How To Make A Bash Script
27 Simple Bash Scripting Tips for Beginners
Don't be afraid of Bash scripting. It will be easier after reading these tips
[Updated on 2021–02-18. Codes changed to Gist and added links]
Table of Contents
Introduction
1. Clean Structure
2. Install ShellCheck on Your Editor
3. Usage Function
4. Error Messages
5. Function Comments
6. How To Concatenate String Variables
7. How To Set a Debug Mode
∘ Other useful set options
8. How To Slice Strings
9. How To Make Sure Users Use a Correct Bash Version
10. How To Transform a String Case
11. Ternary Operator-Like Statement
12. How To Find the Length of a String and an Array
13. How To Unset Variables
14. How To Set a Default Value
15. How To Determine Your Bash Script Name
16. How To Make a Variable Constant
17. How To Find OSs
18. How To Determine Which HTTP Get Tool the System Has Installed
19. How To Determine Which Python the System Has Installed
20. Parameter expansion: How to Find the Script Name and Directory
21. How To Make Status Messages
22. How To Set Locale
23. Type Hinting in Bash?
24. Store Exit Status in a Variable
25. Using Trap for Unexpected Termination
26. Don't Reinvent the Wheel
27. Subshell and Exit Status
Conclusion
Newsletter
References Introduction
After learning the basics of Bash scripting, you can use these practices to make your Bash script look more professional. Here are 27 tips for Bash script beginners.
1. Clean Structure
Your script must start with a bang and describe the purpose of the script. Some examples of how to use your script will be helpful for users. If it's necessary, explain the parameters.
Declare all of the global variables first and then declare all of the functions after the global variables. Use local variables in functions and write the main body after the functions. Use an explicit exit status code in your functions, the if statement, and at the end of the script.
2. Install ShellCheck on Your Editor
ShellCheck is a shell script static analysis tool. By installing ShellCheck on your editor, you can avoid many beginner's pitfalls. After installing it, you can run it on your terminal.
$ shellcheck my_awesome_script Or you can install it on VS Code.
If you are interested in checking the ShellCheck error code list, check this gist.
3. Usage Function
If your script is using positional parameters, add a function explaining how and what users can use the parameters.
$0 outputs the script name. In the above case, the usage function will be called when a user uses h or any letters except f, d , or t.
The above script checks if the number of parameters is two. If it is not, it displays usage.
You can use Bash Heredoc:
4. Error Messages
Google's shell style guide recommends a function to print out messages along with other status information.
The guide suggests that all error messages should go to STDERR because this makes it easier to separate normal status from actual issues.
5. Function Comments
All functions should have comments mentioning description, global variables, arguments, outputs, and returned values, if applicable.
6. How To Concatenate String Variables
In a Bash script, you can concatenate strings in different ways.
Please note that there is no space before and after an equal sign when you are assigning a value to a variable. The first one uses the += sign to concatenate the second value to the first one. The second one uses a double quote with a space between variables. The last one is using a curly bracket since the variable foo is immediately followed by a character.
7. How To Set a Debug Mode
Use set -x. The option being set is -x or xtrace. This displays the expanded commands and variables of a given line of code before the code is run.
The output is a verbose trace of the script's execution.
+ foo=Bash
+ '[' Bash == Bash ']'
+ echo Bash
Bash It is easy to see how the script is run and assigns values.
Other useful set options
set -e exits immediately if a command exits with a non-zero status.
set -u treats unset variables as an error when substituting.
set -C disallows existing regular files to be overwritten.
So you can use set -Ceu at the beginning of your script.
Find more about set using help set.
8. How To Slice Strings
You can use cut -cstart-end to slice a string.
The first one slices the string from the first letter to the third. The second one slices from the second to the fourth. The last one slices the fifth letter.
A parameter expansion performs substring extraction, ${s:off-set-index:length}:
#!/usr/bin/env bash s="abcdefg"
echo ${s:0:3}
echo ${s:1:3}
echo ${s:4:1}
# output
# abc
# bcd
# e
The offset is zero-based.
9. How To Make Sure Users Use a Correct Bash Version
The following script ensures the users use Bash version 4.0 or higher.
if ((BASH_VERSINFO[0] < 4)); then
printf '%s\n' "Error: This requires Bash v4.0 or higher. You have version $BASH_VERSION." 1>&2
exit 2
fi 10. How To Transform a String Case
When you read user input, the input can be in lowercase or uppercase. For Bash lower than version 4, you can change it to uppercase using the tr command with [:lower:] and [:upper:].
Please note that we set the option -x for debugging.
Line 4: Read user input.
Line 5: Pipes, |, let you use the output of a command as the input of another command. We cut the first letter, then transform it from lowercase to uppercase. You can interchange the position of lowercase and uppercase to transform from uppercase to lowercase.
In Bash 4+, you can use case modification operators.
Using these case modification operators, you can create functions.
11. Ternary Operator-Like Statement
Bash doesn't have a ternary operator, but you can do a similar thing for simple variable assignments.
We can write an if statement:
if [ $foo -ge $bar ]; then
baz="Smile!"
else
baz="Sleep!"
fi Using && and || together, we can create a statement similar to a ternary statement:
[ $foo -ge $bar ] && baz="Smile!" || baz="Sleep!" 12. How To Find the Length of a String and an Array
${#string} returns the string length and similarly ${#array[@]} returns the array length.
13. How To Unset Variables
Unsetting variables ensures you are not using predefined variables.
At the beginning of the script:
#!/usr/bin/env bash unset var1 var2 var3
var1=some_value
...
14. How To Set a Default Value
${foo-$DEFAULT} and ${foo=$DEFAULT} evaluate as $DEFAULT if foo is not set.
${foo:-$DEFAULT} and ${foo:=$DEFAULT} evaluates as $DEFAULT if the foo is not set or is empty.
${foo+$OTHER} and ${foo:+OTHER} evaluate as $OTHER if the foo is set, otherwise as a null string.
15. How To Determine Your Bash Script Name
By adding all your Bash scripts to the ~/bin directory and adding its path to your terminal config file, ~/.bashrc or ~/.zshrc, you can run them from any directory. But before you set your script name, it is a good idea for your script name to not be used anywhere else. Use the type or which command to check.
$ type my_script
my_script not found
$ which which
which: shell built-in command There is no script name with my_script, but which is already taken.
16. How To Make a Variable Constant
readonly makes variables and functions read-only.
Line 3: Set a read-only variable.
Line 5: Trying to overwrite the read-only variable. But this returns an error, "line 5: MAX: readonly variable".
Output:
10
/Users/shinokada/bin/ex6: line 5: MAX: readonly variable
10 You can output the readonly variables:
17. How To Find OSs
The different OSs have different commands. For example, Linux uses readlink and macOS uses greadlink.
The following example is rather trivial, but it finds the user's OS and applies the correct command.
Output:
Your script path is /Users/shinokada/bin
Your OS is mac 18. How To Determine Which HTTP Get Tool the System Has Installed
Your Bash script may use an HTTP GET tool. Different systems use different tools. It can be curl, wget, http, fetch , or something else.
&>/dev/null redirects both the standard output stream and the standard error stream to /dev/null. It is the same as >/dev/null 2>&1.
command -v displays the path to the executable or the alias definition of a specific command.
19. How To Determine Which Python the System Has Installed
Similarly, if your script uses Python, you can find the system Python:
20. Parameter expansion: How to Find the Script Name and Directory
Use ${parameter##*/} to get the file name and use ${parameter%/*} to get the directory path.
This Tech.io page explains how to use ${parameter%word}, ${parameter%%word}, ${parameter#word}, ${parameter##word}.
You can use basename "$0" to get the file name as well.
You can find your Bash script directory:
#!/usr/bin/env bash c=$0
echo "${c%/*}"
Outputs:
/Users/shinokada/bin 21. How To Make Status Messages
You may want to inform users what your script is doing.
If you want to output status messages, you should start it with the program name. Use $(basename "$0") to get the program name. The message should be written on the standard error by using echo ... 1>&2.
#!/usr/bin/env bash progname=$(basename "$0")
...
echo "$progname: Running this and that" 1>&2
22. How To Set Locale
You can find your locale environment:
Not all programmers use the same locale. Programmers from France may use fr_CH.UTF-8; geeks from England may be using en_GB.UTF-8. To ensure that the users use the correct locale, you can use LC_ALL to set the locale:
LC_ALL="en_US.UTF-8" 23. Type Hinting in Bash?
Bash script doesn't have type hinting, but you can use declare to make variables indexed arrays, associative arrays, or integers. You can find the declare options by using help declare.
You can use + instead of — to turn off the attribute.
We declared var1 as an integer in line 3. If you try to change it to a string, you get an error (line5). But you can change it to another integer (line7).
Please note that when you use declare in a function, the variable becomes a local variable which you can't access outside of the function.
24. Store Exit Status in a Variable
Rather than using $? in if statements:
mkdir /tmp/tmp_dir
if [ $? = 0 ]; then
# do something
fi Store the exit status in a variable:
The following stores the exit status for checking if a file exists and is a directory.
test -d /tmp/tmp_dir
test_es=$?
if [[ ${test_es} -ne 0 ]]; then
echo "No dir found. Exiting script!" >&2
exit 1
fi 25. Using Trap for Unexpected Termination
Users may terminate your script using Ctrl-c while it is running. If your script is modifying a directory or a file, you need to bring it back to the original state. The command trap is for this situation.
When a user uses Ctrl-c, it generates a signal SIGINT. When a user kills a process, it generates a signal SIGTERM. You can list all the signals using:
# Linux, from terminal
$ trap --list
# macOS, from a script
trap --list You can find the standard signals on man7.org.
The trap command has the form of trap "do something" signal_to_trap. If your cleanup code is long, you can create a function.
Let's trap the Ctrl-c signal:
tempfile=/tmp/myfile
cleanup(){
rm -f $tempfile
}
trap cleanup SIGINT Read more about the trap command.
26. Don't Reinvent the Wheel
If your goal is learning, then you can reinvent the wheel, but if not, use snippets from pure-bash-bible. The pure-bash-bible is a great learning place as well. There are many functions you can use in your scripts. The scripts include strings, arrays, loops, file handling, file paths, and more.
27. Subshell and Exit Status
Can you find the difference between the following codes?
The first one returns:
/Users/myname/bin/ex5: line 34: no_func1: command not found
there is nothing
1 And the second one returns:
/Users/myname/bin/ex5: line 34: no_func2: command not found
there is nothing The difference is the parentheses and braces. Parentheses cause the commands to be run in a subshell, and braces cause the commands to be grouped together but not in a subshell.
Since braces don't create a subshell, exit exits the main shell process, so it never reaches the point where it would run echo $?.
Conclusion
These are 27 Bash scripting tips you can use in your next Bash script project. Many of them are easy to adopt and they make your Bash script look more professional.
Happy coding!
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References
- https://tldp.org/LDP/abs/html/special-chars.html
- https://tldp.org/LDP/abs/html/parameter-substitution.html#PARAMSUBREF
How To Make A Bash Script
Source: https://betterprogramming.pub/27-simple-bash-scripting-tips-for-beginners-d6764c977546
Posted by: lankfordevelve1960.blogspot.com
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