hi, unix gurus.
i am wondering if someone can give me a clear explanation of the differneces between parentheses and brackets, both single and double.
i have heard that double parentheses (( are used for numerical expressions and that single brackets [ are used for strings. but i see very different behavior on this forum and elsewhere. i would love some clarity!
thanks for your help as usual.
First read the man page for test.
[[ and [ are shell builtins (in modern shells)
for test. test or [[ - you have to complete them with ]] - are use either in if statments or in combined statements
if [[ 1 -eq $var ]] ; then
echo "hi there"
fi
# same thing written another way as a one-liner
[[ 1 -eq $var ]] && echo "hi there"
# test if a file exists
[[ -f somefile ]] && echo "found somefile "
the -eq -gt -lt -ge -le comparisons are for comaring integer numbers in [[ ]]
comparing strings- [[ "$string1" = "$string2" ]]
There are other modern operators for testing file times: for example -ot -nt
See the ksh man page
$(( )) does arithmetic operations - whole numbers (long integers).
allowed operators are + - * / and some others like %.
$(( 3 + 1 )) evaluates a 4
$( some command [s]) runs one or more commands in background
it works just like the backticks: `command`
read all of a file into an envrionment variable
myvar=$(< somefile)
put the date in a variable
myvar=$( date )