Arithmetic Operators
$ var=$(( 20 + 5 ))
$ expr 1 + 3 # 4
$ expr 2 - 1 # 1
$ expr 10 / 3 # 3
$ expr 20 % 3 # 2 (remainder)
$ expr 10 \* 3 # 30 (multiply)
String Operators
Expression Meaning
${#str} Length of $str
${str:pos} Extract substring from $str at
$pos
${str:pos:len} Extract $len chars from $str at
$pos
${str/sub/rep} Replace first match of $sub with
$rep
${str//sub/rep} Replace all matches of $sub with
$rep
${str/#sub/rep} If $sub matches front end of $str,
substitute $rep for $sub
${str/%sub/rep} If $sub matches back end of $str,
substitute $rep for $sub
Relational Operators
Num String Test
-eq = Equal to
== Equal to
-ne != Not equal to
-lt \< Less than
-le Less than or equal to
-gt \> Greater than
-ge Greater than or equal to
-z is empty
-n is not empty
File Operators
True if file exists and...
-f file ...is a regular file
-r file ...is readable
-w file ...is writable
-x file ...is executable
-d file ...is a directory
-s file ...has a size greater than zero.
Control Structures
if [ condition ] # true = 0
then
# condition is true
elif [ condition1 ]
then
# condition1 is true
elif condition2
then
# condition2 is true
else
# None of the conditions is true
fi
case expression in
pattern1) execute commands ;;
pattern2) execute commands ;;
esac
while [ true ]
do
# execute commands
done
until [ false ]
do
# execute commands
done
for x in 1 2 3 4 5 # or for x in {1..5}
do
echo "The value of x is $x";
done
LIMIT=10
for ((x=1; x <= LIMIT ; x++))
do
echo -n "$x "
done
for file in *~
do
echo "$file"
done
break [n] # exit n levels of loop
continue [n] # go to next iteration of loop n up
Function Usage
function-name arg1 arg2 arg3 argN
n.b. functions must be defined before use...
Function Definition
function function-name ()
{
# statement1
# statement2
# statementN
return [integer] # optional
}
Functions have access to script variables, and may have
local variables:
$ local var=value
Arrays
$ vars[2]=”two” # declare an array
$ echo ${vars[2]} # access an element
$ fruits=(apples oranges pears) # populate array
$ echo ${fruits[0]} # apples - index from 0
$ declare -a fruits # creates an array
echo "Enter your favourite fruits: "
read -a fruits
echo You entered ${#fruits[@]} fruits
for f in "${fruits[@]}"
do
echo "$f"
done
$ array=( "${fruits[@]}" "grapes" ) # add to end
$ copy="${fruits[@]}" # copy an array
$ unset fruits[1] # delete one element
$ unset fruits # delete array
Array elements do not have to be sequential - indices are
listed in {!fruits[@]}:
for i in ${!fruits[@]}
do
echo fruits[$i]=${fruits[i]}
done
All variables are single element arrays:
$ var=”The quick brown fox”
$ echo {var[0]} # The quick brown fox
String operators can be applied to all the string elements
in an array using ${name[@] ... } notation, e.g.:
$ echo ${arrayZ[@]//abc/xyz} # Replace all
occurrences of abc with xyz
