Is there a way to do something like this
int a = (b == 5) ? c : d;
using Bash?
Is there a way to do something like this
int a = (b == 5) ? c : d;
using Bash?
ternary operator ? :
is just short form of if/else
case "$b" in
5) a=$c ;;
*) a=$d ;;
esac
Or
[[ $b = 5 ]] && a="$c" || a="$d"
Code:
a=$([ "$b" == 5 ] && echo "$c" || echo "$d")
If the condition is merely checking if a variable is set, there's even a shorter form:
a=${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".
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.
(( a = b==5 ? c : d )) # string + numeric
[ $b == 5 ] && { a=$c; true; } || a=$d
This will avoid executing the part after || by accident when the code between && and || fails.
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. :)
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.
We can use following three ways in Shell Scripting for ternary operator :
[ $numVar == numVal ] && resVar="Yop" || resVar="Nop"
Or
resVar=$([ $numVar == numVal ] && echo "Yop" || echo "Nop")
Or
(( numVar == numVal ? (resVar=1) : (resVar=0) ))
There's also a very similar but simpler syntax for ternary conditionals in bash:
a=$(( b == 5 ? 123 : 321 ))
The following seems to work for my use cases:
$ 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 $RESULT"
function show_help()
{
echo ""
echo "usage: BOOLEAN VALUE_IF_TRUE VALUE_IF_FALSE {FALSE_VALUE}"
echo ""
echo "e.g. "
echo ""
echo "tern 1 YES NO => YES"
echo "tern 0 YES NO => NO"
echo "tern "" YES NO => NO"
echo "tern "ANY STRING THAT ISNT 1" YES NO => NO"
echo "ME=$(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" "$3"
Here's a general solution, that
Test with numerical comparison
a=$(if [ "$b" -eq 5 ]; then echo "$c"; else echo "$d"; fi)
Test with String comparison
a=$(if [ "$b" = "5" ]; then echo "$c"; else echo "$d"; fi)
You can use this if you want similar syntax
a=$(( $((b==5)) ? c : d ))
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"`
brew list | grep -q bat && echo 'yes' || echo 'no'
This example will determine if you used homebrew
to install bat
or not yet
If true you will see "yes"
If false you will see "no"
I added the -q
to suppress the grep
ped string output here, so you only see "yes" or "no"
doSomethingAndCheckTruth && echo 'yes' || echo 'no'
Tested with bash
and zsh
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
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 $(( expression ))
A string-oriented alternative, that uses an array:
spec=(IGNORE REPLACE)
for p in {13..15}; do
echo "$p: ${spec[p==14]}";
done
which outputs:
13: IGNORE
14: REPLACE
15: IGNORE
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:
;
, &&
, and ||
operators.t ls / ? ls qqq '||' echo aaa : echo bbb '&&' ls qq
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'