'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
regexbashconditionalnegate


Solution 1:[1]

You can also put the exclamation mark 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.

Solution 2:[2]

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.

Solution 3:[3]

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)

Solution 4:[4]

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

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

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 Community
Solution 2 Guildencrantz
Solution 3 Community
Solution 4 dimir

Related Questions