Master Shell Scripting: From Variables to Advanced Sed Editing

0. Background

The author previously wrote a series of shell practice articles and now provides a concise introduction to shell scripting for beginners, summarizing key concepts learned over time.

1. Syntax

1.1 Variables

Variables are defined without spaces around the equal sign. Example:

str="string"
echo $str
str=123
echo $str

Variables can be referenced with $var or ${var}, and command substitution can assign command output to a variable.

1.2 Printing

Use echo to output strings, variables, or mixed content. Important options: -e enables escape sequences (e.g., \t for tab). -n suppresses the trailing newline.

str4="string4"
echo $str4
echo "str4=$str4"
echo "str4=$str4 str3=$str3"

1.3 Arithmetic

Two common methods:

Using expr (requires escaping *).

Using $[ ... ] which is simpler and does not need escaping.

# expr example
result=$(expr 5 + 5)
echo $result
# $[ ] example
result=$[5 * 5]
echo $result

1.4 Control Structures

Conditional statements use if / else / fi. Numeric comparisons include -eq, -gt, -lt, etc. String comparisons use =, !=, <, >, -n, and -z. File tests include -d, -e, -f, -r, -w, -x, etc.

if [[ 3 -gt 7 ]]; then
echo "yes"
else
echo "no"
fi

1.5 Loops

Supported loops: for (( i=0; i<10; i++ )) – C‑style loop. for item in $(cat file) – iterates over lines.

while [[ condition ]]; do ... done

until [[ condition ]]; do ... done

# C‑style for loop
num=0
for (( i=0; i<10; i++ )); do
  num=$[ $num + $i ]
done
echo "result = $num"

1.6 Arrays (Containers)

Declare indexed arrays with declare -a name. Associative arrays require Bash ≥4.1 ( declare -A name). Example of storing lines in an array and iterating:

declare -a config_content_array
config_content_array[0]="first line"
echo ${config_content_array[0]}

1.7 Functions (Methods)

Define functions using either:

function func1 { ... }

func2() { ... }

Return values can be:

Exit status (0‑255) via return.

Standard output via echo.

Global variables.

function func1 {
  echo "func1 invoked"
  return 280   # actual exit code will be 280 % 256 = 24
}
func2() { echo "return value" }

2. Files

2.1 Reading Files

Read a file line‑by‑line using cat with IFS=$'\n' to preserve line breaks.

file="data"
IFS_OLD=$IFS
IFS=$'
'
for line in $(cat $file); do
  echo "$line"
done
IFS=$IFS_OLD

2.2 Reading Directories

Iterate over directory entries with ls and for, handling sub‑directories recursively.

function read_implement_file_recursively {
  if [[ -d $1 ]]; then
    for item in $(ls $1); do
      itemPath="$1/$item"
      if [[ -d $itemPath ]]; then
        echo "Processing directory $itemPath"
        read_implement_file_recursively $itemPath
      else
        echo "Processing file $itemPath"
      fi
    done
  else
    echo "err: not a directory"
  fi
}
read_implement_file_recursively $(pwd)

2.3 Writing Files

Use output redirection > to overwrite or >> to append. Combine with sort and uniq for sorting and deduplication.

file="subfolder/data2"
destfile="subfolder/data2-p"
sort $file | uniq > $destfile

3. Sed Stream Editing

GNU sed is used for text manipulation (search, replace, insert, delete). Install GNU sed on macOS if needed.

# Check for GNU sed and install via Homebrew if missing
which_sed=$(which sed)
if [[ $(expr "$which_sed" : ".*/gnu-sed/") -gt 0 ]]; then
  echo "GNU sed detected"
else
  brew install gnu-sed --with-default-names
fi

Common sed options: -e add script. -f read script from file. -n suppress automatic printing. -i edit files in‑place.

Common operations: s – substitute. a – append after a line. d – delete a line.

# Append injected content after a method start line
sed -i '/^- \(.*\){$/a\ "${injected_content}"' $file

# Delete lines matching a pattern
sed -i '/${pattern_str}/d' $file

# Replace all occurrences of a class name
sed -i '{ s/${original_class_name}/${result_class_name}/g }' $(grep ${original_class_name} -rl $pbxproj_dir)

4. Modules

4.1 Utility Modules

Separate reusable functions into their own script files and source them.

# Math.sh
power(){
  base=$1
  exp=$2
  result=1
  for (( i=0; i<$exp; i++ )); do
    result=$[ $result * $base ]
  done
  echo $result
}

# Main script
. ./Math.sh
result=$(power 3 5)
echo "3^5 = $result"

4.2 Process Modules

Modules that accept command‑line options via getopts and perform more complex tasks.

# Example of option parsing
while getopts :i:o: opt; do
  case "$opt" in
    i) param_input_dir=$OPTARG;;
    o) param_output_file=$OPTARG;;
    *) echo "Unknown option: $opt";;
  esac
done

5. Input and Menus

5.1 Getting User Input

# Prompt for a directory and validate it
checkInputDestDirRecursive(){
  echo -n "Please enter a directory: "
  read path
  if [[ -d $path ]]; then
    CheckInputDestDirRecursiveReturnValue=$path
  else
    echo -n "Invalid directory, "
    checkInputDestDirRecursive
  fi
}

5.2 Interactive Menu

function genMenu {
  clear
  echo -e "\t\t\tOption Menu
"
  echo -e "\t1. Delete injected content"
  echo -e "\t2. Add injected content"
  echo -e "\t0. Exit menu

"
  echo -en "\t\tEnter option: "
  read -n 1 option
}
while true; do
  genMenu
  case $option in
    0) echo "Bye"; exit 0;;
    1) removeInjectedContent;;
    2) addInjectedContent;;
    *) echo "Wrong!!";;
  esac
  echo
  echo -en "

\tHit any key to continue"
  read -n 1 line
done