Bracket¶

https://stackoverflow.com/questions/2188199/how-to-use-double-or-single-brackets-parentheses-curly-braces

[] Test construct¶

[ is just a different name for the test command, and ] is required as the last parameter, for readability and aesthetic reasons.

x=2
v=x
if [ $v == x ] ; then echo yes ; else echo no ; fi
# if test $v == x ; then echo yes ; else echo no ; fi
yes

[[]] Extended test construct¶

v=x
if [[ $v == 1 ]] ; then echo yes ; else echo no ; fi
no

• Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts

• the &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct

• Arithmetic evaluation of octal / hexadecimal constants takes place automatically within a [[ ... ]] construct

(()) Double Parentheses: arithmetic operations¶

(( i = 78 ))
i = $(( 20 + 5 ))
(( i++ ))

${var}¶

${var} is just a disambiguation mechanism, so ${var}text can be different to $vartext.

echo Variable: ${var}1
var: x1

{ date; top -b -n1 | head ; } >logfile

$(command)¶

$(command) is a modern synonym for command which stands for command substitution; it means run command and put its output here.

round_k() {
    echo $(( ($1 + 500) / 1000 * 1000 ))
}
x=600
y=$(round_k $x)
echo $y

($x)¶

treat as array init by parentheses, each word an element