I have a variable named l and it contains a string of characters
for example
echo $l output will be a1,a2,a3,a4,a5,a6,a7,a8,b1,b2,b3
How do i search for b1 and store it in a new temporary that will be used for condition checking. I cant use awk as the positioning will changed everything.
So my theory will be something like this, getting b1 from that string and i will stored in variable J
$j = b1
if [ $j = 'b1' ]
then
echo "This is a true statment by b1"
else
echo "This is a false statement not by b1"
fi
Is there a faster way to do this as i think i am duplicating the steps.
Thanks
I think this might work for you:
l="a1,a2,a3,a4,a5,a6,a7,a8,b1,b2,b3"
j="b1"
if [[ $l =~ $j ]]; then
echo "This is a true statement by $j"
else
echo "This is a false statement by $j"
fi
or
[[ $l =~ $j ]] && echo "This is a true by $j" || echo "This is false by $j"
Related
#!/bin/bash
var="true"
i=1
while $var
do
read -p "Enter value (true/false): " var
if [[ $var == "true" ]]
then
echo "Iteration : $i"
((i++))
elif [[ $var == "false" ]]
then
echo "Exiting the process"
elif [[ $? -eq 1 ]]
then
echo "Invalid Choice."
echo "Avaialable Choices are true or false"
exit
fi
done
Script is Working Fine. I Enter true the loop will iterate for false the script stops.
I want the script will continue asking "Enter Value" if any other value instead of true or false will be entered.
This would do the same with a more academic syntax:
i=0
while :; do
printf 'Enter value (true/false): '
read -r var
case $var in
true)
i=$((i + 1))
printf 'Iteration : %d\n' $i
;;
false)
printf 'Exiting the process\n'
break
;;
*)
printf 'Invalid Choice.\nAvaialable Choices are true or false\n'
;;
esac
done
You might find this to be a cleaner solution:
i=0
while true; do
read -p "enter value: " myinput
if [[ $myinput = true ]]; then
echo "iteration $i"
i=$((i+1))
elif [[ $myinput = false ]]; then
echo "exiting"
exit
else
echo "invalid input"
fi;
done;
The issue I see with your current code is that it is unclear which command's exit status $? refers to. Does it refer to the echo in the previous elif block? Or the last condition check? Or something else entirely?
I'm new in bash. I tried that:
#!/bin/bash
i=1
while [[ $var != "false" ]]
do
read -p "Enter value (true/false): " var
if [[ $var == "true" ]]
then
echo "Iteration : $i"
((i++))
elif [[ $var == "false" ]]
then
echo "Exiting the process"
elif [[ $? -eq 1 ]]
then
echo "Invalid Choice."
echo "Avaialable Choices are true or false"
fi
done
I changed while $var with while [[ $var ]] because while works like if. It runs the given command. In there it is $var's value.
And I moved exit to first elif expression's end. So if user type false program will exit.
I need to check if the value c exists in an integer array, I'm aware of how to approach this using for loops and if statements:
for c in {1..100};do
sequence=(2 4 6 8 10 12 14 16)
for value in "${sequence[#]}";do
if [[ $value -eq $c ]];then #If c exists in sequence
flag=1
break
fi
done
done
But I don't want this, I tried something similar to this:
[[ ${sequence[*]} =~ $c ]]
But it didn't give me the desired results, I think it works only with string arrays, not integers.
How could I approach this?
Converting my comment to answer so that solution is easy to find for future visitors.
You may use this grep + printf solution:
grep -qFx "$c" <(printf '%s\n' "${sequence[#]}") && echo "found" || echo "nope"
The problem with this method [[ ${sequence[*]} =~ $c ]] is that if $c is 1, than it'll match all instances with 1. Try this approach, make your sequence a regex like this
re=${sequence[*]}
re=${re// /|}
$ echo $re
2|4|6|8|10|12|14|16
Testing
c=1
$ [[ $c =~ $re ]] && echo ok || echo fail
fail
c=11
$ [[ $c =~ $re ]] && echo ok || echo fail
fail
c=12
$ [[ $c =~ $re ]] && echo ok || echo fail
ok
I am attempting to run a block of code if one flag is set to true and the other is set to false. ie
var1=true
var2=false
if [[ $var1 && ! $var2 ]]; then var2="something"; fi
Since that did not evaluate the way that I expected I wrote several other test cases and I am having a hard time understanding how they are being evaluated.
aa=true
bb=false
cc="python"
if [[ "$aa" ]]; then echo "Test0" ; fi
if [[ "$bb" ]]; then echo "Test0.1" ; fi
if [[ !"$aa" ]]; then echo "Test0.2" ; fi
if [[ ! "$aa" ]]; then echo "Test0.3" ; fi
if [[ "$aa" && ! "$bb" ]]; then echo "Test1" ; fi
if [[ "$aa" && ! "$aa" ]]; then echo "Test2" ; fi
if [[ "$aa" ]] && ! [[ "$bb" ]]; then echo "test3" ; fi
if [[ "$aa" ]] && ! [[ "$cc" ]]; then echo "test4" ; fi
if [[ $aa && ! $bb ]]; then echo "Test5" ; fi
if [[ $aa && ! $aa ]]; then echo "Test6" ; fi
if [[ $aa ]] && ! [[ $bb ]]; then echo "test7" ; fi
if [[ $aa ]] && ! [[ $cc ]]; then echo "test8" ; fi
When I run the preceding codeblock the only output I get is
Test0
Test0.1
Test0.2
however, my expectation is that I would get
Test0
Test1
Test3
Test5
Test7
I have tried to understand the best way to run similar tests, however most examples I have found are set up in the format of
if [[ "$aa" == true ]];
which is not quite what I want to do. So my question is what is the best way to make comparisons like this, and why do several of the test cases that I would expect to pass simply not?
Thank you!
Without any operators, [[ only checks if the variable is empty. If it is, then it is considered false, otherwise it is considered true. The contents of the variables do not matter.
Your understanding of booleans in shell context is incorrect.
var1=true
var2=false
Both the above variables are true since those are non-empty strings.
You could instead make use of arithmetic context:
$ a=1
$ b=0
$ ((a==1 && b==0)) && echo y
y
$ ((a==0 && b==0)) && echo y
$
$ ((a && !(b))) && echo y; # This seems to be analogous to what you were attempting
y
The shell does not have Boolean variables, per se. However, there are commands named true and false whose exit statuses are 0 and 1, respectively, and so can be used similarly to Boolean values.
var1=true
var2=false
if $var1 && ! $var2; then var2="something"; fi
The difference is that instead of testing if var1 is set to a true value, you expand it to the name of a command, which runs and succeeds. Likewise, var2 is expanded to a command name which runs and fails, but because it is prefixed with ! the exit status is inverted to indicate success.
(Note that unlike most programming languages, an exit status of 0 indicates success because while most commands have 1 way to succeed, there are many different ways they could fail, so different non-zero values can be assigned different meanings.)
true and false are evaluated as strings ;)
[[ $var ]] is an equivalent of [[ -n $var ]] that check if $var is empty or not.
Then, no need to quote your variables inside [[. See this reminder.
Finally, here is an explication of the difference between && inside brackets and outside.
The closest you can come seems to be use functions instead of variables because you can use their return status in conditionals.
$ var1() { return 0; }
$ var2() { return 1; } # !0 = failure ~ false
and we can test this way
$ var1 && echo "it's true" || echo "it's false"
it's true
$ var2 && echo "it's true" || echo "it's false"
it's false
or this way
$ if var1; then echo "it's true"; else echo "it's false"; fi
it's true
$ if var2; then echo "it's true"; else echo "it's false"; fi
it's false
Hope this helps.
Here is my code:
#!/bin/bash
echo "Letter:"
read a
if [ $a = "a" ]
then
echo "LOL"
fi
if [ $a = "b" ]
then
echo "ROFL"
fi
Is there a way for me to loop this so that, after displaying either LOL or ROFL, I would be asked for a letter again?
Yes.
Oh, you want to know how?
while true; do
echo "Letter:"
read a
if [ $a = "a" ]
then
echo "LOL"
elif [ $a = "b" ]
then
echo "ROFL"
fi
done
Of course, you probably want some way to get out of that infinite loop. The command to run in that case is break. I would write the whole thing like this:
while read -p Letter: a; do
case "$a" in
a) echo LOL;;
b) echo ROFL;;
q) break;;
esac
done
which lets you exit the loop either by entering 'q' or generating end-of-file (control-D).
Don't forget that you always want -r flag with read.
Also there is a quoting error on that line:
if [ $a = "a" ] # this will fail if a='*'
So here is a bit better version(I've also limited the user to input only 1 character):
#!/bin/bash
while true; do
read -rn1 -p 'Letter: ' a
echo
if [[ $a = 'a' ]]; then
echo "LOL"
elif [[ $a = 'b' ]]; then
echo "ROFL"
else
break
fi
done
Or with switch statement:
#!/bin/bash
while read -rn1 -p 'Letter: ' a; do
echo
case $a in
a) echo LOL;;
b) echo ROFL;;
*) break;;
esac
done
I try to test if a string starts with a certain prefix. But my script seems not work (I would expect the "if" branch will not get run). Can some Bash expert help to take a look? thanks!
Here is my code and test result:
$ cat testb.bash
#!/bin/bash
my_var="abcdefg";
if [[ "${my_var:0:5}"=="order" ]]; then
echo "value of my_var is ${my_var}.";
fi;
if [[ "${my_var:0:5}" -eq "order" ]]; then
echo "value of my_var is ${my_var}.";
fi;
if [ "${my_var:0:5}"="order" ]; then
echo "value of my_var is ${my_var}.";
fi;
$ bash -x testb.bash
+ my_var=abcdefg
+ [[ -n abcde==order ]]
+ echo 'value of my_var is abcdefg.'
value of my_var is abcdefg.
+ [[ abcde -eq order ]]
+ echo 'value of my_var is abcdefg.'
value of my_var is abcdefg.
+ '[' abcde=order ']'
+ echo 'value of my_var is abcdefg.'
value of my_var is abcdefg.
$
Whitespace is significant in this case. As you can see in the -x output, it understands the first condition as
[[ -n "${my_var:0:5}==order" ]]
Moreover, to test for a prefix, you can use a pattern:
[[ $my_var == order* ]]
To test the existence of substring, you can use either of these:
if [[ "$j" =~ string1 ]]; then
if [[ $j == *string1* ]]; then
In your particular case, you miss a space surounding ==, so instead of
if [[ "${my_var:0:5}"=="order" ]]; then
it should be
if [[ "${my_var:0:5}" == "order" ]]; then
^ ^
Finally, note that your condition was evaluated as true because it was evaluating if [ "string" ], which is true if string is not empty:
$ [ "a" ] && echo "yes"
yes
Test
$ cat a
#!/bin/bash
my_var="abcdefg";
if [[ "${my_var:0:5}" == "order" ]]; then
echo "value of my_var is ${my_var}."
elif [[ "${my_var:0:5}" == "abcde" ]]; then
echo "yeahaa"
else
echo "is not"
fi
$ ./a
yeahaa
Ok, i tested your code, you shoud such as the following code:
prefix="pre_order";
pre="pre_"
len=${#pre}
echo $len
if [[ "${prefix:0:len}" == "blahvlah" ]] ; then
echo "dddd"
fi;
Notes:
use == for string comparation
for ${} you should initilize a string variable before ${}
use len=${#pre} for lenght of string.
A POSIX-compliant way to test for a prefix is to attempt to remove the prefix, and compare the result to the original string. If the two are the same, the prefix is not present, the removal fails, and the expression expands to the original string.
prefix=foo
string=foobar
if [ "${string#$prefix}" = "$string" ]; then
printf "$string does not start with $prefix\n"
else
printf "$string starts with $prefix\n"
fi