标签：Linux

1、command &>filename redirects both the stdout and the stderr of command to

    filename.

1 if cmp a b &> /dev/null  # Suppress output.        2 then echo "Files a and b are identical."        3 else echo "Files a and b differ."        4 fi

2、|

    pipe. Passes the output of previous command to the input of the next one,

    or to the shell. This is a method of chaining commands together.

12 word=Linux       13 letter_sequence=inu       14 if echo "$word" | grep -q "$letter_sequence"       15 # The "-q" option to grep suppresses output.       16 then       17   echo "$letter_sequence found in $word"       18 else       19   echo "$letter_sequence not found in $word"       20 fi

3、这个嵌套结构有点怪,还有(())和[[]]的有什么区别?

  它的结构是：

  if ()

    if()

    then()

    else()

    endif

  then()

  endif

  总觉得不如这样：

  if ()

  then

    if()

    then()

    else()

    endif

    ()

  endif

 · An if/then construct can contain nested comparisons and tests.

1 if echo "Next *if* is part of the comparison for the first *if*."        2        3   if [[ $comparison = "integer" ]]        4     then (( a < b ))        5   else        6     [[ $a < $b ]]        7   fi        8        9 then       10   echo '$a is less than $b'       11 fi

4、example 7-1

bash里面的测试命令真是多呀？test [] [[]] (()) let ... ，快昏了。

[0]和[1]和[-1]的测试结果都是true，不懂？

[ -n "$xyz" ] 里面的-n什么作用？(

[]就是test命令

在man test中查到如下：

[-n] STRING

              the length of STRING is nonzero

)

#!/bin/bash #  Tip: #  If you're unsure of how a certain condition would evaluate, #+ test it in an if-test. echo echo "Testing \"0\"" if [ 0 ]      # zero then   echo "0 is true." else   echo "0 is false." fi            # 0 is true. echo echo "Testing \"1\"" if [ 1 ]      # one then   echo "1 is true." else   echo "1 is false." fi            # 1 is true. echo echo "Testing \"-1\"" if [ -1 ]     # minus one then   echo "-1 is true." else   echo "-1 is false." fi            # -1 is true. echo echo "Testing \"NULL\"" if [ ]        # NULL (empty condition) then   echo "NULL is true." else   echo "NULL is false." fi            # NULL is false. echo echo "Testing \"xyz\"" if [ xyz ]    # string then   echo "Random string is true." else   echo "Random string is false." fi            # Random string is true. echo echo "Testing \"\$xyz\"" if [ $xyz ]   # Tests if $xyz is null, but...               # it's only an uninitialized variable. then   echo "Uninitialized variable is true." else   echo "Uninitialized variable is false." fi            # Uninitialized variable is false. echo echo "Testing \"-n \$xyz\"" if [ -n "$xyz" ]            # More pedantically correct. then   echo "Uninitialized variable is true." else   echo "Uninitialized variable is false." fi            # Uninitialized variable is false. echo xyz=          # Initialized, but set to null value. echo "Testing \"-n \$xyz\"" if [ -n "$xyz" ] then   echo "Null variable is true." else   echo "Null variable is false." fi            # Null variable is false. echo # When is "false" true? echo "Testing \"false\"" if [ "false" ]              #  It seems that "false" is just a string. then   echo "\"false\" is true." #+ and it tests true. else   echo "\"false\" is false." fi            # "false" is true. echo echo "Testing \"\$false\""  # Again, uninitialized variable. if [ "$false" ] then   echo "\"\$false\" is true." else   echo "\"\$false\" is false." fi            # "$false" is false.               # Now, we get the expected result. #  What would happen if we tested the uninitialized variable "$true"? echo exit 0

5、[[]]比[]更加有用？

The [[ ]] construct is the more versatile Bash version of [ ]. This is the extended test command, adopted from ksh88. ... Tip Using the [[ ... ]] test construct, rather than [ ... ] can prevent many     logic errors in scripts. For example, the &&, ||, <, and > operators work     within a [[ ]] test, despite giving an error within a [ ] construct.

6、

Following an if, neither the test command nor the test brackets ( [ ] or

     [[ ]] ) are strictly necessary.

例如：

[mdx@localhost abs-exercises]$ if dir; then echo "dir success"; else echo "fail" ; fi 3    ch4-2.txt  ch7-0-1.sh  ctrl-m.sh  ex2-2.sh  ex4-5.sh  ex5-2.sh  ex7-1.sh 3]]  ch5-2.txt  ch7-1.txt   env.txt    ex2-3.sh  ex4-6.sh  ex6-1.sh  plan2.txt b]]  ch6.txt    ctrl-h.sh   ex2-1.sh   ex4-2.sh  ex4-7.sh  ex7-0.sh  plan.txt dir success [mdx@localhost abs-exercises]$

[mdx@localhost abs-exercises]$ if Dir; then echo "dir success"; else echo "fail" ; fi bash: Dir: command not found fail [mdx@localhost abs-exercises]$

7、(( 1 / 0 )) 2>/dev/null不懂？

29 (( 1 / 0 )) 2>/dev/null                          # Illegal division by 0.   30 #           ^^^^^^^^^^^   31 echo "Exit status of \"(( 1 / 0 ))\" is $?."     # 1   32   33 # What effect does the "2>/dev/null" have?   34 # What would happen if it were removed?   35 # Try removing it, then rerunning the script.

8、英语疑问:

1)The (( ... )) and let ... constructs also return an exit status of 0 if the

    arithmetic expressions they evaluate expand to a non-zero value.

2)This is in marked contrast to using the test and [ ] constructs previously discussed.

Chapter 6. Exit and Exit Status

1、$? 是脚本中最后一个命令的退出状态。

2、!在起到否定的作用时，其后面需要一个空格，如! true

3、在!用在命令行中时，起到调动最近的历史命令的作用，例如，前面你执行过 man true,

当你执行!man时，就会调用man true.

1、Example 5-2(怎么样才能显示下面这个 $escape ?)

 

54 escape=$'\033'                    # 033 is octal for escape.       55 echo "\"escape\" echoes as $escape"       56 #                                   no visible output.

2、echo -e的用法：

[mdx@localhost abs-exercises]$ echo "\n" \n [mdx@localhost abs-exercises]$ echo -e "\n" [mdx@localhost abs-exercises]$ [mdx@localhost abs-exercises]$ echo "\\" \ [mdx@localhost abs-exercises]$ echo -e "\\" \ [mdx@localhost abs-exercises]$ echo -e '\\' \ [mdx@localhost abs-exercises]$ echo  '\\' \\

3、可是说是一些非常烦人的用法，不用去记它，平常也用不着吧。

Note The behavior of \ depends on whether it is itself escaped, quoted, or

     appearing within command substitution or a here document.

1                       #  Simple escaping and quoting         2 echo \z               #  z         3 echo \\z              # \z         4 echo '\z'             # \z         5 echo '\\z'            # \\z         6 echo "\z"             # \z         7 echo "\\z"            # \z         8         9                       #  Command substitution        10 echo `echo \z`        #  z        11 echo `echo \\z`       #  z        12 echo `echo \\\z`      # \z        13 echo `echo \\\\z`     # \z        14 echo `echo \\\\\\z`   # \z        15 echo `echo \\\\\\\z`  # \\z        16 echo `echo "\z"`      # \z        17 echo `echo "\\z"`     # \z        18        19                       # Here document        20 cat <<EOF        21 \z        22 EOF                   # \z        23        24 cat <<EOF        25 \\z        26 EOF                   # \z        27        28 # These examples supplied by St?phane Chazelas.

4、\可以起到续行符的作用，文中的叙述：

The escape also provides a means of writing a multi-line command. Normally,

each separate line constitutes a different command, but an escape at the end of

a line escapes the newline character, and the command sequence continues on to

the next line.

例如：下面的dir这个命令被分成3行来写，每行一个字符：）

[mdx@localhost abs-exercises]$ d\ > i\ > r ch4-2.txt  ctrl-h.sh  ex2-1.sh  ex2-3.sh  ex4-5.sh  ex4-7.sh  plan2.txt ch5-2.txt  ctrl-m.sh  ex2-2.sh  ex4-2.sh  ex4-6.sh  ex5-2.sh  plan.txt [mdx@localhost abs-exercises]$ dir ch4-2.txt  ctrl-h.sh  ex2-1.sh  ex2-3.sh  ex4-5.sh  ex4-7.sh  plan2.txt ch5-2.txt  ctrl-m.sh  ex2-2.sh  ex4-2.sh  ex4-6.sh  ex5-2.sh  plan.txt

Chapter 5. Quoting

1、那个IFS是什么意思？

Example 5-1. Echoing Weird Variables

1 #!/bin/bash    2 # weirdvars.sh: Echoing weird variables.    3    4 var="'(]\\{}\$\""    5 echo $var        # '(]\{}$"    6 echo "$var"      # '(]\{}$"     Doesn't make a difference.    7    8 echo    9   10 IFS='\'   11 echo $var        # '(] {}$"     \ converted to space. Why?   12 echo "$var"      # '(]\{}$"   13   14 # Examples above supplied by Stephane Chazelas.   15   16 exit 0

2、

[mdx@localhost mdx]$ echo '\'' > > > ...

(回车就显示>，永远不会出现'，除非'没有在两个'之中)

Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result.

1 echo "Why can't I write 's between single quotes"    2    3 echo    4    5 # The roundabout method.    6 echo 'Why can'\''t I write '"'"'s between single quotes'    7 #    |-------|  |----------|   |-----------------------|    8 # Three single-quoted strings, with escaped and quoted single quotes between.    9   10 # This example courtesy of Stéphane Chazelas.

3、太令人难以把握了，没有规律吗？

Of more concern is the inconsistent behavior of "\" within double quotes.

bash$ echo hello\!  hello!      bash$ echo "hello\!"  hello\!          bash$ echo -e x\ty  xty      bash$ echo -e "x\ty"  x       y

4、生词：

Quoting can also suppress echo's  "appetite" for newlines.

Weird

discrete

1、

http://www.linuxsir.org/main/doc/abs/HTML/untyped.html

4.3. Bash Variables Are Untyped

Example 4-4. Integer or string?

20 c=BB34   21 echo "c = $c"            # c = BB34   22 d=${c/BB/23}             # Substitute "23" for "BB".   23                          # This makes $d an integer.   24 echo "d = $d"            # d = 2334

2、

http://www.linuxsir.org/main/doc/abs/HTML/othertypesv.html

The space allotted to the environment is limited. Creating too many environmental variables or ones that use up excessive space may cause problems.

bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`"    bash$ du  bash: /usr/bin/du: Argument list too long

3、

命令:basename(# Strips out path name info (see 'basename'))

[mdx@localhost abs-exercises]$ basename /home/mdx/abs-guide-3.7/ex15.sh

ex15.sh

4、比较：

16 if [ -n "$1" ]              # Tested variable is quoted.   17 then   18  echo "Parameter #1 is $1"  # Need quotes to escape #   19 fi

5、得到最后一个参数：

1 args=$#           # Number of args passed.    2 lastarg=${!args}    3 # Or:       lastarg=${!#}    4 #           (Thanks, Chris Monson.)    5 # Note that lastarg=${!$#} doesn't work.

6、待查

If a script expects a command line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result. One way to prevent this is to append an extra character to both sides of the assignment statement using the expected positional parameter.

1 variable1_=$1_  # Rather than variable1=$1    2 # This will prevent an error, even if positional parameter is absent.    3    4 critical_argument01=$variable1_    5    6 # The extra character can be stripped off later, like so.    7 variable1=${variable1_/_/}    8 # Side effects only if $variable1_ begins with an underscore.    9 # This uses one of the parameter substitution templates discussed later.   10 # (Leaving out the replacement pattern results in a deletion.)   11   12 #  A more straightforward way of dealing with this is   13 #+ to simply test whether expected positional parameters have been passed.   14 if [ -z $1 ]   15 then   16   exit $E_MISSING_POS_PARAM   17 fi   18   19   20 #  However, as Fabian Kreutz points out,   21 #+ the above method may have unexpected side-effects.   22 #  A better method is parameter substitution:   23 #         ${1:-$DefaultVal}   24 #  See the "Parameter Substition" section   25 #+ in the "Variables Revisited" chapter.

7、在Example 4-6. wh, whois domain name lookup中，域名查询的服务器是无效的，可以换成："wh-inn" ) whois $1@whois.internic.net;;

8、生词：

positional

notch

1、echo -n的作用简单的说就是不换行，像java中的System.out.print("something");

19 # In a 'for' loop (really, a type of disguised assignment):     20 echo -n "Values of \"a\" in the loop are: "     21 for a in 7 8 9 11     22 do     23   echo -n "$a "     24 done

结果：Values of "a" in the loop are: 7 8 9 11

2、

下面这段的“!”不懂：

  

10 a=`echo Hello!`   # Assigns result of 'echo' command to 'a'     11 echo $a     12 #  Note that including an exclamation mark (!) within a     13 #+ command substitution construct #+ will not work from the command line,    14 #+ since this triggers the Bash "history mechanism."     15 #  Inside a script, however, the history functions are disabled.

3、``相当于$()

2 R=$(cat /etc/redhat-release)

4、英语生词：

disguised

naked

trailing

1、双引号内的变量可以保留变量值当中的超过一个空格的空格：

(abs-guide-3.7/HTML/variables.html#VARSUBN)

Example 4-1. Variable assignment and substitution

33 hello="A B  C   D"   34 echo $hello   # A B C D   35 echo "$hello" # A B  C   D   36 # As you see, echo $hello   and   echo "$hello"   give different results.   37 #                                      ^      ^   38 # Quoting a variable preserves whitespace.

2、可以在一行中对多个变量赋值，但是在有可能引起版本兼容性问题：

(abs-guide-3.7/HTML/variables.html#VARSUBN)

Example 4-1. Variable assignment and substitution

57 #  It is permissible to set multiple variables on the same line,   58 #+ if separated by white space.   59 #  Caution, this may reduce legibility, and may not be portable.   60   61 var1=21  var2=22  var3=$V3   62 echo   63 echo "var1=$var1   var2=$var2   var3=$var3"   64   65 # May cause problems with older versions of "sh".

3、在没有初始化的变量上进行算术运算不是非法的，当作是"0"，但是在有可能引起版本兼容性问题：

(abs-guide-3.7/HTML/variables.html#VARSUBN)

An uninitialized variable has a "null" value - no assigned value at all (not zero!). Using a variable before assigning a value to it will usually cause problems. It is nevertheless possible to perform arithmetic operations on an uninitialized variable.    1 echo "$uninitialized"                                # (blank line)    2 let "uninitialized += 5"                             # Add 5 to it.    3 echo "$uninitialized"                                # 5    4    5 #  Conclusion:    6 #  An uninitialized variable has no value, however    7 #+ it acts as if it were 0 in an arithmetic operation.    8 #  This is undocumented (and probably non-portable) behavior.

read echo >$2 不懂

(http://www.linuxsir.org/main/doc/abs/HTML/special-chars.html)

Ctl-M

Carriage return.

   1 #!/bin/bash

   2 # Thank you, Lee Maschmeyer, for this example.

   3

   4 read -n 1 -s -p $'Control-M leaves cursor at beginning of this line. Press Enter. \x0d'

   5                                   # Of course, '0d' is the hex equivalent of Control-M.

   6 echo >&2   #  The '-s' makes anything typed silent,

   7            #+ so it is necessary to go to new line explicitly.

   8

   9 read -n 1 -s -p $'Control-J leaves cursor on next line. \x0a'

  10            #  '0a' is the hex equivalent of Control-J, linefeed.

  11 echo >&2

  12

  13 ###

  14

  15 read -n 1 -s -p $'And Control-K\x0bgoes straight down.'

  16 echo >&2   #  Control-K is vertical tab.

  17

  18 # A better example of the effect of a vertical tab is:

  19

  20 var=$'\x0aThis is the bottom line\x0bThis is the top line\x0a'

  21 echo "$var"

  22 #  This works the same way as the above example. However:

  23 echo "$var" | col

  24 #  This causes the right end of the line to be higher than the left end.

  25 #  It also explains why we started and ended with a line feed --

  26 #+ to avoid a garbled screen.

  27

  28 # As Lee Maschmeyer explains:

  29 # --------------------------

  30 #  In the [first vertical tab example] . . . the vertical tab

  31 #+ makes the printing go straight down without a carriage return.

  32 #  This is true only on devices, such as the Linux console,

  33 #+ that can't go "backward."

  34 #  The real purpose of VT is to go straight UP, not down.

  35 #  It can be used to print superscripts on a printer.

  36 #  The col utility can be used to emulate the proper behavior of VT.

  37

  38 exit 0