Master Linux Shell: Variables, Strings, and Script Essentials

Variable Usage

When defining a Bash variable, omit the leading $ and do not place spaces around the equal sign, e.g. your_name="yikoulinux". Variable names may contain letters, digits and underscores, must not start with a digit, must not use punctuation or Bash reserved words, and are conventionally uppercase.

Valid: RUNOOB, LD_LIBRARY_PATH, _var, var2 Invalid: ?var=123, user*name=runoob Shell variables are divided into system variables (e.g. PWD, USER, HOME) and user‑defined variables.

Define a variable: variable=value List all variables: set Remove a variable: unset variable Make a variable read‑only: readonly variable (cannot be unset)

Assign a command’s output: A=$(ls -la) (equivalent to back‑ticks `ls -la`)

String Operations

Bash provides built‑in parameter expansion for common string manipulations, which is faster than invoking external tools. ${#string} – length of

string

${string:position}

– substring from position to the end ${string:position:length} – length characters starting at

position

${string#substring}

– remove shortest matching substring from the beginning ${string##substring} – remove longest matching substring from the beginning ${string%substring} – remove shortest matching substring from the end ${string%%substring} – remove longest matching substring from the end ${string/substring/replacement} – replace first occurrence of

substring

${string//substring/replacement}

– replace all occurrences of

substring

${string/#substring/replacement}

– replace substring if it appears at the start ${string/%substring/replacement} – replace substring if it appears at the end

Examples:

test="I love china"
# Length
echo ${#test}          # 12
# Substring
echo ${test:5}          # e china
echo ${test:5:10}       # e china
# Delete prefix
path="c:/windows/boot.ini"
echo ${path#/}      # c:/windows/boot.ini
echo ${path#*/}     # windows/boot.ini
echo ${path##*/}    # boot.ini
# Replace characters
echo ${path/\//\\}   # c:\windows/boot.ini

Script Creation and Execution

A Bash script is a plain‑text file (usually with a .sh extension) that contains commands. The first line should specify the interpreter, e.g.:

#!/bin/bash

Comments start with #. To run a script you can use any of the following:

sh script.sh

bash script.sh

./script.sh

(after chmod +x script.sh)

Environment Variables

Export a variable to make it available to child processes:

export VAR_NAME=value
source ~/.bashrc   # reload configuration
echo $VAR_NAME

Common environment variables include HOME and PATH:

echo $HOME
echo $PATH

Arithmetic Operations

Use expr for integer arithmetic and comparisons. Example:

num1=30
num2=50
expr $num1 \> $num2   # 0 (false)
expr $num1 \< $num2   # 1 (true)
expr $num1 + $num2    # addition
expr $num1 - $num2    # subtraction
expr $num1 \* $num2   # multiplication
expr $num1 / $num2   # division
expr $num1 % $num2   # remainder

Script Interaction with Users

Positional parameters $1, $2, … hold command‑line arguments. $# gives the number of arguments, $* treats all arguments as a single string, and $@ treats them as separate words.

#!/bin/bash
for ((i=1; i<=$#; i++))
do
  echo "Param $i is: ${!i}"
 done

The read builtin reads input from the user. Useful options: -p "prompt" – display a prompt -t seconds – timeout -n N – read at most N characters

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

Example that validates a positive integer and computes the sum 1+2+…+n:

#!/bin/bash
while true; do
  read -p "please input a positive number: " num
  expr $num + 1 &>/dev/null
  if [ $? -eq 0 ]; then
    if [ $(expr $num \> 0) -eq 1 ]; then
      sum=0
      for i in $(seq 1 $num); do
        sum=$(expr $sum + $i)
      done
      echo "1+2+...+$num = $sum"
      exit
    fi
  fi
  echo "error, input illegal"
  continue
done

Relational Operators

Within [ ] tests Bash supports the following integer comparison operators: -eq – equal -ne – not equal -gt – greater than -lt – less than -ge – greater than or equal -le – less than or equal

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

String Comparison Operators

String tests in [ ] include: = – equal != – not equal -z – length is zero -n – length is non‑zero $var – true if variable is non‑empty

#!/bin/bash
if [ -z "$1" ]; then
  echo "First argument is empty"
else
  if [ "$1" = "$2" ]; then
    echo "$1 equals $2"
  else
    echo "$1 does not equal $2"
  fi
fi

Common File and Directory Operations

Extract directory name: dirname $path Extract file name: basename $path Batch rename files containing spaces:

function processFilePathWithSpace() {
  find $1 -name "* *" | while read line; do
    newFile=$(echo "$line" | sed 's/[ ][ ]*/_/g')
    mv "$line" "$newFile"
  done
}

Read a file line‑by‑line:

while read line; do
  echo "$line"
 done < /tmp/text.txt

Create a file if it does not exist:

[ -f $logFile ] || touch $logFile

Recursive directory traversal:

function getFile() {
  for file in $(ls $1); do
    element="$1/$file"
    if [ -d $element ]; then
      getFile $element
    else
      echo $element
    fi
  done
}

Clear a file’s contents:

cat /dev/null > $filePath