Bash Set Command
Bash has many environment variables for various purposes. The set command of Bash is used to modify or display the different attributes and parameters of the shell environment. This command has many options to perform the different types of tasks. The uses of set command for various purposes are described in this tutorial.
Syntax
set [options] [arguments]
This command can be used with different types of options and arguments for different purposes. If no option or argument is used with this command, the shell variables are printed. The minus sign (-) is used with the command’s option to enable that option and the plus sign (+) is used with the command’s option to disable that option.
Exit Values of Set Command
Three exit values can be returned by this command which are mentioned in the following:
- Zero (0) is returned to complete the task successfully.
- One (1) is returned if a failure occurs for any invalid argument.
- One (1) is returned if a failure occurs for a missing argument.
Different Options of Set Command
The purposes of the most commonly used options of the set command are described in this part of the tutorial.
| Option | Purpose |
|---|---|
| -a | It defines those variables or functions which are created or modified or exported. |
| -b | It informs the job termination. |
| -B | To do the task of the brace expansion. |
| -C | It disables the overwriting feature of the existing file. |
| -e | It exits for non-zero exit status value. |
| -f | It disables the filename generation task. |
| -h | It saves the location of the command where it has been used. |
| -m | It enables job control. |
| -n | It reads the commands. |
| -t | It exits from the command after executing a single command. |
| -u | It traces the unset variables. |
| -v | It prints the shell input lines. |
| -x | It displays the commands and their attributes sequentially. It is mainly used to debug the script. |
Different Examples of the Set Command
The uses of set command with different options are shown in this part of this tutorial.
Example 1: Using the Set Command with -a Option
Create a Bash file with the following script that enables the “set –a” command and initialize three variables named $v1, $v2, and $v3. These variables can be accessed after executing the script.
#Enable -a option to read the values of the variables
set -a
#Initialize three variables
v1=78
v2=50
v3=35
Run the script using the following command:
Read the values of the variable using the “echo” command:
The following output appears after executing the previous commands:
Example 2: Using the Set Command with -C Option
Run the “cat” command to create a text file named testfile.txt. Next, run the “set –C” command to disable the overwriting feature. Next, run the “cat” command again to overwrite the file to check whether the overwriting feature is disabled or not.
$ set -C
$ cat > testfile.txt
The following output appears after executing the previous commands:
Example 3: Using the Set Command with -x Option
Create a Bash file with the following script that declares a numeric array of 6 elements. The values of the array are printed using for loop.
#Declare an array
arr=(67 3 90 56 2 80)
#iterate the array values
for value in ${arr[@]}
do
echo $value
done
Execute the previous script by the following command:
Enable the debugging option using the following command:
The following output appears after executing the provided commands:
Example 4: Using the Set Command with -e Option
Create a Bash file with the following script that reads a file using the “cat” command before and after using the “set –e” command.
#Read a non-existing file without setting set -e
cat myfile.txt
echo "Reading a file..."
#Set the set command with -e option
set -e
#Read a non-existing file after setting set -e
cat myfile.txt
echo "Reading a file..."
The following output appears after executing the provided commands. The first error message is shown because the file does not exist in the current location. The next message is then printed. But after executing the “set –e” command, the execution stops after displaying the error message.
Example 5: Using the Set Command with -u Option
Create a Bash file with the following script that initializes a variable but prints the initialized and uninitialized variable before and after using the “set –u” command.
#Assign value to a variable
strvar="Bash Programming"
printf "$strvar $intvar\n"
#Set the set command with -u option
set -u
#Assign value to a variable
strvar="Bash Programming"
printf "\n$strvar $intvar\n"
The following output appears after executing the previous script. Here, the error is printed for the uninitialized variable:
Example 6: Using the Set Command with -f Option
Run the following command to print the list of all text files of the current location:
Run the following command to disable the globbing:
Run the following command again to print the list of all text files of the current location:
The following output appears after executing the previous script. Based on the output, the “ls *.txt” command did not work after setting “set –f” command:
Example 7: Split the String Using the Set Command with Variable
Create a Bash file with the following script that splits the string value based on the space using the “set – variable” command. The split values are printed later.
#Define a string variable
myvar="Learn bash programming"
#Set the set command without option and with variable
set -- $myvar
#Print the split value
printf "$1\n$2\n$3\n"
The following output appears after executing the previous script. The string value is divided into three parts based on the space that is printed:
Conclusion
The uses of the different options of the “set” command are shown in this tutorial using multiple examples to know the basic uses of this command.
Source: linuxhint.com