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

Question

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

Revolutionize

Answer 1

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

Chaffer

Oye vey! Just when I safely sidestep the intergalactic special character madness of perl, I find myself lost in bash space (placement)! (I feel fear squeezing my gut like a python.) Thanks! – Revolutionize

Are you sure that it's always before [[ and not inside, like if [[ ! "$value" =~ $regex ]];? For example, Sublime Text highlights it weirdly if outside: i.imgur.com/AQWuFtf.png – Selfknowledge

intelliJ gives this warning; You are missing a required space after the !. See SC1035. – Renz

@RichardTylerMiles this answer does have the space after the '!'. You can also put the '!' inside the double-brackets, I just prefer it outside. – Chaffer

Answer 2

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.

Itin

Ah, thanks for the reminder about anchoring. The idea of using lowercase or mixed variable names is confusing to a bash beginner, since the advice I have seen so far is to use uppercase. I understand the point you are making, but I have not seen enough bash scripting examples to feel comfortable deviating from (my understanding of) the cookbook. – Revolutionize

@anyoneis trust us on this one. Use of user-defined uppercase variables should be avoided. All variables in bash are expanded with $ so there is no reason to uppercase them to make them stand out. – Chaffer

I find if [[ ! $foo =~ bar ]] safer than if ! [[ $foo =~ bar ]], because it makes easier to introduce more conditions to the if – Northman

Answer 3

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.

Insolent

Answer 4

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)

Operation

Another reason to use this form it that it won't accidentally match substrings (e.g. fail to add "/bin" to the path because "/usr/bin" is already there). – Compotation

It took me a while to understand how the colons on the left gave me the anchoring I wanted. the idea of reducing the pattern by adding material to the string to be searched is worth remembering. I don't understand why special characters in the path would disrupt the regex solution but not the bash pattern solution. Can you give me an example? – Revolutionize

I don't think this will work reliably in all cases. Regex matching in superior IMHO – Epizoon

Answer 5

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

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

Fogarty