Shell Conditional Statements and Test Operators: Syntax, Examples, and Usage

1. Condition test statements – The article introduces file‑type tests such as -d (directory), -e (exists), -f (regular file) and shows two syntaxes: test -e /root/install.log and [ -e /root/install.log ]. It also covers permission tests ( -r, -w, -x), file‑time comparisons ( -nt, -ot), inode comparison ( -ef), integer comparisons ( -eq, -ne, -gt, -lt, -ge, -le) and string tests ( -z, -n, ==, !=).

2. Single‑branch if statement – Shell’s if syntax differs from most languages; it uses if [ condition ]; then … fi or a newline after then. Logical operators -a (and), -o (or) and negation ! are described.

3. Example: checking whether the current user is root

#!/bin/bash
# script: this is user root
# write: yzc
# date: 20190226
test=$(env | grep "USER" | cut -d "=" -f2)
if [ "$test" == "root" ]; then
    echo -e "dangqianyonghushi root"
fi

This script extracts the USER environment variable, compares it to "root", and prints a message if the comparison succeeds.

4. Example: checking root partition usage

#!/bin/bash
# script: 判断根分区使用率
# write: hahashen
# date: 20190226
test=$(df -h | grep "/dev/mapper/centos-root" | awk '{print $5}' | cut -d "%" -f1)
if [ "$test" -ge "36" ]; then
    echo "/ is full"
fi

The script obtains the usage percentage of the root filesystem, and if it is 36% or higher, it outputs a warning. The article also shows how to make the script executable ( chmod 755 if2.sh) and run it.