Ternary operator in Bash

Question

Is there a way to do something like this  int a    b    5    c   d    using Bash

User · Answer

The let command supports most of the basic operators one would need   let a b  5 c d    Naturally  this works only for assigning variables  it cannot execute other commands

User · Answer

We can use following three ways in Shell Scripting for ternary operator           numVar    numVal    amp  amp  resVar  Yop     resVar  Nop   Or      resVar      numVar    numVal    amp  amp  echo  Yop     echo  Nop    Or         numVar    numVal    resVar 1     resVar 0

User · Answer

The following seems to work for my use cases  Examples   tern 1 YES NO                                                                              YES        tern 0 YES NO                                                                              NO        tern 52 YES NO                                                                             YES        tern 52 YES NO 52                                                                          NO  and can be used in a script like so  RESULT   tern 1 YES NO  echo  quot The result is  RESULT quot   tern function show help       echo  quot  quot    echo  quot usage  BOOLEAN VALUE IF TRUE VALUE IF FALSE  FALSE VALUE  quot    echo  quot  quot    echo  quot e g   quot    echo  quot  quot    echo  quot tern 1 YES NO                              gt  YES quot    echo  quot tern 0 YES NO                              gt  NO quot    echo  quot tern  quot  quot  YES NO                             gt  NO quot    echo  quot tern  quot ANY STRING THAT ISNT 1 quot  YES NO       gt  NO quot    echo  quot ME   tern 0 YES NO                         gt  ME contains NO quot    echo  quot  quot     exit    if    quot  1 quot      quot help quot    then   show help fi if   -z  quot  3 quot    then   show help fi    Set a default value for what is  quot false quot  - gt  0 FALSE VALUE   4 -0   function main     if    quot  1 quot      quot  FALSE VALUE quot     then     echo  3     exit    fi     echo  2    main  quot  1 quot   quot  2 quot   quot  3 quot

User · Answer

This is much like Vladimir s fine answer  If your  ternary  is a case of  if true  string  if false  empty   then you can simply do     c  it was five    b 3   a        b -eq 5     amp  amp  echo   c      echo  a    b 5   a        b -eq 5     amp  amp  echo   c      echo  a it was five

User · Answer

Here are some options   1- Use if then else in one line  it is possible   if      2      raiz             2             then pasta     else pasta   2   fi   2- Write a function like this      Once upon a time  there was an  iif  function in MS VB      function iif        Echoes  2 if 1 banana true etc and  3 if false null 0      case  1 in    false FALSE null NULL 0  echo  3     echo  2  esac     use inside script like this  result  iif   expr   yes   no      or even interpolating  result  iif   expr   positive   negative  because  1 is not true      3- Inspired in the case answer  a more flexible and one line use is    case   expr  in    false FALSE null NULL 0  echo  no    expr      echo  yep  expr   esac     Expression can be something like          expr  expr   var1    gt     var2

User · Answer

A string-oriented alternative  that uses an array   spec  IGNORE REPLACE  for p in  13  15   do   echo   p    spec p  14     done   which outputs   13  IGNORE 14  REPLACE 15  IGNORE

User · Answer

ternary operator     is just short form of if else  case   b  in  5  a  c        a  d    esac   Or       b   5     amp  amp  a   c     a   d

User · Answer

Code   a       b     5    amp  amp  echo   c     echo   d

User · Answer

Here s a general solution  that  works with string tests as well feels rather like an expression avoids any subtle side effects when the condition fails  Test with numerical comparison a   if    quot  b quot  -eq 5    then echo  quot  c quot   else echo  quot  d quot   fi   Test with String comparison a   if    quot  b quot     quot 5 quot     then echo  quot  c quot   else echo  quot  d quot   fi

User · Answer

ping -c1 localhost amp  gt  dev null   amp  amp    echo  true           echo  false

User · Answer

If the condition is merely checking if a variable is set  there s even a shorter form  a   VAR -20   will assign to a the value of VAR if VAR is set  otherwise it will assign it the default value 20 -- this can also be a result of an expression  This approach is technically called  quot Parameter Expansion quot

User · Answer

You can use this if you want similar syntax  a        b  5     c   d

User · Answer

Here is another option where you only have to specify the variable you re assigning once  and it  doesn t matter whether what your assigning is a string or a number   VARIABLE    test    amp  amp  echo VALUE A    echo VALUE B    Just a thought

User · Answer

b    5    amp  amp    a  c  true       a  d   This will avoid executing the part after    by accident when the code between  amp  amp  and    fails

User · Answer

to answer to   int a    b    5    c   d   just write    b 5 c 1 d 2 let a   b  5  c d   echo  a   1  b 6  c 1  d 2  let a   b  5  c d   echo  a   2   remember that   expression   is equivalent to     expression

User · Answer

The top answer     b   5     amp  amp  a  quot  c quot     a  quot  d quot  should only be used if you are certain there will be no error after the  amp  amp   otherwise it will incorrectly excute the part after the     To solve that problem I wrote a ternary function that behaves as it should and it even uses the   and   operators  Edit - new solution Here is my new solution that does not use  IFS nor ev a i l  function executeCmds         declare s s1 s2 i j k     declare -A cmdParts     declare pIFS  IFS     IFS    n      declare results    echo  quot  1 quot    grep -oP                 IFS  pIFS     s  quot  1 quot      for   i 0  i  lt     results      i      do         s  quot   s   results  i     0   quot          results  i   quot   results  i  2    results  i  -3  quot          results  i    echo   results  i    quot   quot         done     s  quot  s amp  amp  quot      let cmdParts t  0     while    do         i   cmdParts t           let cmdParts  i t  0         s1  quot   s   quot  amp  amp  quot      quot          while    do             j   cmdParts  i t               let cmdParts  i  j t  0             s2  quot   s1   quot    quot     quot              while    do                 cmdParts  i  j   cmdParts  i  j t      echo   s2   quot   quot                     s2   s2   quot   quot                   let cmdParts  i  j t                        s2     amp  amp  continue                 break             done             s1   s1   quot    quot               let cmdParts  i t                    s1     amp  amp  continue             break         done         let cmdParts t            s   s   quot  amp  amp  quot               s     amp  amp  continue         break     done     declare lastError 0     declare skipNext false     for   i 0  i  lt    cmdParts t    i        do         let j 0         while    do             let k 0             while    do                 if  skipNext  then                     skipNext false                 else                     if     quot   cmdParts  i  j  k   quot      quot  0 quot      then                          executeCmds  quot   results 0   quot   amp  amp  lastError 0    lastError 1                          results   quot   results    1  quot                       elif     quot   cmdParts  i  j  k  0 1  quot      quot   quot      quot   cmdParts  i  j  k  0 1  quot      quot - quot      then                             cmdParts  i  j  k      amp  amp  lastError 0    lastError 1                     else                           cmdParts  i  j  k                           lastError                        fi                     if    k 1  lt  cmdParts  i  j t      then                         skipNext false                     elif    j 1  lt  cmdParts  i t      then                            lastError  0     amp  amp  skipNext true    skipNext false                     elif    i 1  lt  cmdParts t      then                            lastError  0     amp  amp  skipNext false    skipNext true                     fi                 fi                 let k                       k lt   cmdParts  i  j t         break             done             let j                   j lt   cmdParts  i t         break         done     done     return  lastError    function t         declare commands  quot    quot      find  quot   echo    quot      replace         commands  quot   commands  find  replace  quot      readarray -d     -t statement  lt  lt  lt   quot  commands quot      condition   statement 0       readarray -d     -t statement  lt  lt  lt   quot   statement 1   quot      success  quot   statement 0   quot      failure  quot   statement 1   quot      executeCmds  quot  condition quot       executeCmds  quot  failure quot   return        executeCmds  quot  success quot     executeCmds separates each command individually  apart from the ones that should be skipped due to the  amp  amp  and    operators  It uses    whenever a command starts with   or a flag  There are two ways to pass commands to it   Pass the individual commands unquoted but be sure to quote     amp  amp   and    operators   t ls     ls qqq      echo aaa   echo bbb   amp  amp   ls qq   Pass all the commands quoted   t  ls  a   ls qqq    echo aaa   echo bbb  amp  amp  ls qq   NB I found no way to pass in  amp  amp  and    operators as parameters unquoted  as they are illegal characters for function names and aliases  and I found no way to override bash operators  Old solution - uses ev a i l function t         pIFS  IFS     IFS  quot   quot      read condition success  lt  lt  lt   quot    quot      IFS  quot   quot      read success failure  lt  lt  lt   quot  success quot      IFS  pIFS     eval  quot  condition quot       eval  quot  failure quot    return        eval  quot  success quot    t ls     ls qqq      echo aaa   echo bbb   amp  amp   ls qq t  ls  a   ls qqq    echo aaa   echo bbb  amp  amp  ls qq

User · Answer

if     b  -eq 5    then a   c   else a   d   fi   The cond  amp  amp  op1    op2 expression suggested in other answers has an inherent bug  if op1 has a nonzero exit status  op2 silently becomes the result  the error will also not be caught in -e mode  So  that expression is only safe to use if op1 can never fail  e g      true if a builtin  or variable assignment without any operations that can fail  like division and OS calls     Note the    quotes   The first pair will prevent a syntax error if  b is blank or has whitespace  Others will prevent translation of all whitespace into single spaces

User · Answer

a   b  5   c   d      string   numeric

User · Answer

There s also a very similar but simpler syntax for ternary conditionals in bash  a     b    5   123   321

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