Simple logical operators in Bash

Question

I have a couple of variables and I want to check the following condition  written out in words  then my failed attempt at bash scripting    if varA EQUALS 1 AND   varB EQUALS  t1  OR varB EQUALS  t2    then   do something  done    And in my failed attempt  I came up with   if    varA    1    amp  amp       varB     t1          varC     t2          then     scale 0 05   fi

User · Answer

A very portable version  even to legacy bourne shell    if     varA    1 -a      varB     t1  -o   varB     t2       then    do-something fi   This has the additional quality of running only one subprocess at most  which is the process     whatever the shell flavor   Replace   with -eq if variables contain numeric values  e g    3 -eq 03 is true  but 3   03 is false   string comparison

User · Answer

What you ve written actually almost works  it would work if all the variables were numbers   but it s not an idiomatic way at all          parentheses indicate a subshell  What s inside them isn t an expression like in many other languages  It s a list of commands  just like outside parentheses   These commands are executed in a separate subprocess  so any redirection  assignment  etc  performed inside the parentheses has no effect outside the parentheses    With a leading dollar sign         is a command substitution  there is a command inside the parentheses  and the output from the command is used as part of the command line  after extra expansions unless the substitution is between double quotes  but that s another story            braces are like parentheses in that they group commands  but they only influence parsing  not grouping  The program x 2    x 4     echo  x prints 4  whereas x 2   x 4   echo  x prints 2   Also braces require spaces around them and a semicolon before closing  whereas parentheses don t  That s just a syntax quirk     With a leading dollar sign    VAR  is a parameter expansion  expanding to the value of a variable  with possible extra transformations           double parentheses surround an arithmetic instruction  that is  a computation on integers  with a syntax resembling other programming languages  This syntax is mostly used for assignments and in conditionals    The same syntax is used in arithmetic expressions           which expand to the integer value of the expression             double brackets surround conditional expressions  Conditional expressions are mostly built on operators such as -n  variable to test if a variable is empty and -e  file to test if a file exists  There are also string equality operators    string1       string2   beware that the right-hand side is a pattern  e g      foo    a     tests if  foo starts with a while     foo     a      tests if  foo is exactly a    and the familiar     amp  amp  and    operators for negation  conjunction and disjunction as well as parentheses for grouping  Note that you need a space around each operator  e g       x       y      not      x     y       and a space or a character like   both inside and outside the brackets  e g     -n  foo     not   -n  foo             single brackets are an alternate form of conditional expressions with more quirks  but older and more portable   Don t write any for now  start worrying about them when you find scripts that contain them    This is the idiomatic way to write your test in bash   if     varA    1  amp  amp    varB     t1      varC     t2       then   If you need portability to other shells  this would be the way  note the additional quoting and the separate sets of brackets around each individual test  and the use of the traditional   operator rather than the ksh bash zsh    variant    if     varA    1    amp  amp        varB     t1           varC     t2        then

User · Answer

if     NUM1    1         NUM2    1     amp  amp    -z   STR    then     echo STR is empty but should have a value  fi

User · Answer

Here is the code for the short version of if-then-else statement        a -eq 1         b -eq 2      amp  amp  echo  ok     echo  nok    Pay attention to the following       and  amp  amp  operands inside if condition  i e  between round parentheses  are logical operands  or and     and  amp  amp  operands outside if condition mean then else   Practically the statement says   if  a 1 or b 2  then  ok  else  nok

User · Answer

very close  if     varA -eq 1     amp  amp      varB     t1      varC     t2         then      scale 0 05   fi   should work   breaking it down       varA -eq 1       is an integer comparison where as    varB     t1    is a string comparison   otherwise  I am just grouping the comparisons correctly   Double square brackets delimit a Conditional Expression  And  I find the following to be a good reading on the subject    IBM  Demystify test             and if-then-else

[bash] Simple logical operators in Bash

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