Udemy: Shell Scripting

Quizzes and notes for Udemy online course Shell Scripting: Discover How to Automate Command Line Tasks

Section2: Shell Scripting

• Scripts: A series of commands executed by an interpreter (Shell is the interpreter for shell commands)
• Shebang: #!/bin/bash
  • Current path passed as an argument to the interpreter
  • Does not contain shebang: Executing using current shell
  • Other texts following "#" are comments
• Variables: VARIABLE_NAME="Value"
  • No space around "="
  • Case sensitive (Convention: All upper-case)
  • Use variable: $VARIABLE_NAME or ${VARIABLE_NAME}
  • Assign command to variable: SERVER_NAME=$(hostname)
  • No -/@ signs
• Tests: [ condition-to-test-for ]
  • For example: [ -e /etc/passwd ] checks if this file exists
  • Return status 0 (true) or 1 (false)
  • Other file operators: -d, -f, -r, -s, -w, -x, etc
  • String operators: -z String, -n String, String1 = String2, String1 != String2
  • Arithmetic operators: arg1 -eq/-ne/-lt/-le/-gt/-ge arg2
  • Look up by man test
• If Statements:

  if [ test1 ]
  then
      command 1
  elif [ test2 ]
  then
      command 2
  else
      command N
  fi
  

  • Best practice: Enclose variables in quotes: if [ "$MY_SHELL" = "bash" ]
• For Loop:

  for VARIABLE_NAME in ITEM_1 ITEMN
  do
      command 1
      command 2
      command N
  done
  

  • Lists examples:
    • for COLOR in red green blue
    • COLORS="red green blue"
for COLOR in $COLORS
    • PICTURES=$(ls *jpg)
for PICTURE in $PICTURES
• Positional Parameters:

  $0: script.sh  
  $1: parameter1  
  $2: parameter2
  

  • To access every input variable from $1: for USER in $@
  • Use shift to shift the positional arguments to the left
• STDIN: read -p "PROMPT" VARIABLE

Section3: Exit Statuses and Return Codes

• Exit Status:
  • Every command return a status of 0~255
  • 0 is success, other than 0 are errors
  • $?: Exit code of last command, can assign to variable
  • Explicitly define exit status: exit n (otherwise returns that of previous command)
• Logical Operators:
  • &&: AND (execute second command if only first command succeeds)
  • ||: OR (execute second command if only first command fails)
  • Chain commands together: If previous command exits with 0, command after && will be executed, whereas command after || will not
    • Semicolon ; is same as putting commands on different lines, can also be used outside shell scripts

Section4: Functions

• Define a function:

  function function-name() {
      # Code goes here
  }
  # Or
  function-name() {
      # Code goes here
  }
  

• Call a function:

  function-name
  

• Functions have to be:
  • Defined before it's called
    • Function evaluates at run-time (not pre-compiled), so we can call a function in another function's body (e.g. mutual recursion)
  • No braces around
• Arguments:

  $0: The script itself
  

  • To access all parameters from $1: for USER in $@
• Scopes:
  • By default, variables are global
  • Have to be defined before used
  • Variable defined inside function: Valid after function is executed
  • Local variables:
    • Accessed within the function (only functions can have local variables)
    • Created with: local LOCAL_VAR=1
• Exit status (Return code):
  • Explicitly: return <RETURN_CODE>
  • Implicitly: Exit status of last command
  • 0 is success, other than 0 are errors
  • $?: Exit code of last command, can assign to variable
• PID of current script: $$
• basename: Get only file name of a path

Section5: Shell Script Checklist

• Shebang
• Comments / file header: What the script does, etc.
• Global variables
• Group all functions
  • Use local
• Script body
• Explicitly exit with an exit status
  • Last line with exit 0

Section6: Wildcards

• *: Matches zero or more chars
• ?: Matches one char
• []: A character class: Match any of the chars included between the brackets
  • [!]: Matches any of the chars not included in the bracket (matches exactly one char)
    • [!aeiou]*: Any file not starting with a vowel
  • Ranges: [a-g]*: Any file starting with a, b, c, d, e, f or g
  • Predefined char classes:
    • [[:alpha:]]: Alphabetic letters
    • [[:alnum:]]: Alphanumeric letters
    • [[:digit:]]: Letter in decimal
    • [[:lower:]]: Lowercase letters
    • [[:space:]]: Space, newline and tabs
    • [[:upper:]]: Uppercase letters
• Match a wildcard char:
  • Use an escape char: \? and \*
• Can use in a for loop: for FILE in *.txt

Section7: Case Statements

case "$VAR" in
    pattern_1)
        # Commands
        ;;
    pattern_N)
        # Commands
        ;;
esac

• Use | for multiple patterns
  • Match "yes": [yY]|[yY][eE][sS]
• Use * to catch-all

Section8: Logging

• Who, what, when, where, why
• Syslog standard (Linux)
  • Facilities: kern, user, mail, daemon, auth, local0, local7
  • Severitiese: emerg, alert, crit, err, warning, debug, ...
  • Log locations are configurable:
    • /var/log/messages
    • /var/log/syslog
• Logging with logger

Section9: While Loops

while [CONDITION_IS_TRUE]
do
    command 1
    command 2
    command n
done

• Condition can be a command, continues if exit status is 0
• Increment: ((INDEX++)), this is called arithmetic expansion
• for loop reads file word-by-word. To read the file line-by-line:

  LINE_NUM=1
  while read LINE
  do
      echo "${LINE_NUM}: ${LINE}"
      ((LINE_NUM++))
  done < file_to_read.txt
  

  or pip the file content into while:

  grep * file_to_read.txt | while read LINE
  do
      echo "${LINE}"
  done
  

• Read to multiple variables (like pattern matching): while read FS MP REST
  • First n-1 vars assigned one word each
  • The last command carries all rest words

Section10: Debugging

• -x: Print commands as they execute (after substitution and expansions)
  • Shebang: #!/bin/bash -x
  • Command line: set -x and set +x
  • Options can be combined, for example: #!/bin/bash -ex
• -e: Exit on error
• -v: Print shell input lines as they are read (before substitution or expansions)
  • -vx: Show how commands are written & How they are actually executed
• Manual debugging: For example, use a special variable like DEBUG
  • Use logical Operators:
    $DEBUG && echo "Debug mode ON."
$DEBUG || echo "Debug mode OFF."
  • Set debug to echo:
    DEBUG=echo $DEBUG ls
  • Print args:

  function debug() {
      echo "Executing: $@"
      $@
  }
  debug ls
  

• PS4: Controls what is displayed before a line when using -x option
• Linux and DOS are different in carriage returns
  • Use #!/bin/bash^m or cat -v file to print additional carriage returns
  • Use doc2unix command to convert