Getting Started with Linux shell script programming (i)

Environment variables

Some commands

Various environment variables often appear in shell scripts, so to write a script, you must first understand the environment variables. Common commands for handling environment variables are listed below

• Set is used to display local variables
• env to display environment variables
• Export to display and set environment variables
• Source followed by a file, you can directly execute the script command in the file and update

Common environment variables

• Position parameters

Equivalent to the C language, the main function of the argv array, which is represented here in $num, where Num refers to a number. See the program example below

# Foot.sh#!/bin/shecho "#0" echo "#1" echo "#2 $" echo "#3 $" echo "#4 $4" echo "#5 $" echo "#10 ${10}" # Note the curly braces here

• List some variables

List some common variables that can be viewed directly through ECHO

PWD current path, bash's pwd command is the output of the path oldpwd the previous directory Pathshell the command path home user's home directory Shell current shelluser username UID User ID can refer to/etc/ PASSWDPPID Create the process number of the current process, which is the parent process number PS1 prompt variable

Reference

General strings can be used without quotes, with quotation marks (single or double quotes, to block special characters, such as asterisks)

"1 is $"	Double quotes	A string that references a backslash (\) In addition to the dollar sign ($) backslash (\) can output single quotation marks
' 1 is $ '	Single quotation marks	The strings inside the single quotation mark are output as is, including double quotes, but cannot output single quotes
Vara= ' pwd '	Anti-Quote	The contents of the inverted quotation mark are interpreted as shell commands.

Note: ${pwd} and ' pwd ' mean the same, the returned result is a string, and the newline character is deleted

Comparison

Place the expression in brackets, such as ["$num 1"-eq 20] and return 0 if the condition satisfies.

Comparison operators

-eq	Euqal	==
-ge	Greater or equal	>=
-gt	Greater	>
-le	Less or equal	<=
-lt	Less than	<
-ne	Not equal	!=

File operators

-D File	Directory	is a directory
-E File	Exist	Whether there is
-F File	File	is a normal file
-R File	Readable	Whether it is readable
-S file		Whether the file length is not zero
-W File	Writable	Whether it can be written
-X File	Executive	is executable
-L File	Linkable	Whether it is a symbolic link

logical operators

!expression	Logical Non-
Expression1-a expression2	Logic and
Expression1-o expression2	Logical OR OR

Getting Started with Linux shell script programming (i)