[Unix / Linux Shell Tutorial ] Function

1. #!/bin/bash
2. # Basic function
4. print_something () {
5. echo Hello I am a function
6. }
8. print_something
9. print_something

1. ./function_example.sh
2. Hello I am a function
3. Hello I am a function

Passing Arguments

It is often the case that we would like the function to process some data for us. We may send data to the function in a similar way to passing command line arguments to a script. We supply the arguments directly after the function name. Within the function they are accessible as $1, $2, etc.

1. #!/bin/bash
2. # Passing arguments to a function
4. print_something () {
5. echo Hello $1
6. }
8. print_something Mars
9. print_something Jupiter

1. ./arguments_example.sh
2. Hello Mars
3. Hello Jupiter

Return Values

Most other programming languages have the concept of a return value for functions, a means for the function to send data back to the original calling location. Bash functions don't allow us to do this. They do however allow us to set a return status. Similar to how a program or command exits with an exit status which indicates whether it succeeded or not. We use the keyword return to indicate a return statu

1. #!/bin/bash
2. # Setting a return status for a function
4. print_something () {
5. echo Hello $1
6. return 5
7. }
9. print_something Mars
10. print_something Jupiter
11. echo The previous function has a return value of $?

Line 11 - Remember that the variable $? contains the return status of the previously run command or function.

1. ./return_status_example.sh
2. Hello Mars
3. Hello Jupiter
4. The previous function has a return value of 5

Example2

1. #!/bin/bash
2. # Setting a return value to a function
4. lines_in_file () {
5. cat $1 | wc -l
6. }
8. num_lines=$( lines_in_file $1 )
10. echo The file $1 has $num_lines lines in it.

Let's break it down:

• Line 5 - This command will print the number of lines in the file referred to by $1.
• Line 8 - We use command substitution to take what would normally be printed to the screen and assign it to the variable num_lines

1. cat myfile.txt
2. Tomato
3. Lettuce
4. Capsicum
5. 
   
6. ./return_hack.sh myfile.txt
7. The file myfile.txt has 3 lines in it.

Example3

1. #!/bin/bash
2. # Experimenting with variable scope
4. var_change () {
5. local var1='local 1'
6. echo Inside function: var1 is $var1 : var2 is $var2
7. var1='changed again'
8. var2='2 changed again'
9. }
11. var1='global 1'
12. var2='global 2'
14. echo Before function call: var1 is $var1 : var2 is $var2
16. var_change
18. echo After function call: var1 is $var1 : var2 is $var2

1. ./local_variables.sh
2. Before function call: var1 is global 1 : var2 is global 2
3. Inside function: var1 is local 1 : var2 is global 2
4. After function call: var1 is global 1 : var2 is 2 changed again