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sources/tech/20170519 10 Useful Tips for Writing Effective Bash Scripts in Linux.md

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+10 Useful Tips for Writing Effective Bash Scripts in Linux
+============================================================
+
+[Shell scripting][4] is the easiest form of programming you can learn/do in Linux. More so, it is a required skill for [system administration for automating tasks][5], developing new simple utilities/tools just to mention but a few.
+
+In this article, we will share 10 useful and practical tips for writing effective and reliable bash scripts and they include:
+
+### 1\. Always Use Comments in Scripts
+
+This is a recommended practice which is not only applied to shell scripting but all other kinds of programming. Writing comments in a script helps you or some else going through your script understand what the different parts of the script do.
+
+For starters, comments are defined using the `#` sign.
+
+```
+#TecMint is the best site for all kind of Linux articles
+```
+
+### 2\. Make a Script exit When Fails
+
+Sometimes bash may continue to execute a script even when a certain command fails, thus affecting the rest of the script (may eventually result in logical errors). Use the line below to exit a script when a command fails:
+
+```
+#let script exit if a command fails
+set -o errexit 
+OR
+set -e
+```
+
+### 3\. Make a Script exit When Bash Uses Undeclared Variable
+
+Bash may also try to use an undeclared script which could cause a logical error. Therefore use the following line to instruct bash to exit a script when it attempts to use an undeclared variable:
+
+```
+#let script exit if an unsed variable is used
+set -o nounset
+OR
+set -u
+```
+
+### 4\. Use Double Quotes to Reference Variables
+
+Using double quotes while referencing (using a value of a variable) helps to prevent word splitting (regarding whitespace) and unnecessary globbing (recognizing and expanding wildcards).
+
+Check out the example below:
+
+```
+#!/bin/bash
+#let script exit if a command fails
+set -o errexit 
+#let script exit if an unsed variable is used
+set -o nounset
+echo "Names without double quotes" 
+echo
+names="Tecmint FOSSMint Linusay"
+for name in $names; do
+echo "$name"
+done
+echo
+echo "Names with double quotes" 
+echo
+for name in "$names"; do
+echo "$name"
+done
+exit 0
+```
+
+Save the file and exit, then run it as follows:
+
+```
+$ ./names.sh
+```
+ [![Use Double Quotes in Scripts](https://www.tecmint.com/wp-content/uploads/2017/05/Use-Double-Quotes-in-Scripts.png)][6] 
+
+Use Double Quotes in Scripts
+
+### 5\. Use functions in Scripts
+
+Except for very small scripts (with a few lines of code), always remember to use functions to modularize your code and make scripts more readable and reusable.
+
+The syntax for writing functions is as follows:
+
+```
+function check_root(){
+command1; 
+command2;
+}
+OR
+check_root(){
+command1; 
+command2;
+}
+```
+
+For single line code, use termination characters after each command like this:
+
+```
+check_root(){ command1; command2; }
+```
+
+### 6\. Use = instead of == for String Comparisons
+
+Note that `==` is a synonym for `=`, therefore only use a single `=` for string comparisons, for instance:
+
+```
+value1=”tecmint.com”
+value2=”fossmint.com”
+if [ "$value1" = "$value2" ]
+```
+
+### 7\. Use $(command) instead of legacy ‘command’ for Substitution
+
+[Command substitution][7] replaces a command with its output. Use `$(command)` instead of backquotes ``command`` for command substitution.
+
+This is recommended even by [shellcheck tool][8] (shows warnings and suggestions for shell scripts). For example:
+
+```
+user=`echo “$UID”`
+user=$(echo “$UID”)
+```
+
+### 8\. Use Read-only to Declare Static Variables
+
+A static variable doesn’t change; its value can not be altered once it’s defined in a script:
+
+```
+readonly passwd_file=”/etc/passwd”
+readonly group_file=”/etc/group”
+```
+
+### 9\. Use Uppercase Names for ENVIRONMENT Variables and Lowercase for Custom Variables
+
+All bash environment variables are named with uppercase letters, therefore use lowercase letters to name your custom variables to avoid variable name conflicts:
+
+```
+#define custom variables using lowercase and use uppercase for env variables
+nikto_file=”$HOME/Downloads/nikto-master/program/nikto.pl”
+perl “$nikto_file” -h  “$1”
+```
+
+### 10\. Always Perform Debugging for Long Scripts
+
+If you are writing bash scripts with thousands of lines of code, finding errors may become a nightmare. To easily fix things before executing a script, perform some debugging. Master this tip by reading through the guides provided below:
+
+1.  [How To Enable Shell Script Debugging Mode in Linux][1]
+
+2.  [How to Perform Syntax Checking Debugging Mode in Shell Scripts][2]
+
+3.  [How to Trace Execution of Commands in Shell Script with Shell Tracing][3]
+
+That’s all! Do you have any other best bash scripting practices to share? If yes, then use the comment form below to do that.
+
+--------------------------------------------------------------------------------
+
+作者简介：
+
+
+Aaron Kili is a Linux and F.O.S.S enthusiast, an upcoming Linux SysAdmin, web developer, and currently a content creator for TecMint who loves working with computers and strongly believes in sharing knowledge.
+
+----------------
+
+via: https://www.tecmint.com/useful-tips-for-writing-bash-scripts-in-linux/
+
+作者：[ Aaron Kili][a]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]:https://www.tecmint.com/author/aaronkili/
+[1]:https://www.tecmint.com/enable-shell-debug-mode-linux/
+[2]:https://www.tecmint.com/check-syntax-in-shell-script/
+[3]:https://www.tecmint.com/trace-shell-script-execution-in-linux/
+[4]:https://www.tecmint.com/category/bash-shell/
+[5]:https://www.tecmint.com/using-shell-script-to-automate-linux-system-maintenance-tasks/
+[6]:https://www.tecmint.com/wp-content/uploads/2017/05/Use-Double-Quotes-in-Scripts.png
+[7]:https://www.tecmint.com/assign-linux-command-output-to-variable/
+[8]:https://www.tecmint.com/shellcheck-shell-script-code-analyzer-for-linux/