Shell test Command

testThe command is a Shell built-in conditional judgment tool used to evaluate expressions and return a boolean value (true/false). It is usually used withifstatements, and is the foundation for implementing logical control in Shell scripts.

The test command in Shell is used to check whether a condition holds true. It can perform tests in three areas: numeric values, strings, and files.

Syntax Format

test EXPRESSION
# 或
[ EXPRESSION ]  # 注意方括号内必须有空格

File Test Operations

testThe command is most commonly used to check file attributes. Below are common file test options:

Operator Description Example
-e File exists [ -e file.txt ]
-f Is a regular file [ -f /path/to/file ]
-d Is a directory [ -d /path/to/dir ]
-r Readable [ -r file.txt ]
-w Writable [ -w file.txt ]
-x Executable [ -x script.sh ]
-s File size > 0 [ -s logfile ]
-L Is a symbolic link [ -L symlink ]

Example Script：

Examples

#!/bin/bash

file="/etc/passwd"

if [ -e "$file" ]; then
    echo "$fileexists"
    if [ -r "$file" ]; then
        echo "and is readable"
    fi
else
    echo "$filedoes not exist"
fi

The output result is:

/etc/passwd 存在
并且可读

String Comparison

testProvides multiple string comparison methods:

Operator Description Example
-z STRING String is empty [ -z "$var" ]
-n STRING String is not empty [ -n "$var" ]
STRING1 = STRING2 Strings are equal [ "$var1" = "$var2" ]
STRING1 != STRING2 Strings are not equal [ "$var1" != "$var2" ]

Important Note: String variables should always be enclosed in double quotes to prevent empty variables from causing syntax errors.

Example：

Examples

#!/bin/bash

read -p "Enter username: " username

if [ -z "$username" ]; then
    echo "Error: username cannot be empty"
    exit 1
elif [ "$username" = "root" ]; then
    echo "Warning: using the root account is not recommended"
else
    echo "Welcome,$username"
fi

After execution, we enter example in the terminal, and the output is similar to the following:

输入用户名: example
欢迎, example

Numeric Comparison

For numeric comparison,testdifferent operators are used:

Operator Description Example
-eq Equal to [ "$a" -eq "$b" ]
-ne Not equal to [ "$a" -ne "$b" ]
-gt Greater than [ "$a" -gt "$b" ]
-ge Greater than or equal to [ "$a" -ge "$b" ]
-lt Less than [ "$a" -lt "$b" ]
-le Less than or equal to [ "$a" -le "$b" ]

Example：

Examples

#!/bin/bash

read -p "Enter age: " age

if [ "$age" -lt 0 ]; then
    echo "Age cannot be negative"
elif [ "$age" -lt 18 ]; then
    echo "Minor"
elif [ "$age" -ge 18 ] && [ "$age" -lt 60 ]; then
    echo "Adult"
else
    echo "Senior citizen"
fi

After execution, we enter 12 in the terminal, and the output is similar to the following:

输入年龄: 12
未成年人

Logical Operators

testSupports logical combinations:

Operator Description Example
! Logical NOT [ ! -f "$file" ]
-a Logical AND [ "$a" -eq 1 -a "$b" -eq 2 ]
-o Logical OR [ "$a" -eq 1 -o "$b" -eq 2 ]

Modern recommended syntax: Use&&and||instead of-aand-o, which is more POSIX-compliant:

Examples

[ "$a" -eq 1 ] && [ "$b" -eq 2 ]  # AND
[ "$a" -eq 1 ] || [ "$b" -eq 2 ]  # OR

Advanced Usage: [[ ]] and (( ))

Bash provides more powerful test syntax:

Double Brackets [[ ]]

  • Supports pattern matching:[[ "$var" == *.txt ]]
  • Supports regular expressions:[[ "$var" =~ ^[0-9]+$ ]]
  • Safer string handling

Arithmetic Comparison (( ))

  • Designed specifically for numeric comparison:(( a > b ))
  • Supports more complex arithmetic expressions

Example：

Examples

if [[ "$file" == *.log ]]; then
    echo "This is a log file"
fi

if (( $count > 10 )); then
    echo "The count exceeds 10"
fi

Practical Application Examples

1. Check if a service is running

Examples

#!/bin/bash

service="nginx"

if systemctl is-active --quiet "$service"; then
    echo "$service"is running"
else
    echo "$service"is not running"
    # You can add a command to start the service here
fi

2. Backup File Check

Examples

#!/bin/bash

backup_file="/backups/data_$(date +%Y%m%d).tar.gz"

if [ ! -f "$backup_file" ]; then
    echo "Error: backup file$backup_filedoes not exist"
    exit 1
elif [ ! -s "$backup_file" ]; then
    echo "Warning: backup file is empty"
else
    echo "Backup verification succeeded"
fi

Common Errors and Debugging Tips

  1. Missing space：[ "$a"="$b" ]is wrong, the correct form is[ "$a" = "$b" ]
  2. Unquoted variable：[ -f $file ]should be[ -f "$file" ]
  3. Confusing string and numeric comparison: Use=for comparing strings,-eqfor comparing numeric values

Debugging tip: Add at the beginning of the scriptset -xto enable debug mode, or useechoto print the test expression:

Examples

echo "Test expression: [$a -eq $b ]"
[ "$a" -eq "$b" ] && echo "is true" || echo "is false"
Other Extensions