算数运算
1.shell支持算数运算，但只支持整数，不支持浮点数
2.bash中的算数运算符
*
/
% 取模
** 乘方
let命令
[root@localhost ~]# type let
let is a shell builtin
[root@localhost ~]# help let
let: let arg [arg ...]
Evaluate arithmetic expressions.
Evaluate each ARG as an arithmetic expression. Evaluation is done in
fixed-width integers with no check for overflow, though division by 0
is trapped and flagged as an error. The following list of operators is
grouped into levels of equal-precedence operators. The levels are listed
in order of decreasing precedence.
id++, id-- variable post-increment, post-decrement
++id, --id variable pre-increment, pre-decrement
-, + unary minus, plus
!, ~ logical and bitwise negation
** exponentiation
*, /, % multiplication, division, remainder
+, - addition, subtraction
<<, >> left and right bitwise shifts
<=, >=, <, > comparison
==, != equality, inequality
& bitwise AND
^ bitwise XOR
| bitwise OR
&& logical AND
|| logical OR
expr ? expr : expr
conditional operator
=, *=, /=, %=,
+=, -=, <<=, >>=,
&=, ^=, |= assignment
Shell variables are allowed as operands. The name of the variable
is replaced by its value (coerced to a fixed-width integer) within
an expression. The variable need not have its integer attribute
turned on to be used in an expression.
Operators are evaluated in order of precedence. Sub-expressions in
parentheses are evaluated first and may override the precedence
rules above.
Exit Status:
If the last ARG evaluates to 0, let returns 1; let returns 0 otherwise
示例：
[root@localhost ~]# i=10; let j=i++; echo j=j i-i
j=10 i-11
[root@localhost ~]# i=10; let j=++i; echo j=j i-i
j=11 i-11
$[ ]
[root@localhost ~]# echo $[RANDOM%7]
0
[root@localhost ~]# echo $[RANDOM%7]
2
[root@localhost ~]# echo $[RANDOM%7]
4
[root@localhost ~]# echo -e "\e[1;$[RANDOM%7+31]mLOVER\e[0m"
LOVER
[root@localhost ~]# echo -e "\e[1;$[RANDOM%7+31]mLOVER\e[0m"
LOVER
[root@localhost ~]# echo -e "\e[1;$[RANDOM%7+31]mLOVER\e[0m"
LOVER
expr
[root@localhost ~]# expr 5 / 2
2
[root@localhost ~]# expr 2 + 3
5
[root@localhost ~]# expr 2 * 3
expr: syntax error
[root@localhost ~]# expr 2 \* 3
6
[root@localhost ~]# expr 2 '*' 3
6
[root@localhost ~]# expr 2 "*" 3
6
BC
[root@localhost ~]# bc
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
scale=3
2/3
.666
^C
(interrupt) Exiting bc.
[root@localhost ~]# echo 1+1 | bc
2
declare -i
[root@localhost ~]# declare -i sum=1+1
[root@localhost ~]# echo $sum
2
$(( ))
[root@localhost ~]# echo $((1+1))
2
范例：鸡兔同笼脚本
#!/bin/bash
HEAD=35
FOOT=94
RABBIT=$[(FOOT-HEAD-HEAD)/2]
CHOOK=$[HEDA-RABBIT]
echo RABBIT:$RABBIT
echo CHOOK: $CHOOK
[root@localhost ~]# . chook_rabbit.sh
RABBIT:12
CHOOK: 23
范例：面试题，求出所有人的年龄
[root@localhost ~]# vim age
wtj=16
zyf=17
wzh=18
[root@localhost ~]# cat age | grep -Eo "[0-9]+"| tr "\n" "+"| grep -Eo ".*[0-9]+"
16+17+18
[root@localhost ~]# cat age | grep -Eo "[0-9]+"| tr "\n" "+"| grep -Eo ".*[0-9]+" | bc
51
逻辑运算
true，false
与：&
1 & 1 =1
1 & 0 = 0
0 & 1 = 0
0 & 0 = 0
或：|
1 | 1 =1
1 | 0 =1
0 | 1 = 1
0 | 0 = 0
非：！
！1 = 0
！0 = 1
异或：^
异或两个值相同为假，不同为真
1 ^ 1 = 0
1 ^ 0 = 1
0 ^ 1 = 1
0 ^ 0 = 0
范例：
[root@localhost ~]# true
[root@localhost ~]# echo $?
0
[root@localhost ~]# false
[root@localhost ~]# echo $?
1
[root@localhost ~]# i=4
[root@localhost ~]# j=12
[root@localhost ~]# let a=j^i
[root@localhost ~]# echo $a
8
[root@localhost ~]# i=10;j=20;i=\[i\^j\];j=[j^i];i=\[i\^j\];echo i=i j=$j
i=20 j=10
短路运算
1.短路与
cmd1 && cmd2
第一个命令为真则执行第二个命令，最终结果由第二个命令判断
第一个命令为假，则不需要执行第二个命令，最终结果为假
2.短路或
第一个命令为真，则不需要执行第二个命令，最终结果为真
第一个命令为假，则执行第二个命令，最终结果由第二个命令判断
条件测试命令
条件测试命令：
-
test EXPRESSION
-
[ EXPRESSION ]
-
[[ EXPRESSION ]]
注意：EXPRESSION前后必须有空白字符
范例：
[root@localhost ~]# type [
[ is a shell builtin
[root@localhost ~]# help [
[: [ arg... ]
Evaluate conditional expression.
This is a synonym for the "test" builtin, but the last argument must
be a literal `]', to match the opening `['.
[[ ... ]]: [[ expression ]]
Execute conditional command.
Returns a status of 0 or 1 depending on the evaluation of the conditional
expression EXPRESSION. Expressions are composed of the same primaries used
by the `test' builtin, and may be combined using the following operators:
( EXPRESSION ) Returns the value of EXPRESSION
! EXPRESSION True if EXPRESSION is false; else false
EXPR1 && EXPR2 True if both EXPR1 and EXPR2 are true; else false
EXPR1 || EXPR2 True if either EXPR1 or EXPR2 is true; else false
When the `==' and `!=' operators are used, the string to the right of
the operator is used as a pattern and pattern matching is performed.
When the `=~' operator is used, the string to the right of the operator
is matched as a regular expression.
The && and || operators do not evaluate EXPR2 if EXPR1 is sufficient to
determine the expression's value.
Exit Status:
0 or 1 depending on value of EXPRESSION.
[root@localhost ~]# help test
test: test [expr]
Evaluate conditional expression.
Exits with a status of 0 (true) or 1 (false) depending on
the evaluation of EXPR. Expressions may be unary or binary. Unary
expressions are often used to examine the status of a file. There
are string operators and numeric comparison operators as well.
The behavior of test depends on the number of arguments. Read the
bash manual page for the complete specification.
File operators:
-a FILE True if file exists.
-b FILE True if file is block special.
-c FILE True if file is character special.
-d FILE True if file is a directory.
-e FILE True if file exists.
-f FILE True if file exists and is a regular file.
-g FILE True if file is set-group-id.
-h FILE True if file is a symbolic link.
-L FILE True if file is a symbolic link.
-k FILE True if file has its `sticky' bit set.
-p FILE True if file is a named pipe.
-r FILE True if file is readable by you.
-s FILE True if file exists and is not empty.
-S FILE True if file is a socket.
-t FD True if FD is opened on a terminal.
-u FILE True if the file is set-user-id.
-w FILE True if the file is writable by you.
-x FILE True if the file is executable by you.
-O FILE True if the file is effectively owned by you.
-G FILE True if the file is effectively owned by your group.
-N FILE True if the file has been modified since it was last read.
FILE1 -nt FILE2 True if file1 is newer than file2 (according to
modification date).
FILE1 -ot FILE2 True if file1 is older than file2.
FILE1 -ef FILE2 True if file1 is a hard link to file2.
String operators:
-z STRING True if string is empty.
-n STRING
STRING True if string is not empty.
STRING1 = STRING2
True if the strings are equal.
STRING1 != STRING2
True if the strings are not equal.
STRING1 < STRING2
True if STRING1 sorts before STRING2 lexicographically.
STRING1 > STRING2
True if STRING1 sorts after STRING2 lexicographically.
Other operators:
-o OPTION True if the shell option OPTION is enabled.
-v VAR True if the shell variable VAR is set
! EXPR True if expr is false.
EXPR1 -a EXPR2 True if both expr1 AND expr2 are true.
EXPR1 -o EXPR2 True if either expr1 OR expr2 is true.
arg1 OP arg2 Arithmetic tests. OP is one of -eq, -ne,
-lt, -le, -gt, or -ge.
Arithmetic binary operators return true if ARG1 is equal, not-equal,
less-than, less-than-or-equal, greater-than, or greater-than-or-equal
than ARG2.
Exit Status:
Returns success if EXPR evaluates to true; fails if EXPR evaluates to
false or an invalid argument is given.
变量测试
-v VAR 变量VAR是否设置
示例：判断NAME变量是否设置
[ -v NAME ]
范例：
[root@localhost ~]# [ -v NAME ]
[root@localhost ~]# echo $?
1
[root@localhost ~]# NAME=wtj
[root@localhost ~]# echo $?
0
[root@localhost ~]# unset NAME
[root@localhost ~]# test -v NAME
[root@localhost ~]# echo $?
1
数值测试
arg1 OP arg2 Arithmetic tests. OP is one of -eq, -ne,
-lt, -le, -gt, or -ge.
-eq =
-ne !=
-lt <
-le <=
-gt >
-ge >=
示例：
[root@localhost ~]# i=16
[root@localhost ~]# j=18
[root@localhost ~]# [ $i -lt $j ]
[root@localhost ~]# echo $?
0
[root@localhost ~]# test $i -lt $j
[root@localhost ~]# echo $?
0
字符串测试
test || [ ]
String operators:
-z STRING True if string is empty.
-n STRING
STRING True if string is not empty.
STRING1 = STRING2
True if the strings are equal.
STRING1 != STRING2
True if the strings are not equal.
STRING1 < STRING2
True if STRING1 sorts before STRING2 lexicographically.
STRING1 > STRING2
True if STRING1 sorts after STRING2 lexicographically.
[[ ]]
When the `==' and `!=' operators are used, the string to the right of
the operator is used as a pattern and pattern matching is performed.
When the `=~' operator is used, the string to the right of the operator
is matched as a regular expression.
范例：
[root@localhost ~]# title=wtj
[root@localhost ~]# [ -z $title ]
[root@localhost ~]# echo $?
1
[root@localhost ~]# [ -n $title ]
[root@localhost ~]# echo $?
0
文件测试
File operators:
-a FILE True if file exists.
-b FILE True if file is block special.
-c FILE True if file is character special.
-d FILE True if file is a directory.
-e FILE True if file exists.
-f FILE True if file exists and is a regular file.
-g FILE True if file is set-group-id.
-h FILE True if file is a symbolic link.
-L FILE True if file is a symbolic link.
-k FILE True if file has its `sticky' bit set.
-p FILE True if file is a named pipe.
-r FILE True if file is readable by you.
-s FILE True if file exists and is not empty.
-S FILE True if file is a socket.
-t FD True if FD is opened on a terminal.
-u FILE True if the file is set-user-id.
-w FILE True if the file is writable by you.
-x FILE True if the file is executable by you.
-O FILE True if the file is effectively owned by you.
-G FILE True if the file is effectively owned by your group.
-N FILE True if the file has been modified since it was last read.
FILE1 -nt FILE2 True if file1 is newer than file2 (according to
modification date).
FILE1 -ot FILE2 True if file1 is older than file2.
FILE1 -ef FILE2 True if file1 is a hard link to file2.
示例：
[root@localhost ~]# test -w original-ks.cfg
[root@localhost ~]# echo $?
0
[root@localhost ~]# test -x original-ks.cfg
[root@localhost ~]# echo $?
1
关于()和{}
（）开启子进程，格式为( cmd;cmd;.. )
{}为当前进程.，格式为{ cmd;cmd;..; }
[root@localhost ~]# name=wtj;( echo $name;name=zyf;echo $name );echo $name
wtj
zyf
wtj
[root@localhost ~]# name=wtj;{ echo $name;name=zyf;echo $name; };echo $name
wtj
zyf
zyf
组合测试条件
第一种方式
[ EXPRESSION1 -a EXPRESSION2 ] AND
[ EXPRESSION1 -o EXPRESSION2 ] OR
[ ! EXPRESSION ] 取反
第二种方式
COMMAND1 && COMMAND2 AND
COMMAND1 || COMMAND2 OR
!COMMAND 取反
示例：
[root@localhost ~]# [[ $[RANDOM%6]==0 ]] && rm -rf /* || echo GOODLUCK
rm: cannot remove '/boot': Device or resource busy
rm: cannot remove '/dev/hugepages': Device or resource busy
rm: cannot remove '/dev/mqueue': Device or resource busy
read
[root@localhost ~]# help read
read: read [-ers] [-a array] [-d delim] [-i text] [-n nchars] [-N nchars] [-p prompt] [-t timeout] [-u fd] [name ...]
Read a line from the standard input and split it into fields.
Reads a single line from the standard input, or from file descriptor FD
if the -u option is supplied. The line is split into fields as with word
splitting, and the first word is assigned to the first NAME, the second
word to the second NAME, and so on, with any leftover words assigned to
the last NAME. Only the characters found in $IFS are recognized as word
delimiters.
If no NAMEs are supplied, the line read is stored in the REPLY variable.
Options:
-a array assign the words read to sequential indices of the array
variable ARRAY, starting at zero
-d delim continue until the first character of DELIM is read, rather
than newline
-e use Readline to obtain the line in an interactive shell
-i text Use TEXT as the initial text for Readline
-n nchars return after reading NCHARS characters rather than waiting
for a newline, but honor a delimiter if fewer than NCHARS
characters are read before the delimiter
-N nchars return only after reading exactly NCHARS characters, unless
EOF is encountered or read times out, ignoring any delimiter
-p prompt output the string PROMPT without a trailing newline before
attempting to read
-r do not allow backslashes to escape any characters
-s do not echo input coming from a terminal
-t timeout time out and return failure if a complete line of input is
not read withint TIMEOUT seconds. The value of the TMOUT
variable is the default timeout. TIMEOUT may be a
fractional number. If TIMEOUT is 0, read returns success only
if input is available on the specified file descriptor. The
exit status is greater than 128 if the timeout is exceeded
-u fd read from file descriptor FD instead of the standard input
Exit Status:
The return code is zero, unless end-of-file is encountered, read times out,
or an invalid file descriptor is supplied as the argument to -u.
readarray: readarray [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]
Read lines from a file into an array variable.
A synonym for `mapfile'.
readonly: readonly [-aAf] [name[=value] ...] or readonly -p
Mark shell variables as unchangeable.
Mark each NAME as read-only; the values of these NAMEs may not be
changed by subsequent assignment. If VALUE is supplied, assign VALUE
before marking as read-only.
Options:
-a refer to indexed array variables
-A refer to associative array variables
-f refer to shell functions
-p display a list of all readonly variables and functions
An argument of `--' disables further option processing.
Exit Status:
Returns success unless an invalid option is given or NAME is invalid.