As the title said what does the "=~" do in the bash shell script running on Linux? I googled online and found that "==" checks equality and "!=" checks inequality. How about "=~"? I guess it may be some regular expression matching?
The =~ does a bash regular expression match
Example
$ [[ 45 =~ [0-9]+ ]] && echo "45 contains digits"
45 contains digits
$ [[ "hello" =~ [0-9]+ ]] && echo "hello doesnt contains digits"
$ [[ "hello" =~ [a-z]+ ]] && echo "hello contains alphabets"
hello contains alphabets
Yes, it's regular expression matching. It's in the bash manual:
An additional binary operator, =~, is available, with the same precedence as == and !=. When it is used, the string to the right of the operator is considered an extended regular expression and matched accordingly (as in regex3)).
Related
This question already has answers here:
How to compare strings in Bash
(12 answers)
Closed 4 years ago.
I'm trying to get an if statement to work in Bash (using Ubuntu):
#!/bin/bash
s1="hi"
s2="hi"
if ["$s1" == "$s2"]
then
echo match
fi
I've tried various forms of the if statement, using [["$s1" == "$s2"]], with and without quotes, using =, == and -eq, but I still get the following error:
[hi: command not found
I've looked at various sites and tutorials and copied those, but it doesn't work - what am I doing wrong?
Eventually, I want to say if $s1 contains $s2, so how can I do that?
I did just work out the spaces bit... :/ How do I say contains?
I tried
if [[ "$s1" == "*$s2*" ]]
but it didn't work.
For string equality comparison, use:
if [[ "$s1" == "$s2" ]]
For string does NOT equal comparison, use:
if [[ "$s1" != "$s2" ]]
For the a contains b, use:
if [[ $s1 == *"$s2"* ]]
(and make sure to add spaces between the symbols):
Bad:
if [["$s1" == "$s2"]]
Good:
if [[ "$s1" == "$s2" ]]
You should be careful to leave a space between the sign of '[' and double quotes where the variable contains this:
if [ "$s1" == "$s2" ]; then
# ^ ^ ^ ^
echo match
fi
The ^s show the blank spaces you need to leave.
You need spaces:
if [ "$s1" == "$s2" ]
I suggest this one:
if [ "$a" = "$b" ]
Notice the white space between the openning/closing brackets and the variables and also the white spaces wrapping the '=' sign.
Also, be careful of your script header. It's not the same thing whether you use
#!/bin/bash
or
#!/bin/sh
Here's the source.
Bash 4+ examples. Note: not using quotes will cause issues when words contain spaces, etc. Always quote in Bash IMO.
Here are some examples Bash 4+:
Example 1, check for 'yes' in string (case insensitive):
if [[ "${str,,}" == *"yes"* ]] ;then
Example 2, check for 'yes' in string (case insensitive):
if [[ "$(echo "$str" | tr '[:upper:]' '[:lower:]')" == *"yes"* ]] ;then
Example 3, check for 'yes' in string (case sensitive):
if [[ "${str}" == *"yes"* ]] ;then
Example 4, check for 'yes' in string (case sensitive):
if [[ "${str}" =~ "yes" ]] ;then
Example 5, exact match (case sensitive):
if [[ "${str}" == "yes" ]] ;then
Example 6, exact match (case insensitive):
if [[ "${str,,}" == "yes" ]] ;then
Example 7, exact match:
if [ "$a" = "$b" ] ;then
This question has already great answers, but here it appears that there is a slight confusion between using single equal (=) and double equals (==) in
if [ "$s1" == "$s2" ]
The main difference lies in which scripting language you are using. If you are using Bash then include #!/bin/bash in the starting of the script and save your script as filename.bash. To execute, use bash filename.bash - then you have to use ==.
If you are using sh then use #!/bin/sh and save your script as filename.sh. To execute use sh filename.sh - then you have to use single =. Avoid intermixing them.
I would suggest:
#!/bin/bash
s1="hi"
s2="hi"
if [ $s1 = $s2 ]
then
echo match
fi
Without the double quotes and with only one equals.
$ if [ "$s1" == "$s2" ]; then echo match; fi
match
$ test "s1" = "s2" ;echo match
match
$
I don't have access to a Linux box right now, but [ is actually a program (and a Bash builtin), so I think you have to put a space between [ and the first parameter.
Also note that the string equality operator seems to be a single =.
This is more a clarification than an answer! Yes, the clue is in the error message:
[hi: command not found
which shows you that your "hi" has been concatenated to the "[".
Unlike in more traditional programming languages, in Bash, "[" is a command just like the more obvious "ls", etc. - it's not treated specially just because it's a symbol, hence the "[" and the (substituted) "$s1" which are immediately next to each other in your question, are joined (as is correct for Bash), and it then tries to find a command in that position: [hi - which is unknown to Bash.
In C and some other languages, the "[" would be seen as a different "character class" and would be disjoint from the following "hi".
Hence you require a space after the opening "[".
Use:
#!/bin/bash
s1="hi"
s2="hi"
if [ "x$s1" == "x$s2" ]
then
echo match
fi
Adding an additional string inside makes it more safe.
You could also use another notation for single-line commands:
[ "x$s1" == "x$s2" ] && echo match
For a version with pure Bash and without test, but really ugly, try:
if ( exit "${s1/*$s2*/0}" )2>/dev/null
then
echo match
fi
Explanation: In ( )an extra subshell is opened. It exits with 0 if there was a match, and it tries to exit with $s1 if there was no match which raises an error (ugly). This error is directed to /dev/null.
I have a string: dev/2.0 or dev/2.0-tymlez. How can I extract the string after the last - hyphen in bash? If there is no -, then the variable should be empty else tymlez and I want to store the result in $STRING. After that I would like to check the variable with:
if [ -z "$STRING" ]
then
echo "\$STRING is empty"
else
echo "\$STRING is NOT empty"
fi
Is that possible?
I recommend against calling your variable STRING. All-uppercase variables are used by the system (e.g. HOME) or the shell itself (e.g. PWD, RANDOM).
That said, you could do something like
string='dev/2.0-tymlez'
case "$string" in
*-*) string="${string##*-}";;
*) string='';;
esac
It's a bit clunky: It first checks whether there are any - at all, and if so, it removes the longest prefix matching *-; otherwise it just sets string to empty (because *- wouldn't have matched anything then).
You could use the =~ operator:
string="dev/2.0-tymlez"
[[ $string =~ -([^-]+)$ ]]; string=${BASH_REMATCH[1]}
BASH_REMATCH is a special array where the matches from [[ ... =~ ... ]] are assigned to.
You can use sed:
for string in "dev/2.0" "dev/2.0-1-2-3" "dev/2.0-tymlez"; do
string=$(sed 's/[^-]*[-]*//' <<< "${string}")
echo "string=[${string}]"
done
Result
string=[]
string=[1-2-3]
string=[tymlez]
This question already has answers here:
How to compare strings in Bash
(12 answers)
Closed 4 years ago.
I'm trying to get an if statement to work in Bash (using Ubuntu):
#!/bin/bash
s1="hi"
s2="hi"
if ["$s1" == "$s2"]
then
echo match
fi
I've tried various forms of the if statement, using [["$s1" == "$s2"]], with and without quotes, using =, == and -eq, but I still get the following error:
[hi: command not found
I've looked at various sites and tutorials and copied those, but it doesn't work - what am I doing wrong?
Eventually, I want to say if $s1 contains $s2, so how can I do that?
I did just work out the spaces bit... :/ How do I say contains?
I tried
if [[ "$s1" == "*$s2*" ]]
but it didn't work.
For string equality comparison, use:
if [[ "$s1" == "$s2" ]]
For string does NOT equal comparison, use:
if [[ "$s1" != "$s2" ]]
For the a contains b, use:
if [[ $s1 == *"$s2"* ]]
(and make sure to add spaces between the symbols):
Bad:
if [["$s1" == "$s2"]]
Good:
if [[ "$s1" == "$s2" ]]
You should be careful to leave a space between the sign of '[' and double quotes where the variable contains this:
if [ "$s1" == "$s2" ]; then
# ^ ^ ^ ^
echo match
fi
The ^s show the blank spaces you need to leave.
You need spaces:
if [ "$s1" == "$s2" ]
I suggest this one:
if [ "$a" = "$b" ]
Notice the white space between the openning/closing brackets and the variables and also the white spaces wrapping the '=' sign.
Also, be careful of your script header. It's not the same thing whether you use
#!/bin/bash
or
#!/bin/sh
Here's the source.
Bash 4+ examples. Note: not using quotes will cause issues when words contain spaces, etc. Always quote in Bash IMO.
Here are some examples Bash 4+:
Example 1, check for 'yes' in string (case insensitive):
if [[ "${str,,}" == *"yes"* ]] ;then
Example 2, check for 'yes' in string (case insensitive):
if [[ "$(echo "$str" | tr '[:upper:]' '[:lower:]')" == *"yes"* ]] ;then
Example 3, check for 'yes' in string (case sensitive):
if [[ "${str}" == *"yes"* ]] ;then
Example 4, check for 'yes' in string (case sensitive):
if [[ "${str}" =~ "yes" ]] ;then
Example 5, exact match (case sensitive):
if [[ "${str}" == "yes" ]] ;then
Example 6, exact match (case insensitive):
if [[ "${str,,}" == "yes" ]] ;then
Example 7, exact match:
if [ "$a" = "$b" ] ;then
This question has already great answers, but here it appears that there is a slight confusion between using single equal (=) and double equals (==) in
if [ "$s1" == "$s2" ]
The main difference lies in which scripting language you are using. If you are using Bash then include #!/bin/bash in the starting of the script and save your script as filename.bash. To execute, use bash filename.bash - then you have to use ==.
If you are using sh then use #!/bin/sh and save your script as filename.sh. To execute use sh filename.sh - then you have to use single =. Avoid intermixing them.
I would suggest:
#!/bin/bash
s1="hi"
s2="hi"
if [ $s1 = $s2 ]
then
echo match
fi
Without the double quotes and with only one equals.
$ if [ "$s1" == "$s2" ]; then echo match; fi
match
$ test "s1" = "s2" ;echo match
match
$
I don't have access to a Linux box right now, but [ is actually a program (and a Bash builtin), so I think you have to put a space between [ and the first parameter.
Also note that the string equality operator seems to be a single =.
This is more a clarification than an answer! Yes, the clue is in the error message:
[hi: command not found
which shows you that your "hi" has been concatenated to the "[".
Unlike in more traditional programming languages, in Bash, "[" is a command just like the more obvious "ls", etc. - it's not treated specially just because it's a symbol, hence the "[" and the (substituted) "$s1" which are immediately next to each other in your question, are joined (as is correct for Bash), and it then tries to find a command in that position: [hi - which is unknown to Bash.
In C and some other languages, the "[" would be seen as a different "character class" and would be disjoint from the following "hi".
Hence you require a space after the opening "[".
Use:
#!/bin/bash
s1="hi"
s2="hi"
if [ "x$s1" == "x$s2" ]
then
echo match
fi
Adding an additional string inside makes it more safe.
You could also use another notation for single-line commands:
[ "x$s1" == "x$s2" ] && echo match
For a version with pure Bash and without test, but really ugly, try:
if ( exit "${s1/*$s2*/0}" )2>/dev/null
then
echo match
fi
Explanation: In ( )an extra subshell is opened. It exits with 0 if there was a match, and it tries to exit with $s1 if there was no match which raises an error (ugly). This error is directed to /dev/null.
String:
name#gmail.com
Checking for:
#
.com
My code
if [[ $word =~ "#" ]]
then
if [[ $word =~ ".com" || $word =~ ".ca" ]]
My problem
name#.com
The above example gets passed, which is not what I want. How do I check for characters (1 or more) between "#" and ".com"?
You can use a very very basic regex:
[[ $var =~ ^[a-z]+#[a-z]+\.[a-z]+$ ]]
It looks for a string being exactly like this:
at least one a-z char
#
at least one a-z char
.
at least one a-z char
It can get as complicated as you want, see for example Email check regular expression with bash script.
See in action
$ var="a#b.com"
$ [[ $var =~ ^[a-z]+#[a-z]+\.[a-z]+$ ]] && echo "kind of valid email"
kind of valid email
$ var="a#.com"
$ [[ $var =~ ^[a-z]+#[a-z]+\.[a-z]+$ ]] && echo "kind of valid email"
$
why not go for other tools like perl:
> echo "x#gmail.com" | perl -lne 'print $1 if(/#(.*?)\.com/)'
gmail
The glob pattern would be: [[ $word == ?*#?*.#(com|ca) ]]
? matches any single character and * matches zero or more characters
#(p1|p2|p3|...) is an extended globbing pattern that matches one of the given patterns. This requires:
shopt -s extglob
testing:
$ for word in #.com #a.ca a#.com a#b.ca a#b.org; do
echo -ne "$word\t"
[[ $word == ?*#?*.#(com|ca) ]] && echo matches || echo does not match
done
#.com does not match
#a.ca does not match
a#.com does not match
a#b.ca matches
a#b.org does not match
Is there any bash command to do something similar to:
if [[ $string =~ $pattern ]]
but that it works with simple wild cards (?,*) and not complex regular expressions ??
More info:
I have a config file (a sort of .ini-like file) where each line is composed of a wild card pattern and some other data.
For any given input string that my script receives, I have to find the first line in the config file where the wild card pattern matches the input string and then return the rest of the data in that line.
It's simple. I just need a way to match a string against wild card patterns and not RegExps since the patterns may contain dots, brackets, dashes, etc. and I don't want those to be interpreted as special characters.
The [ -z ${string/$pattern} ] trick has some pretty serious problems: if string is blank, it'll match all possible patterns; if it contains spaces, the test command will parse it as part of an expression (try string="x -o 1 -eq 1" for amusement). bash's [[ expressions do glob-style wildcard matching natively with the == operator, so there's no need for all these elaborate (and trouble-prone) tricks. Just use:
if [[ $string == $pattern ]]
There's several ways of doing this.
In bash >= 3, you have regex matching like you describe, e.g.
$ foo=foobar
$ if [[ $foo =~ f.ob.r ]]; then echo "ok"; fi
ok
Note that this syntax uses regex patterns, so it uses . instead of ? to match a single character.
If what you want to do is just test that the string contains a substring, there's more classic ways of doing that, e.g.
# ${foo/b?r/} replaces "b?r" with the empty string in $foo
# So we're testing if $foo does not contain "b?r" one time
$ if [[ ${foo/b?r/} = $foo ]]; then echo "ok"; fi
You can also test if a string begins or ends with an expression this way:
# ${foo%b?r} removes "bar" in the end of $foo
# So we're testing if $foo does not end with "b?r"
$ if [[ ${foo%b?r} = $foo ]]; then echo "ok"; fi
# ${foo#b?r} removes "b?r" in the beginning of $foo
# So we're testing if $foo does not begin with "b?r"
$ if [[ ${foo#b?r} = $foo ]]; then echo "ok"; fi
ok
See the Parameter Expansion paragraph of man bash for more info on these syntaxes. Using ## or %% instead of # and % respectively will achieve a longest matching instead of a simple matching.
Another very classic way of dealing with wildcards is to use case:
case $foo in
*bar)
echo "Foo matches *bar"
;;
bar?)
echo "Foo matches bar?"
;;
*)
echo "Foo didn't match any known rule"
;;
esac
John T's answer was deleted, but I actually think he was on the right track. Here it is:
Another portable method which will work in most versions of bash is
to echo your string then pipe to grep. If no match is found, it will
evaluate to false as the result will be blank. If something is returned,
it will evaluate to true.
[john#awesome]$string="Hello World"
[john#awesome]$if [[ `echo $string | grep Hello` ]];then echo "match";fi
match
What John didn't consider is the wildcard requested by the answer. For that, use egrep, a.k.a. grep -E, and use the regex wildcard .*. Here, . is the wildcard, and * is a multiplier meaning "any number of these". So, John's example becomes:
$ string="Hello World"
$ if [[ `echo $string | egrep "Hel.*"` ]]; then echo "match"; fi
The . wildcard notation is fairly standard regex, so it should work with any command that speaks regex's.
It does get nasty if you need to escape the special characters, so this may be sub-optimal:
$ if [[ `echo $string | egrep "\.\-\$.*"` ]]; then echo "match"; fi