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[ Linux Command Manual](#) * * * `test` is a built-in command in Linux/Unix systems used for conditional evaluation. It is mainly used for various testing and comparison operations in shell scripts. It can check file attributes, compare strings and numbers, making it an indispensable tool when writing shell scripts. * * * ## Basic Syntax test EXPRESSION # Or use bracket format (more commonly used) > **Note**: When using bracket format, there must be spaces between the expression and brackets, i.e., `` * * * ## Main Function Categories ### 1. File Testing Check various attributes of files or directories: | Expression | Meaning | | --- | --- | | `-e file` | File exists | | `-f file` | Is a regular file (not a directory or device file) | | `-d file` | Is a directory | | `-s file` | File size is not empty | | `-r file` | File is readable | | `-w file` | File is writable | | `-x file` | File is executable | | `-L file` | File is a symbolic link | **Example**: ## Example if[-f"/etc/passwd"]; then echo"This is a regular file" fi * * * ### 2. String Comparison Compare the relationship between two strings: | Expression | Meaning | | --- | --- | | `-z "string"` | String length is 0 | | `-n "string"` | String length is not 0 | | `"str1" = "str2"` | Strings are equal | | `"str1" != "str2"` | Strings are not equal | **Example**: ## Example read-p"Enter username: " username if[-z"$username"]; then echo"Username cannot be empty" fi * * * ### 3. Numeric Comparison Compare the size relationship between two integers: | Expression | Meaning | | --- | --- | | `n1 -eq n2` | Equal | | `n1 -ne n2` | Not equal | | `n1 -gt n2` | Greater than | | `n1 -ge n2` | Greater than or equal | | `n1 -lt n2` | Less than | | `n1 -le n2` | Less than or equal | **Example**: ## Example if[ $(id -u)-eq 0]; then echo"Current user is root" fi * * * ### 4. Logical Operators Combine multiple test conditions: | Operator | Meaning | Example | | --- | --- | --- | | `!` | Logical NOT | `[ ! -f file ]` | | `-a` | Logical AND | `` | | `-o` | Logical OR | `` | **Modern Recommended Usage** (using `&&` and `||`): ## Example &&# File exists and is readable ||# Is a directory or is a file * * * ## Advanced Usage ### 1. Compound Condition Evaluation ## Example if[-f"/etc/passwd"-a-r"/etc/passwd"]; then echo"File exists and is readable" fi ### 2. Combined with if Statement ## Example if["$1" = "start"]; then echo"Starting service..." elif["$1" = "stop"]; then echo"Stopping service..." else echo"Invalid parameter" fi ### 3. Use in Variable Assignment ## Example [-z"$PATH"]&&PATH="/usr/bin:/bin"# Set default value if PATH is empty * * * ## Common Errors and Notes **Space Issues**: ## Example ["$var" = "value"]# Correct ["$var"="value"]# Incorrect, missing spaces **String Variable Quoting**: ## Example [-n"$var"]# Correct, prevents errors when variable is empty [-n$var]# Will cause error when $var is empty **Numeric vs String Comparison**: ## Example # Correct, numeric comparison ["10">"2"]# Incorrect, this is string comparison (lexicographic order) **File Testing Paths**: ## Example [-f"$file"]# Correct, variable is quoted [-f$file]# Will cause error when path contains spaces * * * ## Practical Application Examples ### Example 1: Check if Backup Directory Exists ## Example backup_dir="/var/backups" if[!-d"$backup_dir"]; then mkdir-p"$backup_dir" echo"Created backup directory: $backup_dir" fi ### Example 2: Validate User Input ## Example read-p"Please enter age: " age if["$age"-lt 0-o"$age"-gt 120]; then echo"Invalid age input" exit 1 fi ### Example 3: Service Start Script ## Example #!/bin/bash if["$1" = "start"]; then if[-f"/var/run/service.pid"]; then echo"Service is already running" else echo"Starting service..." # Start command fi elif["$1" = "stop"]; then # Stop logic else echo"Usage: $0 {start|stop}" fi * * * ## Summary The `test` command is a fundamental tool in shell script programming. Mastering it allows you to: * Implement conditional branching logic * Verify file and directory status * Compare strings and numbers * Build complex evaluation conditions Remember key points: 1. Bracket format is more commonly used but pay attention to spaces 2. Variable references should be enclosed in double quotes 3. Distinguish between string comparison and numeric comparison operators 4. Use modern `&&` and `||` instead of `-a` and `-o` [ Linux Command Manual](#)