Introduction to comparison symbols and special symbols in Linux shells

Shell string comparison, judging whether it is a numeric   two-dollar comparison operator, compare variables or compare numbers. Note the difference between a number and a string.   Integer comparison  -eq  equals, such as:if [  "$a"  -eq  "$b"  ] -ne  not equal to, such as: if [   $a  -ne  $b  ] -gt  greater than, such as:if [  "$a"  -gt  "$b"  ] -ge   greater than equals, such as:if [  "$a"  -ge  "$b"  ] -lt  less than, such as:if [  "$a"  -lt   "$b"  ] -le  less than equals, such as:if [  "$a"  -le  "$b"  ] <  less than (requires double brackets), such as: (("$a"  <  "$b")  <=  less than equals (requires double brackets), such as: (("$a"  <=  "$b")  >  greater than ( Double brackets are required), such as: (("$a"  >  "$b")  >=  greater than equals (requires double brackets), such as: (("$a"  >=  "$b")   string comparison  =  equals, such as:if [  "$a"  =  "$b"  ] ==  equals, such as:if [  "$a"  ==   "$b" &NBSP;], and = equivalent   NOTE: the function of = = is different in [[]] and [], as follows: 1 [[  $a  == z* ]] #   If $ A starts with "Z" (pattern match) then it will be true 2 [[  $a  ==  "z*"  ]] #  if $ A equals z* (character match), then the result is true 3 4 [  $a  == z* ] # file  globbing  and word splitting will occur  5 [  "$a"  ==  "z*"  ] #  if $ A is equal to z* (character match), then the result is true  a little explanation, about file globbing is a shorthand for the document, such as "*.c" is, again like .  but file  Globbing is not a strict regular expression, although in most cases the structure is less like . != , such as:if [  "$a"  !=  "$b"  ]  This operator will use pattern matching in the [[]] structure . <  less than, in ASCII alphabetical order. such as: if [[  "$a"  <  "$b" &NBSP;]  if [  "$a"  \<  "$b"  ]  Note: in [] structure "<" needs to be escaped . >  greater than, In ASCII alphabetical order. such as: if [[  "$a"  >  "$b"  ]] if [  "$a"  \>  "$b"  ]  Note: in [] structure ">" needs to be escaped .  specific reference example 26-11 to see an example of this operator's application . -z  string "null". That is, the length is 0. -n  string is not "null"   note:  using-n in the [] structure must be used "" to cause the variable to be used. Use a string that is not "" with! -z  or just unused "" The referenced string itself, placed in the [] structure. Although it is generally possible to   work, it is unsafe. Accustomed to using "" toTesting strings is a good habit . awk  ' {print $2} '  class.txt | grep  ' ^[0-9. '  > res shell Comparison of numbers and calculation   comparison:  method one: if [ ${a} -lt ${b} ];  then ...  This is the most basic comparison method, using LT (less than), GT (greater than), le (less than equals), GE (greater than equals), advantages: not found; disadvantage: can only compare integers, using LT,GT and other non-intuitive   method two:  if  ((${a} < ${b}))  then ...  This is Cshell style comparison, Pros: Don't use LT,GT and other hard-to-remember strings ; disadvantage: Still can only compare integer   method three: if  (echo ${a} ${b} | awk  '! $1>$2) {exit 1} ')  then ...  this is the advantage of using awk comparison: you can compare decimals; disadvantage: the expression is too complex, difficult to remember   method four: if  (Echo  ${A} - ${B} | bc -q | grep -q  "^-");  then    This is a comparison using BC calculations, advantages: you can compare decimals; Cons: more complex expressions, difficult to remember   calculate:  method One: typeset c=$ (expr ${a} + ${b});  shell Basic Tools, advantages: Convenient to detect whether the variable is a number; disadvantage: only integers can be computed, and only add and subtract can be computed, multiplication method   method two:let  "C=${a}+${b}";  or  let is not calculated   "c=a+b"   embedded command calculation, advantages: Can calculate multiplication method and bit arithmetic, etc. disadvantage: Only integer   method three:typeset c=$ ((a+b))  cshell style calculation, advantages: Can calculate multiplication method and bit operation, introduction, easy to write; disadvantage: cannot calculate decimal   method four: Typeset c=${echo ${a} ${b } | awk  ' {print $1+$2} ')   using awk calculation, the advantage: can calculate the decimal, can achieve a variety of calculation methods, computational flexibility; disadvantage: the expression is too complex   method five: typeset &NBSP;C=${ECHO&NBSP;${A}&NBSP;+&NBSP;${B}&NBSP;|&NBSP;BC&NBSP;-Q)   The advantage of using AWK calculation: the ability to calculate decimals, more computing than awk, flexible computing ; Disadvantage: The expression is too complex, the number of digits after the decimal point must be set using Scale=n, otherwise the result may be truncated to integers   special characters   symbols   Use  ;  general situation we output a command to press a carriage return, If you want to execute multiple commands on one line, the middle can be used; number split  cd /home ; ls *  denotes any character (regular)  ?  any character  [abc]   One of the list items  [^abc]  for list take non   can also use range  [a-z] [0-9] [a-z] (all characters and numbers)  {}  loop list  touch_{1,2,3} will be established Touch_1,touch_2,touch_3 loop out the three files, will also use the  echo ${ab}c ~ home directory cd ~   (normal calls into the home directory of the user's own directory)  $  extract variable values   '  $ ()   command replace touch  ' date +%f_\ ' date  +%t\ '  touch $ (date +%f_$ (date +%t))  $[]  integer calculation  echo $[2+3] -  * / %&nbSP; floating point  echo  "SCALE=3;&NBSP;10/3"  | bc -l  (BC for calculation)  \  escaping the following string   echo \\  output \  Escape special characters to prevent the shell from interpreting special characters in bash   " "   with a space string   spaces as part of a string  echo   "abc xyz"  echo  ' abc xyz '   '   command replacement   command execution Results  $ ()   Ibid, but it makes up ' Nested defects  @  No special meanings  #  annotations (general programming needs to be annotated to let other team members know about their own program functions)  $  variable Value  $ ()   command substitution  ${}  variable name range  %  Kill background often jobs number, modulo operation (everyone should not be unfamiliar with the model)  ^  take non   and  ! Duplicate  &  process, && for logic and  *  match arbitrary strings; compute multiplication   ()   sub-process execution  -  minus, interval ,cd -  back to Upper directory, kill current jobs _  (underscore) no special meaning  +  plus;  kill current jobs (process)  =  assignment  |   Piping, | |   Logic or  \  escape   when some special symbols such as $ are a variable that need to be escaped before Bash Parsing  {}  command list  {ls;cd /;}  []  character wildcard, [] also used for test commands  :  null command   Truth  ;  command Terminator   "  soft citation  "   Hard lead   <  input redirection  >  output redirection  >&  Merge 2 and 1 output  ,  enum delimiter  .  current directory  /  directory delimiter  ?  single character   enter   command execution 

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