How to build a conditional assignment in bash?

Bash Problem Overview

I'm looking a way to build conditional assignments in bash:

In Java it looks like this:

int variable= (condition) ? 1 : 0;

Bash Solutions

Solution 1 - Bash

If you want a way to define defaults in a shell script, use code like this:

: ${VAR:="default"}

Yes, the line begins with ':'. I use this in shell scripts so I can override variables in ENV, or use the default.

This is related because this is my most common use case for that kind of logic. ;]

Solution 2 - Bash

As per Jonathan's comment:

variable=$(( 1 == 1 ? 1 : 0 ))

EDIT:

I revised the original answer which just echo'd the value of the condition operator, it didn't actually show any assignment.

Solution 3 - Bash

myvar="default" && [[ <some_condition_is_true> ]]  && myvar="non-default"

real examples:

DELIM="" && [[ "$APP_ENV_RESOLVED" != "" ]] && DELIM=$INNER_DELIM

The condition can be "(( ... ))" as well:

filepath=/proc/drbd && (( $# > 0 )) && filepath=$1

Solution 4 - Bash

In addition to the other more general answers (particularly as per Jonathan's comment and Kevin's more general answer [which also supports strings]) I'd like to add the following two solutions:

setting the variable to either `0` or `1` based on the condition:

(as the question's example suggests.)

The general form would read

(condition); variable=$?;

where $variable results in being either 0 or 1 and condition can be any valid conditional expression.

E.g. checking a variable ...

[[ $variableToCheck == "$othervariable, string or number to match" ]]
variable=$?

... or checking a file's existence ...

[ -f "$filepath" ]
fileExists=$?

... or checking the nummerical value of $myNumber:

(( myNumber >= 1000000000 ))
is_huge_number=$?

The advantages of this solution is that

it supports arbitrary conditional expressions, including strings
(which are not supported in arithmetic expressions of Jonathan's solution)
that variable gets declared in any case, unlike in griffon's answer:
[ -z "$variable" ] && variable="defaultValue"
Which would matter in case you want to nameref it later on (e.g. from within a function).

Please note: In Bash, the special variable $? always contains the exit code of the previously executed statement (or statement block; see the man bash for more details). As such, a positive result is generally represented by the value 0, not 1 (See my comment below, thanks Assimilater for pointing it out). Thus, if the condition is true (e.g [[2 eq 2]]) then $?=0.

If instead you need 0 or 1 in your variable (e.g. to print or do mathematical calculations) then you need to employ boolean negation using a leading exclamation mark (as pointed out by GypsySpellweaver in the comments below): ( ! condition ); variable=$? or ! ( condition ); variable=$?. (However, readability in terms of what happens might be a bit less obvious.)

Another possible solution by Jonathan would be variable=$(( 1 == 1 ? 1 : 0 )) - which, however, is creating a subshell.

If you want to avoid the creation of a subshel, keep good readability or have arbitrary conditions, use one of the following solutions.

setting the variable to arbitrary values:

as it is done in most other answers, it could adapted as follows:

(condition) \
    && variable=true \
    || variable=false

e.g as in

[[ $variableToCheck == "$othervariable, string or number to match" ]] \
    && variable="$valueIfTrue" \
    || variable="$valueIfFalse"

or to get 1 in a positive check, and 0 upon failure (like in the question's example):

[[ $variableToCheck == "$othervariable, string or number to match" ]] \
    && variable=1 \
    || variable=0

(for the last example, - as already mentioned in the notes above - the same can be achieved with boolean negation using a leading exclamation mark:

[[ ! $variableToCheck == "$othervariable, string or number to match" ]]
variable=$?

The advantages of this solution is that

it might be considered a bit better readable than Kevin's answer
myvar="default" && [[ <some_condition_is_true> ]] && myvar="non-default", and
the $valueIfTrue is conditionally evaluated only if needed,
which would matter in case you'd do something
with side-effect, like
- variable=$((i++)), or
- { variable=$1; shift; }
high computation, like
- variable=$(find / -type f -name ListOfFilesWithThisNameOnMySystem)
is a bit shorter than ghostdog74's answer
(which, however is great if you have multiple conditions!)
does not open a subshell as in Pierre's answer
and as above:
it supports arbitrary conditional expressions, including strings
(which are not supported in arithmetic expressions of Jonathan's solution)
that variable gets declared in any case, unlike in griffon's answer:
[ -z "$variable" ] && variable="defaultValue"
Which would matter in case you want to nameref it later on (e.g. from within a function).

Solution 5 - Bash

I wanted to do a conditional assignment with strings and I ended up with:

VAR=$([ "$MYVALUE" = "value" ] && echo "YES" || echo "NO")

Which are:

VAR=$([ condition ] && value_when_true || value_when_false)

Solution 6 - Bash

Big ups to @Demosthenex and especially @Dennis Williamson for the shortest and easiest solution I've seen. Leave it to bash to require a bunch of parentheses for a simple ternary assignment. Ahh, the 60s! And to put it all together in an example...

echo $BASHRULES;             # not defined
                             # no output
: ${BASHRULES:="SCHOOL"}     # assign the variable
echo $BASHRULES              # check it
SCHOOL                       # correct answer
: ${BASHRULES="FOREVER?"}    # notice the slightly different syntax for the conditional assignment
echo $BASHRULES              # let's see what happened!
SCHOOL                       # unchanged! (it was already defined)

I wrote that a long time ago.. these days I'd probably get more excited over a solution like...

PLATFORM=iphonesimulator
OTHERSDK=iphone && [[ $PLATFORM=~os ]]      \
                &&     OTHERSDK+=simulator  \
                ||     OTHERSDK+=os

> $OTHERSDK ⇥ iphoneos

Solution 7 - Bash

another way using a case/switch

case "$variable" in
  condition) result=1 ;;
          *) result=0 ;;
esac

Solution 8 - Bash

If you want to assign a value unless variable is empty use this:

[ -z "$variable" ] && variable="defaultValue"

You can put as well, each other condition on the []

Content Type	Original Author	Original Content on Stackoverflow
Question	Neuquino	View Question on Stackoverflow
Solution 1 - Bash	Demosthenex	View Answer on Stackoverflow
Solution 2 - Bash	Anthony Forloney	View Answer on Stackoverflow
Solution 3 - Bash	Kevin Little	View Answer on Stackoverflow
Solution 4 - Bash	Martin Rüegg	View Answer on Stackoverflow
Solution 5 - Bash	Pierre Maoui	View Answer on Stackoverflow
Solution 6 - Bash	Alex Gray	View Answer on Stackoverflow
Solution 7 - Bash	ghostdog74	View Answer on Stackoverflow
Solution 8 - Bash	griffon vulture	View Answer on Stackoverflow

How to build a conditional assignment in bash?

Bash Problem Overview

Bash Solutions

Solution 1 - Bash

Solution 2 - Bash

EDIT:

Solution 3 - Bash

Solution 4 - Bash

setting the variable to either `0` or `1` based on the condition:

The advantages of this solution is that

setting the variable to arbitrary values:

The advantages of this solution is that

Solution 5 - Bash

Solution 6 - Bash

Solution 7 - Bash

Solution 8 - Bash

Alternative to a goto statement in Java

Is it a good practice to always use smart pointers?

Attributions

Bash Problem Overview

Bash Solutions

Solution 1 - Bash

Solution 2 - Bash

EDIT:

Solution 3 - Bash

Solution 4 - Bash

setting the variable to either 0 or 1 based on the condition:

The advantages of this solution is that

setting the variable to arbitrary values:

The advantages of this solution is that

Solution 5 - Bash

Solution 6 - Bash

Solution 7 - Bash

Solution 8 - Bash

Alternative to a goto statement in Java

Is it a good practice to always use smart pointers?

Attributions

setting the variable to either `0` or `1` based on the condition: