CODEWITHSUNDEEP
regexbashconditional-statementsnegate
David Rogers
David Rogers

Reputation: 4280

How do I negate a test with regular expressions in a bash script?

Using GNU bash (version 4.0.35(1)-release (x86_64-suse-linux-gnu), I would like to negate a test with Regular Expressions. For example, I would like to conditionally add a path to the PATH variable, if the path is not already there, as in:

TEMP=/mnt/silo/bin
if [[ ${PATH} =~ ${TEMP} ]] ; then PATH=$PATH; else PATH=$PATH:$TEMP; fi
TEMP=/mnt/silo/Scripts:
if [[ ${PATH} =~ ${TEMP} ]] ; then PATH=$PATH; else PATH=$PATH:$TEMP; fi
TEMP=/mnt/silo/local/bin
if [[ ${PATH} =~ ${TEMP} ]] ; then PATH=$PATH; else PATH=$PATH:$TEMP; fi
export PATH

I'm sure there are a million ways to do this, but what I would like to know is if the conditional can be negated somehow, as in (the erroneous):

TEMP=/mnt/silo/bin
if ![[ ${PATH} =~ ${TEMP} ]] ; then PATH=$PATH:$TEMP; fi
TEMP=/mnt/silo/Scripts:
if ![[ ${PATH} =~ ${TEMP} ]] ; then PATH=$PATH:$TEMP; fi
TEMP=/mnt/silo/local/bin
if ![[ ${PATH} =~ ${TEMP} ]] ; then PATH=$PATH:$TEMP; fi
export PATH

Upvotes: 293

Views: 237734

Answers (5)

Dennis Williamson
Dennis Williamson

Reputation: 360143

You can also put the exclamation point inside the brackets:

if [[ ! $PATH =~ $temp ]]

but you should anchor your pattern to reduce false positives:

temp=/mnt/silo/bin
pattern="(^|:)$temp(:|$)"
if [[ ! $PATH =~ $pattern ]]

which looks for a match at the beginning or end with a colon before or after it (or both). I recommend using lowercase or mixed case variable names as a habit to reduce the chance of name collisions with shell variables.

Upvotes: 201

dimir
dimir

Reputation: 793

I like to simplify the code without using conditional operators in such cases:

TEMP=/mnt/silo/bin
[[ ${PATH} =~ ${TEMP} ]] || PATH=$PATH:$TEMP

Upvotes: 7

none of your business
none of your business

Reputation: 291

the safest way is to put the ! for the regex negation within the [[ ]] like this: 

if [[ ! ${STR} =~ YOUR_REGEX ]]; then

otherwise it might fail on certain systems.

Upvotes: 29

Mark Byers
Mark Byers

Reputation: 838376

Yes you can negate the test as SiegeX has already pointed out.

However you shouldn't use regular expressions for this - it can fail if your path contains special characters. Try this instead:

[[ ":$PATH:" != *":$1:"* ]]

(Source)

Upvotes: 11

SiegeX
SiegeX

Reputation: 140377

You had it right, just put a space between the ! and the [[ like if ! [[

Upvotes: 339

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