Beginner-Friendly Guide to Linux Shell Programming

1. Variables

Shell variables are assigned without a dollar sign, e.g. your_name="yikoulinux". Rules: use letters, numbers, and underscores; cannot start with a digit; no spaces around the equal sign; usually uppercase. Valid examples:

RUNOOB
LD_LIBRARY_PATH
_var
var2

. Invalid examples:

?var=123
user*name=runoob

. Common commands: set to list all variables, unset VAR to delete, readonly VAR to make static (cannot unset).

2. String Operations

Built‑in parameter expansion allows length, substring, and replacement. Examples: ${#string} – length of

string

${string:position}

– substring from

position

${string:position:length}

– substring of

length

${string#substring}

– remove shortest matching prefix ${string##substring} – remove longest matching prefix ${string%substring} – remove shortest matching suffix ${string%%substring} – remove longest matching suffix ${string/old/new} – replace first occurrence ${string//old/new} – replace all occurrences

Example to get length:

test='I love china'
echo ${#test}   # prints 12

Example to extract substring:

echo ${test:5}      # e china
echo ${test:4:10}   # ve china

Example to delete prefix/suffix:

test='c:/windows/boot.ini'
echo ${test#/}      # c:/windows/boot.ini
echo ${test#*/}    # windows/boot.ini
echo ${test##*/}   # boot.ini
echo ${test%/*}    # c:/windows
echo ${test%%/*}   # c:

3. Script Creation and Execution

A shell script is a collection of commands saved in a file, usually with .sh extension. The first line (shebang) specifies the interpreter, e.g. #!/bin/bash or #!/bin/sh. Example script first.sh:

#cd /usr/local/sbin
#vim first.sh
#!/bin/bash
## this is my first shell script
# written by 一口Linux 2021.5.3
date
echo "Hello world"

Execute with sh first.sh, bash first.sh, ./first.sh (after chmod +x first.sh).

4. Environment Variables

System variables (e.g. HOME, PWD, SHELL, USER) and user‑defined variables. Set with export VAR=value to make it part of the environment, source a file with source file, and view with echo $VAR. Example:

export PATH=$PATH:/home/peng/yikou/
source /etc/bash.bashrc

5. Arithmetic

Use expr for integer arithmetic and comparisons. Example:

num1=30
num2=50
expr $num1 + $num2   # addition
expr $num1 - $num2   # subtraction
expr $num1 \* $num2   # multiplication
expr $num1 / $num2   # division
expr $num1 % $num2   # remainder
expr $num1 > $num2   # comparison (returns 1 if true)

For loops and conditionals, remember spaces around operators and escape special characters.

6. User Interaction

Positional parameters $0 … $9 hold script name and arguments. $# gives the number of arguments. Use read to get input:

read -p "Please enter your name: " name
echo "Hello $name."

Reading multiple values:

read -p "Enter the values: " val1 val2 val3
echo "$val1"
echo "$val2"
echo "$val3"

Loop over arguments with $* (single string) or $@ (preserves quoting).

7. Logical Operators

Numeric comparisons: -eq, -ne, -gt, -lt, -ge, -le. Example:

a=10
b=20
if [ $a -gt $b ]; then
  echo "a greater than b"
else
  echo "a not greater than b"
fi

8. String Operators

String comparisons: =, !=, -z (zero length), -n (non‑zero length), and testing if a variable is set with [ $var ]. Example script checks two arguments and reports equality, emptiness, etc.

9. File and Directory Operations

Common tasks:

Extract directory: dirname $path Extract filename: basename $path Batch rename files with spaces using find and sed to replace spaces with underscores.

Read file line‑by‑line: cat file | while read line; do echo $line; done Create a file if it does not exist: [ -f $logFile ] || touch $logFile Recursive directory traversal with a function that calls itself on sub‑directories.

Clear file content: cat /dev/null > $filePath All code snippets above are presented exactly as they appear in the original article, and the accompanying screenshots illustrate the command output.