[bash] Ternary operator (?:) in Bash

Is there a way to do something like this

int a = (b == 5) ? c : d;

using Bash?

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 ))

The top answer [[ $b = 5 ]] && a="$c" || a="$d" should only be used if you are certain there will be no error after the &&, 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 "$1" | grep -oP '{ .*? }'))
    IFS=$pIFS
    s="$1"
    for ((i=0; i < ${#results[@]}; i++)); do
        s="${s/${results[$i]}/'\0'}"
        results[$i]="${results[$i]:2:${#results[$i]}-3}"
        results[$i]=$(echo ${results[$i]%%";"*})
    done
    s="$s&&"
    let cmdParts[t]=0
    while :; do
        i=${cmdParts[t]}
        let cmdParts[$i,t]=0
        s1="${s%%"&&"*}||"
        while :; do
            j=${cmdParts[$i,t]}
            let cmdParts[$i,$j,t]=0
            s2="${s1%%"||"*};"
            while :; do
                cmdParts[$i,$j,${cmdParts[$i,$j,t]}]=$(echo ${s2%%";"*})
                s2=${s2#*";"}
                let cmdParts[$i,$j,t]++
                [[ $s2 ]] && continue
                break
            done
            s1=${s1#*"||"}
            let cmdParts[$i,t]++
            [[ $s1 ]] && continue
            break
        done
        let cmdParts[t]++
        s=${s#*"&&"}
        [[ $s ]] && continue
        break
    done
    declare lastError=0
    declare skipNext=false
    for ((i=0; i < ${cmdParts[t]}; i++ )) ; do
        let j=0
        while :; do
            let k=0
            while :; do
                if $skipNext; then
                    skipNext=false
                else
                    if [[ "${cmdParts[$i,$j,$k]}" == "\0" ]]; then
                         executeCmds "${results[0]}" && lastError=0 || lastError=1
                         results=("${results[@]:1}")
                    elif [[ "${cmdParts[$i,$j,$k]:0:1}" == "!" || "${cmdParts[$i,$j,$k]:0:1}" == "-" ]]; then
                        [ ${cmdParts[$i,$j,$k]} ] && lastError=0 || lastError=1
                    else
                        ${cmdParts[$i,$j,$k]}
                        lastError=$?
                    fi
                    if (( k+1 < cmdParts[$i,$j,t] )); then
                        skipNext=false
                    elif (( j+1 < cmdParts[$i,t] )); then
                        (( lastError==0 )) && skipNext=true || skipNext=false
                    elif (( i+1 < cmdParts[t] )); then
                        (( lastError==0 )) && skipNext=false || skipNext=true
                    fi
                fi
                let k++
                [[ $k<${cmdParts[$i,$j,t]} ]] || break
            done
            let j++
            [[ $j<${cmdParts[$i,t]} ]] || break
        done
    done
    return $lastError
}

function t()
{
    declare commands="$@"
    find="$(echo ?)"
    replace='?'
    commands="${commands/$find/$replace}"
    readarray -d '?' -t statement <<< "$commands"
    condition=${statement[0]}
    readarray -d ':' -t statement <<< "${statement[1]}"
    success="${statement[0]}"
    failure="${statement[1]}"
    executeCmds "$condition" || { executeCmds "$failure"; return; }
    executeCmds "$success"
}

executeCmds separates each command individually, apart from the ones that should be skipped due to the && and || operators. It uses [] whenever a command starts with ! or a flag.

There are two ways to pass commands to it:

1. Pass the individual commands unquoted but be sure to quote ;, &&, and || operators.

t ls / ? ls qqq '||' echo aaa : echo bbb '&&' ls qq

2. Pass all the commands quoted:

t 'ls /a ? ls qqq || echo aaa : echo bbb && ls qq'

NB I found no way to pass in && 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=$IFS
    IFS="?"
    read condition success <<< "$@"
    IFS=":"
    read success failure <<< "$success"
    IFS=$pIFS
    eval "$condition" || { eval "$failure" ; return; }
    eval "$success"
}
t ls / ? ls qqq '||' echo aaa : echo bbb '&&' ls qq
t 'ls /a ? ls qqq || echo aaa : echo bbb && ls qq'

(ping -c1 localhost&>/dev/null) && { echo "true"; } || {  echo "false"; }

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.

There's also a very similar but simpler syntax for ternary conditionals in bash:

a=$(( b == 5 ? 123 : 321  ))

(( a = b==5 ? c : d )) # string + numeric

You can use this if you want similar syntax

a=$(( $((b==5)) ? c : d ))

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=$(if [ "$b" -eq 5 ]; then echo "$c"; else echo "$d"; fi)

Test with String comparison

a=$(if [ "$b" = "5" ]; then echo "$c"; else echo "$d"; fi)

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

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 "$var1" '>' "$var2"`

[ $b == 5 ] && { a=$c; true; } || a=$d

This will avoid executing the part after || by accident when the code between && and || fails.

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="$([[ $b -eq 5 ]] && echo "$c")"
$ echo $a

$ b=5
$ a="$([[ $b -eq 5 ]] && echo "$c")"
$ echo $a
it was five

Code:

a=$([ "$b" == 5 ] && echo "$c" || echo "$d")

if [ "$b" -eq 5 ]; then a="$c"; else a="$d"; fi

The cond && 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.

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 ] && echo VALUE_A || echo VALUE_B`

Just a thought. :)

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 "Parameter Expansion".

We can use following three ways in Shell Scripting for ternary operator :

    [ $numVar == numVal ] && resVar="Yop" || resVar="Nop"

Or

    resVar=$([ $numVar == numVal ] && echo "Yop" || echo "Nop")

Or

    (( numVar == numVal ? (resVar=1) : (resVar=0) ))

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 "The result is $RESULT"

tern

function show_help()
{
  echo ""
  echo "usage: BOOLEAN VALUE_IF_TRUE VALUE_IF_FALSE {FALSE_VALUE}"
  echo ""
  echo "e.g. "
  echo ""
  echo "tern 1 YES NO                            => YES"
  echo "tern 0 YES NO                            => NO"
  echo "tern "" YES NO                           => NO"
  echo "tern "ANY STRING THAT ISNT 1" YES NO     => NO"
  echo "ME=$(tern 0 YES NO)                      => ME contains NO"
  echo ""

  exit
}

if [ "$1" == "help" ]
then
  show_help
fi
if [ -z "$3" ]
then
  show_help
fi

# Set a default value for what is "false" -> 0
FALSE_VALUE=${4:-0}

function main
{
  if [ "$1" == "$FALSE_VALUE" ]; then
    echo $3
    exit;
  fi;

  echo $2
}

main "$1" "$2" "$3"