Regarding Bash substring comparison

Question

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.
$

choroba · Accepted Answer · 2013-10-19 23:10:28Z

2

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* ]]

answered Oct 19, 2013 at 23:10

choroba

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Comments

fedorqui · Accepted Answer · 2013-10-22 07:41:24Z

2

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

edited Oct 22, 2013 at 7:41

answered Oct 19, 2013 at 23:08

fedorqui

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3 Comments

Aleks-Daniel Jakimenko-A. Over a year ago

Also you can remove useless semicolons

fedorqui Over a year ago

I guess just from my_var="abcdefg". In the if condition they are necessary before then.

fedorqui Over a year ago

You have an eagle-eye, @Aleks-DanielJakimenko :D

PersianGulf · Accepted Answer · 2013-10-19 23:29:45Z

1

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.

answered Oct 19, 2013 at 23:29

PersianGulf

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Comments

chepner · Accepted Answer · 2013-10-20 15:08:42Z

0

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

answered Oct 20, 2013 at 15:08

chepner

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