Readonly command

• Use the readonly command to make variables and functions readonly i.e. you cannot change the value of variables.

Make variable readonly

readonly var
readonly var=value
readonly p=/tmp/toi.txt
# error
p=/tmp/newvale

Make function readonly

• You need to use the -f option to make corresponding function readonly and syntax is:

readonly -f functionName

• For example, write a function called hello() at a shell prompt, enter:

function hello() { echo "Hello world"; }
# invoke it 
hello

• Make it readonly:

readonly -f hello
# invoke it 
hello

• Now, try to update the hello(), enter:

function hello() { echo "Hello $1, let us be friends."; }

Sample outputs:

bash: hello: readonly function

Display all readonly variables

If no arguments are given, or if -p is given to the readonly buitin, a list of all readonly names is printed on screen:

readonly

OR

readonly -p

Sample outputs:

readonly 
declare -ar BASH_VERSINFO='([0]="3" [1]="2" [2]="39" [3]="1" [4]="release" [5]="i486-pc-linux-gnu")'
declare -ir EUID="1000"
declare -ir PPID="7628"
declare -r SHELLOPTS="braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor"
declare -ir UID="1000"
declare -r pwdfile="/etc/passwd"

Display all readonly functions

Type the following command:

readonly -f

Sample outputs:

hello () 
{ 
    echo "Hello world"
}
declare -fr hello

See also

• Declare command
• Variables in shell