dollar 命令

常用示例

说明

$ is the shell's expansion operator for variables, commands, and arithmetic. It triggers substitution of values before command execution. Variable expansion $VAR or ${VAR} retrieves the variable's value. Braces allow modifiers and are required for array access and complex expansions. Command substitution $(command) executes the command and substitutes its output. This replaces the older backtick syntax. Arithmetic expansion $((expr)) evaluates mathematical expressions.

参数

${var}

Value of variable.

${var:-default}

Use default if var is unset or empty.

${var:=default}

Assign default if var is unset or empty.

${var:+alternate}

Use alternate value if var is set.

${var:?error}

Exit with error message if var is unset.

${#var}

String length of var.

${var%pattern}

Remove shortest suffix match.

${var%%pattern}

Remove longest suffix match.

${var#pattern}

Remove shortest prefix match.

${var##pattern}

Remove longest prefix match.

${var/old/new}

Replace first occurrence of old with new.

${var//old/new}

Replace all occurrences of old with new.

${var:offset:length}

Substring extraction.

$?

Exit status of last command.

$$

PID of the current shell.

$!

PID of last background job.

$0

Script or shell name.

$1-$9

Positional parameters.

$@

All positional parameters as separate words.

$*

All positional parameters as a single word.

$#

Number of positional parameters.