.218#53(209.132.186.218)

;; WHEN: Thu Jan 12 10:22:11 2012

;; MSG SIZE  rcvd: 180

9. Bulk DNS Query Using dig -f (and command line)

Query multiple websites using a data file:

You can perform a bulk DNS query based on the data from a file.

First,create a sample names.txt file that contains the website that you want to query.

$ vi names.txt
redhat.com
centos.org

Next,execute dig -f as shown below,which will perform DNS query for the websites listed in the names.txt file and display the output.

$ dig -f names.txt +noall +answer
redhat.com.             60      IN      A       209.132.183.81
centos.org.             60      IN      A       72.232.194.162

You can also combine record type with the -f option. The following example displays the MX records of multiple websites that are located in the names.txt file.

$ dig -f names.txt MX +noall +answer
redhat.com.             600     IN      MX      10 mx2.redhat.com.
redhat.com.             600     IN      MX      5 mx1.redhat.com.
centos.org.             3600    IN      MX      10 mail.centos.org.

Query multiple websites from dig command line:

You can also query multiple websites from the dig command line as shown below. The following example queries MX record for redhat.com,and NS record for centos.org from the command line

$ dig redhat.com mx +noall +answer centos.org ns +noall +answer

; <<>> DiG 9.7.3-RedHat-9.7.3-2.el6 <<>> redhat.com mx +noall +answer centos.org ns +noall +answer
;; global options: +cmd
redhat.com. 332 IN MX 10 mx2.redhat.com.
redhat.com. 332 IN MX 5 mx1.redhat.com.
centos.org. 3778 IN NS ns3.centos.org.
centos.org. 3778 IN NS ns4.centos.org.
centos.org. 3778 IN NS ns1.centos.org.

10. Use $HOME/.digrc File to Store Default dig Options

If you are always trying to view only the ANSWER section of the dig output,you don’t have to keep typing “+noall +answer” on your every dig command. Instead,add your dig options to the .digrc file as shown below.

$ cat $HOME/.digrc
+noall +answer

Now anytime you execute dig command,it will always use +noall and +answer options by default. Now the dig command line became very simple and easy to read without you have to type those options every time.

$ dig redhat.com redhat.com. 60 IN A 209.132.183.81

$ dig redhat.com MX
redhat.com. 52 IN MX 5 mx1.redhat.com.
redhat.com. 52 IN MX 10 mx2.redhat.com.

Linux provides several powerful administrative tools and utilities which will help you to manage your systems effectively. If you don’t kNow what these tools are and how to use them,you Could be spending lot of time trying to perform even the basic administrative tasks. The focus of this course is to help you understand system administration tools,which will help you to become an effective Linux system administrator

10 Questions To Make Programming Interviews Less Expensive--reference

Conducting sql Expert and next one is the full stack developer you are looking for. If you have trust them blindly and invite disappointed. One of the first thing you should do is to filter candidates who claims to have certain skills e.g. sql but doesn't have them,the faster you can weed out those candidates the cheaper will be the hiring process. A is just for that purpose,it doesn't cost you much and also suitable for candidate,as they don't have to take off and come down to your office. It's flexible for both the parties. When I phone interview someone,I spent fist few minutes to listen them and then I go for my list of weed out programming question to see if candidate is good enough to spend another 30 to 40 minutes. They have saved a lot of time,where I found out that candidate having words like "Strong knowledge of Java","Exceptional in SQL" and "Programming gurus" fail to answer these simple questions. If you are a candidate and gone through couple of interviews,you might have noticed that almost all interviewers make up their minds in the first 10 minutes. The rest of the interview gives them reasons supporting said decision,but not all is lost. If you ever feel that you have messed up with your chance,try coming of some really good answers on rest of questions,if you can impress interviewer to an extent that encourage you to go deep,you may be able to change his initial decision. To get some feedback and improve upon my method,I have decided to share my list of weed out programming questions (don't bother about sharing questions,I have many similar questions on my secret question bank and you can create them easily as well). I have chosen one or two question from

decoration: underline;">10 Questions to Start Your Programming Interview

sql expert who can't write JOIN queries.,My first weed out question is asking them to describe a . They don't have to get it exactly right,I just Developer the first weed out question is to explain . Here at minimum I want then to kNow is that a GET is what you generally see in the URL and a POST is commonly what you see in HTML Forms. Again depending upon their answer,you can also further question about limitation,security and usage of GET vs POST method. This question will give you enough hint that whether they really kNow something about internet or not.,one of the popular weed out question is rather simple,how do you find a particular process and kill it? Here I expect them to tell me about ps, grep and kill. Also to gauge their level of understanding you can ask them about ps options e.g. what does a,f and e means in ps -afe command. Second level weed out question in UNIX can be about e.g. files which are greater than 2GB etc. Don't get me wrong but if a person cannot answer these question,it would be difficult for him to work in a project which has tons of process and connected to tons of other server. One counter argument question against my weed out question,I always hear that it would take just 5 minutes to learn those commands,but they fail to answer me,when I said why they didn't spent those five minutes before coming to interview.

(Object Oriented Programming),my weed out question is Here I expect slightly more than the popular deFinition of classes are blue print to create objects,yes that's correct but how do you kNow that he understood the concept and not just have mugged it,Ask him to give examples,and then cross question him on that e.g. where does object get created,who creates it etc. ,particularly when it comes to code,the most popular question to weed out the non-programming programmer is . If a programmer cannot write a Fizz-buzz in 10 to 15 minutes,he probably needs more practice and not ready yet. This is something I don't ask on phone interview but on written test I have before face to face interviews. There has been instances in past before we had a proper interview process of multiple rounds where I had literally asked Fizzbuzz,and their answer took the better part of an hour. Another weed out question in my list for programming is to have them write and ask them to optimize it. Fibonacci is very common but you would be surprise with number of programmers failing to write in using pen and paper and even on IDE. It also weed out programmers who understand recursion than who doesn't. My experience is the programmer who understand recursion are usually better than those who doesn't. This is where most of natural programmers comes in.,my weed out question is Some one may say that it slightly harsh to judge someone's XML skill with just one question,but you would agree that this is a fundamental. I kNow there are many programmer who has worked in XML and can work in XML but doesn't familiar with this fundamental but shouldn't it's their responsibility to learn fundamental like this,just working is not enough,you also need to fill your gap.,my weed out question is It's such a fundamental that I expect anyone who has worked or learned Java should know about it. Here I expect that they should mention about some tools which comes with JDK,at least javac (the Java compiler) and JVM,which actually runs every Java program. One more question in my list to weed out non Java programmer is I have hard time teaching this fundamental to couple of people and have found that if you don't kNow difference between these two,you will struggle to set-up your project,debugging and fixing those nightmarish ClassNotFoundException and NoClassDefFoundError. It's again a must kNow detail for any one who claims to work in Java. be it in Java or any other language,one of the good weed out question is asking candidate to . You can ask this question differently either by giving him a practical scenario or just asking about how to code so that deadlock doesn't happen. If you have not done many interviews,you will be surprised with how many programmers,with professional experience of 2 to 4 years fail to answer this question correctly.,the first question I ask to candidate is about ,because I believe that as a programmer you must know array,linked list,set,map and string algorithms. If you want to add another level of cushion than you can also ask about without using any library function. This will give you enough idea whether to proceed further or not.

trivia is not a good way to find a programmers,but questions which are closely related to practical experience are good way to weed out someone who claims to kNow something but not there yet. The best way to find a programmer is to sit down with them and examine their projects,or have them to pair program with you. Ask them what part are they most proud of and ask them what part they would change,why they would change it and how they would change it. Once you do this,other than personality questions their is nothing more that you need to ask to gauge their ability to program. But if you do this with 100 programmers,you are not wasting lot of your time but also your organization time and money. Before you invite programmer for face to face interview,you must ensure they deserved to be there. It's not practical to call all the guys based upon their agents claim only. Let me kNow what are you set of weed out questions,what do you ask to C, C++, Ruby, Python or JavaScript developer to check whether they deserve your time or not.Read more:

费波那契数列（：Successione di Fibonacci），又译费波拿契数、斐波那契数列、斐波那契数列、黄金分割数列。

在上，费波那契数列是以的方法来定义：

(n≧2)

用文字来说，就是费波那契数列由0和1开始，之后的费波那契系数就由之前的两数相加。首几个费波那契系数是（

）：

, , , , , , , , , , , , ……

特别指出：不是第一项，而是第零项。

源自：http://zh.wikipedia.org/wiki/%E6%96%90%E6%B3%A2%E9%82%A3%E5%A5%91%E6%95%B0%E5%88%97

15 Advanced PostgreSQL Commands with Examples

转自：http://www.thegeekstuff.com/2009/04/15-practical-postgresql-database-adminstration-commands/

1. 如何找到postgresql数据库中占空间最大的表？

$ /usr/local/pgsql/bin/psql test
Welcome to psql 8.3.7,the Postgresql interactive terminal.

Type:  \copyright for distribution terms
       \h for help with sql commands
       \? for help with psql commands
       \g or terminate with semicolon to execute query
       \q to quit

test=# SELECT relname,relpages FROM pg_class ORDER BY relpages DESC;
              relname              | relpages
-----------------------------------+----------
 pg_proc                           |       50
 pg_proc_proname_args_nsp_index    |       40
 pg_depend                         |       37
 pg_attribute                      |       30

如果你只想要最大的那个表，可以用limit参数来限制结果的数量，就像这样：

# SELECT relname,relpages FROM pg_class ORDER BY relpages DESC limit 1;
 relname | relpages
---------+----------
 pg_proc |       50
(1 row)

relname- 关系名/表名
relpages- 关系页数（默认情况下一个页大小是8kb）
pg_class- 系统表,维护着所有relations的详细信息
limit 1- 限制返回结果只显示一行

2. 如何计算postgresql数据库所占用的硬盘大小？

pg_database_size这个方法是专门用来查询数据库大小的，它返回的结果单位是字节（bytes）。:

# SELECT pg_database_size('geekdb');
pg_database_size
------------------
         63287944
(1 row)

如果你想要让结果更直观一点，那就使用**pg_size_pretty**方法，它可以把字节数转换成更友好易读的格式。

# SELECT pg_size_pretty(pg_database_size('geekdb'));
 pg_size_pretty
----------------
 60 MB
(1 row)

3. 如何计算postgresql表所占用的硬盘大小？

下面这个命令查出来的表大小是包含索引和toasted data的，如果你对除去索引外仅仅是表占的大小感兴趣，可以使用后面提供的那个命令。

# SELECT pg_size_pretty(pg_total_relation_size('big_table'));
 pg_size_pretty
----------------
 55 MB
(1 row)

如何查询不含索引的postgresql表的大小？

使用**pg_relation_size**而不是**pg_total_relation_size**方法。

# SELECT pg_size_pretty(pg_relation_size('big_table'));
 pg_size_pretty
----------------
 38 MB
(1 row)

scanf() Token	Regular Expression
%c	.
%5c	.{5}
%d	[-+]?\d+
%e, %E, %f, %g	[-+]?(\d+(\.\d*)?\|\.\d+)([eE][-+]?\d+)?
%i	[-+]?(0[xX][\dA-Fa-f]+\|0[0-7]*\|\d+)
%o	0[0-7]*
%s	\S+
%u	\d+
%x, %X	0[xX][\dA-Fa-f]+

Pattern	Text	Match
A	X
a	-
AB	X

表达式	可匹配
\\	代表 "\" 本身
\.	匹配小数点（.）本身
\Q...\E	中间的字符作为普通字符

元字符	功能	示例	匹配对象
^	行首定位符	‘^user''	匹配所有以user开头的行
$	行尾定位符	’user$''	匹配所以以user结尾的行
。	匹配一个字符	‘u.r''	匹配包含一个u,后跟一个字符，再跟一个r的行
*	匹配两个或多个前导字符	’u*ser''	匹配包含零个或多个u后，跟ser模式的行
[]	匹配一组字符中的人一个	‘[uU]ser''	匹配包含user或者User的行
[^]	匹配不在指定字符组里的字符	’[^A-S]ser‘	匹配一个不在A到S之间的字符，并且该字符后紧跟着ser的行
\<	词首定位符	’\<user‘	匹配包含以user开头的词的行
\>	词尾定位符	’user\>‘	匹配包含以user结尾的词的行
\<..\>	标记匹配到的字符	’\<user\>''	匹配包含<user>的行
{M}{M,}{M,N}	匹配重复出现的次数M次匹配出现的次数至少M次匹配出现的次数至少M次，但不超过N次	''u\{4\}u\{5\} u\{5,8\}''	匹配连续出现4个u的行匹配连续出现最少5个u的行匹配连续出现最少5个，最多8个u的行

Regular Expressions --正则表达式官方教程（正则表达式 详解）

Regular Expressions in Grep Command with 10 Examples --reference

Example 1. Beginning of line ( ^ )

Example 2. End of the line ( $)

Example 3. Count of empty lines ( ^$ )

Example 4. Single Character (.)

Example 5. Zero or more occurrence (*)

Example 6. One or more occurrence (\+)

Example 7. Zero or one occurrence (\?)

Example 8.Escaping the special character (\)

Example 9. Character Class ([0-9])

Example 10. Exception in the character class

总结

Express 相关文章

10 Linux DIG Command Examples for DNS Lookup--reference

1. Simple dig Command Usage (Understand dig Output)

2. display Only the ANSWER SECTION of the Dig command Output

3. Query MX Records Using dig -t MX

4. Query NS Records Using dig -t NS

5. View ALL DNS Records Types Using dig -t ANY

6. View Short Output Using dig +short

7. DNS Reverse Look-up Using dig -x

8. Use a Specific DNS server Using dig @dnsserver

9. Bulk DNS Query Using dig -f (and command line)

Query multiple websites using a data file:

Query multiple websites from dig command line:

10. Use $HOME/.digrc File to Store Default dig Options

10 Questions To Make Programming Interviews Less Expensive--reference

decoration: underline;">10 Questions to Start Your Programming Interview

15 Advanced PostgreSQL Commands with Examples

1. 如何找到postgresql数据库中占空间最大的表？

2. 如何计算postgresql数据库所占用的硬盘大小？

3. 如何计算postgresql表所占用的硬盘大小？

4. 如何查看postgresql表的索引？

5. 如何创建一个指定类型的索引？

6. 如何在postgresql中使用事务？

7. 如何查看postgresql数据库对某个SQL查询的执行计划？

8. 如何通过在服务端执行查询来显示执行计划？

9. 怎么生成一个序列的数字并把它们插入到一个表中？

10. 如何统计postgresql表里面的行数？

11. 如何查询表中某列**第二大**的值？

12. 如何查询表中某列**第二小**的值？

13. 如何列出postgresql数据库中基本的数据类型？

14. 如何把某一次查询的结果保存为一个文件？

15. 存储加密后的密码

原文后的评论

在第13个命令中，那个typtype='b'是什么意思？

获取第二大/小的值效率问题

COUNT(*)效率问题

15 Linux Split and Join Command Examples to Manage Large Files--reference

Linux Split Command Examples

1. Basic Split Example

2.Change the Suffix Length using -a option

3.Customize Split File Size using -b option

4. Create Split Files with Numeric Suffix using -d option

5. Customize the Number of Split Chunks using -C option

6. Avoid Zero Sized Chunks using -e option

7. Customize Number of Lines using -l option

Get Detailed information using –verbose option

Linux Join Command Examples

8. Basic Join Example

9. Join works on Sorted List

10. Ignore Case using -i option

11. Verify that Input is Sorted using –check-order option

12. Do not Check the Sortness using –nocheck-order option

13. Print Unpairable Lines using -a option

14. Print Only Unpaired Lines using -v option

15. Join Based on Different Columns from Both Files using -1 and -2 option