Mastering conditional expression bash script

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Introduction

Learn how to use conditional expressions in Bash scripts with this comprehensive guide. From basic examples to advanced usage, this article covers everything you need to know to master conditional expressions in Bash scripting.

Conditional expressions in Bash scripting are crucial for creating powerful and flexible scripts. Whether you are checking the existence of a file, comparing strings, or evaluating numeric conditions, mastering conditional expressions will make your scripts more robust and versatile. This article will take you through the basics to advanced concepts, complete with examples to illustrate each point.

Understanding Conditional Expressions

Conditional expressions allow Bash scripts to make decisions and execute different commands based on those decisions. These expressions evaluate to true or false, determining the flow of script execution.

Basic Conditional Expressions

File Tests

File tests are used to check the status of files and directories. Here are some common file tests:

  • -e: Check if a file exists.
  • -d: Check if a directory exists.
  • -f: Check if a regular file exists.
  • -r: Check if a file is readable.
  • -w: Check if a file is writable.
  • -x: Check if a file is executable.
#!/bin/bash

file="example.txt"

if [ -e "$file" ]; then
    echo "File exists."
else
    echo "File does not exist."
fi

String Comparisons

String comparisons are used to compare text strings. Here are some common string comparisons:

  • =: Check if strings are equal.
  • !=: Check if strings are not equal.
  • <: Check if the first string is less than the second string (lexicographically).
  • >: Check if the first string is greater than the second string (lexicographically).
#!/bin/bash

string1="hello"
string2="world"

if [ "$string1" = "$string2" ]; then
    echo "Strings are equal."
else
    echo "Strings are not equal."
fi

Numeric Comparisons

Numeric comparisons are used to compare integer values. Here are some common numeric comparisons:

  • -eq: Equal to.
  • -ne: Not equal to.
  • -lt: Less than.
  • -le: Less than or equal to.
  • -gt: Greater than.
  • -ge: Greater than or equal to.
#!/bin/bash

num1=5
num2=10

if [ "$num1" -lt "$num2" ]; then
    echo "$num1 is less than $num2."
else
    echo "$num1 is not less than $num2."
fi

Advanced Conditional Expressions

Combining Expressions

You can combine multiple conditional expressions using logical operators:

  • &&: Logical AND.
  • ||: Logical OR.
#!/bin/bash

file="example.txt"
string="hello"

if [ -e "$file" ] && [ "$string" = "hello" ]; then
    echo "File exists and string is hello."
else
    echo "Condition not met."
fi

Pattern Matching

Pattern matching allows you to check if a string matches a specific pattern.

#!/bin/bash

string="hello world"

if [[ "$string" =~ ^hello ]]; then
    echo "String starts with hello."
else
    echo "String does not start with hello."
fi

Practical Examples

Example 1: File Existence Check

#!/bin/bash
# author: HuuPV
# for example basic for check file exists!
FILE="/home/huupv/scripts/menu_list"
if [ -f $FILE ]; then
    echo -e "$FILE \t File exists!"
else
    echo -e "$FILE \t File not exists!"
fi

Example 2: User Input Validation

#!/bin/bash

read -p "Enter your age: " age

if [[ "$age" =~ ^[0-9]+$ ]]; then
    echo "Valid age."
else
    echo "Invalid age. Please enter a number."
fi

Example 3: Directory Backup Script

#!/bin/bash

source_dir="/path/to/source"
backup_dir="/path/to/backup"

if [ -d "$source_dir" ]; then
    cp -r "$source_dir" "$backup_dir"
    echo "Backup completed."
else
    echo "Source directory does not exist."
fi

File Operators Reference

Here is a comprehensive list of file operators you can use in your Bash scripts:

  • -a FILE: True if file exists.
  • -b FILE: True if file is block special.
  • -c FILE: True if file is character special.
  • -d FILE: True if file is a directory.
  • -e FILE: True if file exists.
  • -f FILE: True if file exists and is a regular file.
  • -g FILE: True if file is set-group-id.
  • -h FILE: True if file is a symbolic link.
  • -L FILE: True if file is a symbolic link.
  • -k FILE: True if file has its `sticky' bit set.
  • -p FILE: True if file is a named pipe.
  • -r FILE: True if file is readable by you.
  • -s FILE: True if file exists and is not empty.
  • -S FILE: True if file is a socket.
  • -t FD: True if FD is opened on a terminal.
  • -u FILE: True if the file is set-user-id.
  • -w FILE: True if the file is writable by you.
  • -x FILE: True if the file is executable by you.
  • -O FILE: True if the file is effectively owned by you.
  • -G FILE: True if the file is effectively owned by your group.
  • -N FILE: True if the file has been modified since it was last read.
  • FILE1 -nt FILE2: True if file1 is newer than file2 (according to modification date).
  • FILE1 -ot FILE2: True if file1 is older than file2.
  • FILE1 -ef FILE2: True if file1 is a hard link to file2.

String Operators Reference

Here are some common string operators you can use in your Bash scripts:

  • -z STRING: True if string is empty.
  • -n STRING: True if string is not empty.
  • STRING1 = STRING2: True if the strings are equal.
  • STRING1 != STRING2: True if the strings are not equal.
  • STRING1 < STRING2: True if STRING1 sorts before STRING2 lexicographically.
  • STRING1 > STRING2: True if STRING1 sorts after STRING2 lexicographically.

Frequently Asked Questions (FAQs)

What is a conditional expression in Bash scripting?

A conditional expression in Bash scripting is used to evaluate a condition and determine the flow of the script based on whether the condition is true or false.

How do you compare strings in Bash?

Strings in Bash can be compared using the = and != operators for equality and inequality, respectively. Lexicographical comparisons can be done using < and >.

Can you use logical operators in Bash conditional expressions?

Yes, logical operators such as && (AND) and || (OR) can be used to combine multiple conditional expressions in Bash.

What are file tests in Bash?

File tests are expressions that check the status of files and directories, such as whether they exist, are readable, writable, or executable.

How do you perform numeric comparisons in Bash?

Numeric comparisons in Bash are done using operators like -eq, -ne, -lt, -le, -gt, and -ge.

Conclusion

Mastering conditional expressions in Bash scripting is essential for writing efficient and powerful scripts. By understanding and using basic and advanced conditional expressions, you can create scripts that handle various scenarios and conditions effectively. Practice with the examples provided and explore further to enhance your scripting skills.

By following the guidelines in this article, you can write scripts that are both robust and versatile, ensuring they meet your needs and handle different situations gracefully. Keep experimenting with different expressions and combinations to deepen your understanding and become proficient in Bash scripting. Thank you for reading the huuphan.com page!

conditional expression bash script

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