Script Execution Methods

Shell scripts are text files containing Linux commands executed by an interpreter.

Using sh

sh script.sh

Direct Execution

To execute a script directly:

1. Begin the script with #!/bin/bash
2. Grant execution permissions:

chmod u+x script.sh
./script.sh

This method internally uses the /bin/bash interpreter.

Common Symbols and Commands

• Command prompt ending with $ indicates a regular user; # indicates root (highest privileges)
• In scripts, # denotes comments
• View shell version: $SHELL or $0
• Root user ID is 0: echo $UID
• PS1 defines the command prompt format, e.g., PS1="\\e\\u@\\h~\\w"

Output Commands

• echo: outputs text with newline
• printf: outputs text without newline

# Standard output with newline

echo hello world

# No newline

echo -n "hello world"

# With escape sequences

echo -e "\t\n"

# Colored output

echo -e "\e[1;42m hello world \e[0m"

# printf syntax similar to C
printf "%-20s\n" jack

File Operations

• cat script.sh: view contents
• head script.sh: view beginning
• tail -f script.sh: view end continuously
• vim script.sh: edit file
• touch script.sh: create file
• ls -l script.sh: detailed file info

Text Replacement

• tr: replace characters

# Replace 'a' with 'b'
echo xxx | tr "a" "b"

# Convert lowercase to uppercase
echo "abc" | tr "a-z" "A-Z"

# Delete specific characters
echo "hello" | tr -d "l"

# Keep only digits
echo "abc123def" | tr -d -c "0-9"

Arithmetic Operations

• let: perform calculations
• $((expression)): arithmetic expansion
• expr: evaluate expressions

# Basic arithmetic
result=$((a + b))

# Floating point
echo "2*0.5" | bc

# Decimal precision
echo "scale=2;5/6" | bc

# Binary conversion
num=10
echo "obase=2;$num" | bc

# Square root
echo "sqrt(100)" | bc

# Exponentiation
echo "2^3" | bc

File Descriptors

• File descriptors are integers referring to open files
• 0: standard input, 1: standard output, 2: standard error

# Redirect standard output
echo "hello" > output.txt

# Append to file
echo "world" >> output.txt

# Redirect stderr to file
ls 2> error.txt

# Redirect stderr to stdout
ls 2>&1

# Discard stderr
ls 2>/dev/null

# Tee command
echo "data" | tee -a output.txt | cat

# Input redirection
cat < input.txt

# Here document
cat << EOF > log.txt
content here
EOF

# Custom file descriptor
exec 3<input.txt
exec 4>output.txt

File Checksums

• Generate checksums: md5sum file1 file2 > checksum.md5
• Verify integrity: md5sum -c checksum.md5

Example: generate checksums for all C file

find . -type f -name "*.c" -print0 | xargs -0 md5sum >> c.md5

Sorting

• sort: sort lines
• Combine with uniq for deduplication

# Sort numerically
sort -n data.txt

# Sort by column
sort -k 2 data.txt

# Reverse order
sort -r data.txt

# Dictionary order
sort -d data.txt

# Deduplicate
sort data.txt | uniq

# Count duplicates
sort -k 3 data.txt | uniq -c

Find Command

Find files recursively based on criteria:

# Basic usage
find ./ -maxdepth 3 -type f -name "*.txt"

# File types: f (file), d (directory), l (link)

# Time-based filters
find . -type f -name "*.c" -atime 7   # exactly 7 days ago
find . -type f -name "*.c" -atime +7  # older than 7 days
find . -type f -name "*.c" -atime -7  # newer than 7 days

# Size filters
find . -type f -size +20k   # larger than 20KB
find . -type f -size -20k   # smaller than 20KB

# Permission filters
find . -type f -perm 644

# Execute actions
find . -type f -name "*.py" -exec rm {} \;
find . -type f -name "*.py" -exec wc -l {} \;

Command Line Arguments

• Use xargs to convert input into command-line arguments

# Default delimiter is whitespace
printf "a\nb\nc" | xargs -n 2 echo

# Custom delimiter
printf "a,b,c" | xargs -d , echo

# Replace placeholders
printf "a\nb" | xargs -I {} echo "{}"

# Process files with null delimiters
find . -type f -print0 | xargs -0 rm -f

File Permissions

• Modify permissions using chmod

# Set specific permissions
chmod u=rwx,g=r,o=r script.sh

# Add execute permission for all
chmod a+x script.sh

# Remove write and execute for owner
chmod u-wx script.sh

# List all files with details
ls -la

Variables

Regular Variables

# Assign values
name='jack'

# Multi-line strings
pyCode='import os\nimport csv\nxxxx'

# Use variables
state="name is $name, current dir $(pwd)"

Environment Variables

Per-user

Edit ~/.bashrc:

export PATH=$PATH:/home/user/bin
export LD_LIBRARY_PATH=/usr/local/lib

Reload configuration:

source ~/.bashrc

System-wide

Edit /etc/profile:

export PATH=$PATH:/home/user/bin

Apply changes:

source /etc/profile

Script Parameters

# Pass arguments when running
./script.sh param1 param2

# Access parameters
$1, $2, ..., $9
${10}

# Total count
$#

# All parameters
$*
$@

# Last command exit status
$?

Conditional Statements

Test Conditions

# Numeric comparisons
-eq, -gt, -lt, -ge, -le

# String comparisons
=, ==, !=, -z (empty), -n (not empty)

# File tests
-e, -f, -d, -r, -w, -x

# Examples
if test 5 -gt 3; then
    echo "true"
fi

if [[ $1 = '' ]]; then
    echo "no params"
elif [[ $1 = 'lauf' ]] || [[ $1 = 'tom' ]]; then
    echo "lauf or tom params"
else
    echo "other params"
fi

# Short-circuit evaluation
[ $? -eq 0 ] && cd lauf

Loops

# Numeric range
for i in {1..5}; do
    echo $i
done

# String iteration
IFS=";"
for i in "jack;lucy;lili"; do
    echo $i
done

# Directory traversal
for file in *; do
    echo $file
done

Functions

Functions resemble JavaScript syntax:

function func1() {
    echo $1
    echo $#
    echo $@
}

func2() {
    echo "func2"
}

# Call function
func1 "tom" 4

Practical Example

Recursive directory file creation:

#!/bin/bash

function addFile() {
    if [ -d "$1" ]; then
        echo "adding file to $1"
        touch "$1/lauf.txt"
        dealDir "$1"
    fi
}

function dealDir() {
    cd "$1"
    curFiles=$(ls)
    for item in $curFiles; do
        addFile "$item"
    done
}

dealDir "$1"