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Variables in bash

Read only variables​

readonly someVariable=someValue

someVariable is assigned a value someValue. This value cannot be modified. An attempt to do this will result in an error:

line 5: someVariable: readonly variable

Variables scope​

Reduce a variable scope to a function

Variable default value​

Supposing, we need to implement the following requirement:

  • if the ${HOST_NAME} environment has a non-empty value, assign it to host variable,
  • otherwise, assign localhost value to host variable.
host={HOST_NAME:localhost}

Output a variable value with modified letters case​

Output 'someVariable' value with the first letter capitalized

someVariable=somevalue

echo ${someVariable^}

Outputs:
Somevalue

Output 'someVariable' value with all letters capitalized

someVariable=someValue

echo ${someVariable^^}

Outputs:
SOMEVALUE

Output 'someVariable' value with the first letter in lower case

someVariable=SOMEvalue

echo ${someVariable,}

Outputs:
sOMEvalue

Output 'someVariable' value with all letters in lower case

someVariable=SOMEvalue

echo ${someVariable,,}

Outputs:
somevalue

Output a variable value with some parts dynamically replaced by other values​

Output a variable value with all occurences of a substring replaced with some other string

Supposing, there exists a variable someVariable with value some variable value. We need to ouput the someVariable value with all whitespaces replaced by dashes:

someVariable="some variable value"

echo ${someVariable// /-}

Outputs:
some-variable-value

Output a variable value with only the first occurence of substring replaced with some other string

Supposing, there exists a variable someVariable with value project,module1,module2,module3. We need to ouput the someVariable value with the first comma replaced by a colon:

someVariable="project,module1,module2,module3"

echo ${someVariable/,/:}

Outputs:
project:module1,module2,module3

Output a variable value with a special character replaced by another character

Supposing, there exists a variable someVariable with value path/dir1/dir2/file. We need to ouput the someVariable value with the first slash replaced by a colon:

someVariable="path/dir1/dir2/file"

echo ${someVariable/\//:}

Outputs:
path:dir1/dir2/file

A backslash is used to escape a forward slash in the regular expression.

Get a slice of a variable value​

Examples are based on the bash substring expansion:

${parameter:offset}
${parameter:offset:length}

The first element index is 0. As an example let's take all characters from the 4th till the last one:

s=200730
echo ${s:4}
Output: 30

In another example we will take two characters starting from the 2nd one:

s=200730
echo ${s:2:2}
Output: 07

Get a value of a variable which name is specified in another variable​

Supposing, there exists a variable someVariable with value someValue. In case the someVariable is constructed dynamically in the script, and we need to get someValue by someVariable, we can use:

someVariable=someValue

firstPartOfVariableName=some
secondPartOfVariableName=Variable

variableName="${firstPartOfVariableName}${secondPartOfVariableName}"

echo ${!variableName}

Outputs:
someValue