Bash:Variables

Arithmetics

Bash and ksh only work with integers.

+, -, *, /, %

add, substract, multiply, divide, modulo.

**

For bash, Exponentiation

10#00012

For bash, force the figure (00012) to be interpreted as decimal. Figures starting with 0 are considered octal by default.

expr <expr>

Voer expr uit, bij boolean wordt 1 (onwaar) of 0 (waar) gegeven

((VAR=x+y))

let VAR=x+y

VAR=$((x+y))

VAR=$(expr x + y)

Calculate and assign to VAR

let VAR+=2

VAR=$((VAR += 2))

Add two to VAR

Special Parameters

$0

Name current procedure

$1

First passed argument ($2 is the second, $3... )

$*

All arguments given as one string

$@

All arguments given as separate strings per parameter

$#

The number of passed arguments

$?

The returnstate for the last executed command. 0 = OK, all others not.

$$

ProcessID of current process, handy to use for temporary files

$!

ProcessID of last spawned process.

Nested variables

var2=${!var1}

var2 will get the value of the evaluation of var1 (value of var0)

var0=1
var1=var0
var2=${!var1}
echo $var2
1

Loop over a range

for i in {00..59}
 do
  echo ${i}
 done

for i in $(seq ${START} ${END})
 do
  echo $i
 done

Operations on variables

• # searches from the beginning
• % searches from the end

${var#??}

Remove the first 2 chars

${var%??}

Remove the last 2 chars

${var%e*}

Remove everything from and including the last 'e'

${var#*e}

Remove everything up to the first 'e'

${var#* * }

Remove the first 2 words from a line (carefully check the spaces)

${var% * *}

Remove the last 2 words from a line (carefully check the spaces)

${#var}

Length of the string

Check if a string is in the variable

The trick is that if the variable does not have the searchstring the variable will be unchanged.

if [ "${var#*searchstring}" != "${var}" ]
 then
  echo "searchstring is in $var"
fi