If statement in linxu shell

Be careful of the blank in the following statements.

 

[[]] is a new feature in bash 3.0 for string comparision.

{{}} is a new feature in bash 3.0 for integer comparision.

so we can use "==, >=, <=" supersede "-eq, -gt, -lt"

 

Example:

 

if [[ "1" == "1" ]]; then

    echo "1 equals 1"

fi

 

exit

 

 

$(()) expression can be used to do the operations with the integer, like

 

sum=$((1+1))

echo $sum

 

Then the output is 2.

 

Other way to the same result is:

declare -i sum=1+1 or sum=$[1+1]

 

 

others:

accounts=cat "/etc/passwd" | cut -d ":" -f1

for account in accounts

do

    echo "$account"

done

 

 

if [ ! -e "somefile"]; then

fi

 

rm file

rmdir dir

touch file

 

echo $$ show curent pid

 

echo $? show the last return code.

 

export | cut -c 12- which will show 12+ chars

 

exercise:

cat /etc/passwd | sort -t ":" -k 3      sort according to 3rd column separated by ":"

uniq -c show the count which occurs multiply.

 

cat /etc/passwd | tr -d ':' remove ":"