I am new to shell scripting and I am trying to learn while loop in an ubuntu shell script. Script.sh
#!/bin/bash
#Initializing two variables
#while loop
echo while loop
a=0
while [ $a -lt 10 ]
do
echo $a
a=$(a + 1)
done
Output is -
vscoder@vscoder-VirtualBox:~$ ./script.sh
while loop
0
./script.sh: line 10: a: command not found
./script.sh: line 7: [: -lt: unary operator expected
vscoder@vscoder-VirtualBox:~$
I have no idea what is the error. Somebody please help me with the issue.
This is what's happening:
a == 0 so [ $a -lt 10 ] is truea=$(a + 1)a with arguments + and 1a: command not found error is throwna: this is an empty string$a is an empty string, and because we're using [ not [[ , and because $a is unquoted, the shell sees this: [ -lt 10 ]-lt is not a unary operator (see the test command and Bash Conditional Expressions ), and the error message is thrown[ exits with a non-zero status, so while stops looping.While comparison need to change and assignment of a variable
#!/bin/bash
echo while loop
a=0
while [[ $a -lt 10 ]]
do
echo $a
a=$(($a+1))
done
-1 and then does a string comparison of "-1" < "10".
a=$(( a + 1 )). Note you need to count upwards to meet the exit condition. A for loop would be a better construct here:for a in {0..9}; do ... doneorfor (( a = 0; a < 10; a++ )); do ... done.-lt(less than) and similar.<is being interpreted as a file redirection, with 10 as the filename.shellcheck.netis your friend here.<is a redirection (in[ ... ], not in[[ ... ]]). Quoting it makes it compare sort order.