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BASH Guide Summary

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BASH Guide Summary

  1. 1. BASH Guide Summary Ohgyun Ahn
  2. 2. Summary of BASH Guide• This is a PERSONAL summary of Gregs BASH Guide.• The official front page URL of original guide is
  3. 3. BASH is,• an acronym for "Bourne Again Shell".• command interperters.• a layer between system function calls and user.
  4. 4. BASH is NOT,• your operating system.• your window manager.• your terminal.(but it often runs inside terminal)• control your mouse and keyboard.• configure your system.• activate your screensaver.• open your files when you double-click them.• involved in launching applications.
  5. 5. BASH is,• ONLY an interface for you to execute statements, using BASH syntax.• a (possibly interactive) command interpreter, acting as a layer between the user and the system.
  6. 6. BASH mode• Interactive: A mode of operation where a prompt asks you for one command at a time.• Script: A file that contains a sequence of commands.
  7. 7. Prompt types• $ : BASH shell (POSIX, Korn shell, BASH)• % : C shell (csh, tcsh)• # : Running as the systems superuser account
  8. 8. Commands and Arguments$ # This is comment.$ # touch:$ # A COMMAND that changes the Last Modified time.$ # If the filename does not exist,$ # it simply creates that file.$ touch a b c$ lsa b c$ # There MUST be spaces between command and arguments.$ # The amount of whitespace between arguments$ # does not matter.$ echo a b ca b c
  9. 9. Strings• In BASH programming, almost everything is a string.• When you type a command, the commands name is a string.• Each argument is a string.• Variable name are strings.• The contents of variables are strings.• A filename is a string, and most files contains strings.
  10. 10. Types of Commands• Aliases: A way of shortening commands. (only used in interactive shells, not in scripts)$ ls -laF$ alias ll=ls -laF$ ll• Functions: Somewhat like aliases, but can be used in scripts.$ sum () { echo "$1 $2"; }$ sum foo barfoo bar
  11. 11. Types of Commands• Builtins: Basic commands built into BASH$ cd ..$ echo "echo is a bulitin"• Keywords: Quite like builtins, but special parsing rules apply to them$ [[ a < b ]]
  12. 12. Types of Commands• Executables (Application): Also called an external command or application. Invoked by using a pathname.$ ./myprogram
  13. 13. How to figure out the type?• Command Types: aliases, functions, builtins, keywords, executables$ type rmrm is /bin/rm$ type cdcd is a function
  14. 14. Scripts• A script is basically a sequence of commands.• How to make a script: 1. Create a new file. 2. Put this in it at the top: #!/usr/bin/env bash
  15. 15. Shebang• The top-most header of file.• Also called a sha-bang, hashbang, pound-bang.• The way this header works: 1. When the kernel executes a non-binary, it looks at the first line of the file. 2. If the line begins with #!, the kernel uses the line to determine the interpreter that the code should be passed to.
  16. 16. #!What??• #!<interpreter> <argument> This tries to run <interpreter> <argument> to read and run the rest of file.• #!/bin/bash tells the kernel to run the program /bin/bash, and give this script as an argument.• #!/usr/bin/env bash tells the kernel to run the program, /usr/bin/env, and give it two arguments, bash, and this script.• Using /usr/bin/env is more potable.
  17. 17. sh is NOT bash• Do not fooled by examples on the Internet that uses /bin/sh as interperter.• sh is NOT bash.• Refrain from giving scripts .sh extension. It serves no purpose, and its completely misleading. (since its going to be a bash scripts, not a sh scripts)
  18. 18. Creating a script[myscript]#!/usr/bin/env bashecho "Hello, BASH"[prompt]$ # Execute script using bash command.$ bash myscript$ # Or, give your script executable permissions,$ # so you can execute the script as an application.$ chmod +x myscript$ ./myscript
  19. 19. Special Characters• [whitespace] Whitespace (spaces, tabs and newlines). BASH uses whitespace to determine each word. The first word of each command is a command name. Any additional words become arguments to that command.• $ Expansion character. This character is used in most substitutions, including parameter expansion.• # Comment character. Any word beginning with # begins a comment.
  20. 20. Special Characters• Quotes protect the text inside from being split into multiple words or arguments.• text (Single quotes) Prevent the all special characters.• "text" (Double quotes) This permit substitutions to occur. Except for the $, prevent the special characters.$ echo "$HOME"/Users/ohgyun$ echo $HOME$HOME
  21. 21. Special Characters• Escape Character.• ~ Shortcut for home directory.• > or < Redirection characters. There character are used modify (redirect) the input and/or output of a command.• | Pipelines allow you to send to output of one command as the input to another command.
  22. 22. Special Characters• [[ expression ]] Test Expression. This evaluates the conditional expression.• { command; } Command grouping. The commands inside the braces treated as though they were only one command.• `command`, $(command) Command substitution. This executes the inner command first, and then replace the whole `...` or $(...0 with that commands standard ouptut.
  23. 23. Special Characters• (command) Subshell Execution. This executes the command in a new bash shell.• ((expression)) Arithemtic Command. Inside the parentheses, operator such as +,-,*,/ are seen as mathmatical operators.• $((expression)) Arithematic Substitution. This expression is replaced with the result of its arithmetic evaluation.
  24. 24. Special Characters• (command) Subshell Execution. This executes the command in a new bash shell. A subshell is a child process, and changes in subshell causes no effect on the current shell.$ pwd/Users/ohgyun$ (cd ..)$ pwd/Users/ohgyun
  25. 25. Special Characters• ((expression)) Arithemtic Command. Inside the parentheses, operator such as +,-,*,/ are seen as mathmatical operators.• $((expression)) Arithematic Substitution. This expression is replaced with the result of its arithmetic evaluation.$ echo $((5 + 5))10$ (( 5 > 0 )) && echo "True"True
  26. 26. Parameters• Parameters are a sort of named space in memory.• Parameters come in two flavors. - Special parameters: read-only, pre-set by BASH. - Variables: parameters you can create and update.$ echo $$ # Special parameter that retreive the PID70724$ # Variable assignment$ # NO space between the = sign$ foo=bar$ echo $foobar
  27. 27. Parameters is "$XX"?• Parameters do NOT start with a $-sign.• The $-sign means Parameter expansion, and replaces the parameter to its content.$ foo=bar$ # Parameter Name: foo$ # Usage: "$foo"
  28. 28. Parameter Expansion• Parameter expansion is the substitution of a parameter by its value.$ foo=bar$ echo "Foo is $foo" # Parameter expands to its value.Foo is bar$ # Remember?$ # Single quote does not expand parameters.$ echo Foo is $fooFoo is $foo
  29. 29. "Double Quote it!"• Parameter expansion causes the $parameter to be replaced by its contents. Note the following!$ song="My song.mp3"$ rm $songrm: My: No such file or directoryrm: song.mp3: No such file or directory• Because Bash replaced $song by its content, being My song.mp3; then it performed word splitting; and only THEN executed the command. It was as if you had typed this:$ rm My song.mp3
  30. 30. "Double Quote it!"• How do we fix this? Put double quotes around every parameter expansion!$ rm "$song"
  31. 31. Special Parameters• Parameters that arent variables.• Parameter Name: 0 / Usage: "$0" Contains the name, or the path, of the script.• Parameter Name: ? / Usage: $? Expands to the exit code of the most recently completed foreground command.• Parameter Name: $ / Usage: $$ Expands to the PID of the current shell. (PID = process ID number)• Parameter Name: ! / Usage: $! Expands to the PID of the command most recently executed in the background.
  32. 32. Special Parameters• Parameter Name: 1 2 etc. / Usage: "$1" Positional Parameters contain the arguments that were passed to the current script or function.• Parameter Name: * / Usage: "$*" Expands to words of all the positional parameters. It expands to a single string containing them all.• Parameter Name: @ / Usage: "$@" Expands to words of all the positional parameters. It expands to a list of them all as individual words.• Parameter Name: _ / Usage: "$_" Expands to the last argument of the last command that was executed.
  33. 33. Reserved variables• The variables that the shell provides:• BASH_VERSION: The version of Bash• HOSTNAME: The hostname of your computer.• PWD: Current working directory• PPID: Contains the PID of the parent process.• RANDOM: A random number between 0 and 32767.• UID: The ID number of the current user.• HOME: The current users home directory.• PATH: A colon-seperated list of paths that will be searched to find a command.• PS1: The format of your shell prompt.• TMPDIR: The directory for temporary files.
  34. 34. Variable Types• Although Bash is not a typed language, it does have a few different types of variables.• Array: declare -a variable The variable is an array of strings.• Integer: declare -i variable The variable holds an integer. It automatically triggers Arithmetic Evaluation.• Read Only: declare -r variable The variable can no longer be modifed or unset.• Export: declare -x variable The variable will be inherited by any child process.
  35. 35. Variable Types$ a=5; a+=2; echo $a; unset a52$ declare -i a=5; a+=2; echo $a; unset a7$ a=5+2; echo $a; unset a5+2$ declare -i a=5+2; echo $a; unset a7$ declare -r a=5$ a=10bash: a: readonly variable
  36. 36. ${Parameter expansion}• ${parameter} You can use curly braces to mark the beginning and the end of the parameter name.$ foo=bar$ echo "$foos, ${foo}s", bars• ${#parameter} The length in characters of the value.$ foo=abcde$ echo ${#foo}5
  37. 37. ${Parameter expansion}• ${parameter:-word} Use default value, if parameter is unset or null.$ echo "${xxx:-bar}"bar$ echo "$xxx"• ${parameter:=word} Assign default value, if parameter is unset or null.$ echo "${xxx:=bar}"bar$ echo "$xxx"bar
  38. 38. ${Parameter expansion}• ${parameter:+word} Use alternate value, if parameteris NOT unset or null.$ echo "${xxx:+bar}"$ xxx=foo$ echo "${xxx:+bar}"bar$ echo "$xxx"foo
  39. 39. ${Parameter expansion}• ${parameter:offset:length} Substring expansion. Expands to up to length characters of parameter starting at the offset(0-indexed)$ foo=abcde$ echo ${foo:1:3}bcd$ echo ${foo:1}bcde$ # If offset is negative (use parentheses!),$ # count backword from the end of parameter$ echo ${foo:(-3):2}cd
  40. 40. ${Parameter expansion}• Substring removal: Remove the value matching the pattern.• ${parameter#pattern} From the beginning, the shortest match is deleted.• ${parameter##pattern}• From the beginning, the longest match is deleted.• ${parameter%pattern} From the end, the shorted match is deleted.• ${parameter%%pattern} From the end, the longest math is deleted.
  41. 41. ${Parameter expansion}$ foo="a b c d e"$ # Pattern: * $ echo ${foo#* } # greedyb c d e$ echo ${foo##* }e$ # Pattern: *$ echo ${foo% *}a b c d• echo ${foo%% *}a
  42. 42. ${Parameter expansion}• ${parameter/pattern/string} The first match of pattern replaced by string.• ${parameter//pat/string} Every match of pattern is replaced.$ foo="a a a"$ echo ${foo/a/x}x a a$ echo ${foo//a/x}x x x
  43. 43. Patterns• Pattern matching serves two roles in the shell: 1. Globs: Selecting filenames within a directory. 2. Regular Expression: Pattern matching for strings, not for filename.
  44. 44. Globs• Globs can be used to match filenames.• Meta characters: * : Mathces any string, including the null string. ? : Mathces any single character. [...] : Mathces any one of the enclosed characters.• Globs must match a WHOLE string.• Use globs instead of ls to enumerate files
  45. 45. Globs$ lsa ab abc bc bcd$ echo *a ab abc bc bcd$ echo b*bc bcd$ echo ?a$ echo ?bab$ echo *[c-z]abc bc bcd
  46. 46. Extended Globs• Bash also supports a feature called Extended Globs.• This feature is turend off by default. You can turn it on using shopt command. (shopt = shell options)• Patterns: ?(pattern-list) : Matches zero or one occurence *(pattern-list) : zero or more +(pattern-list) : one or more @(pattern-list) : one !(pattern-list) : anything except (pattern-list is a list of items sperated by |)
  47. 47. Extended Globs$ shopt -s extglob # turn on extended globs$ lsaaab.jpg aab.jpg ab.jpg ab.txt$ echo a?(a)b*aab.jpg ab.jpg ab.txt$ echo a*(a)b*aaab.jpg aab.jpg ab.jpg ab.txt$ echo a+(a)b*aaab.jpg aab.jpg$ echo a@(a)b*aab.jpg$ echo !(*jpg)ab.txt
  48. 48. Regular Expressions• Regular Expression can only be used for pattern matching, not for filename matching.• Bash supports the =~ operator to the [[ keyword. When the string matches the pattern, [[ returns with an exit code of 0 ("true"). Exit code 1 ("false") is returned if not matched.
  49. 49. Regular Expressions$ regex=a(b)(c) # put your regex in variable$ [[ abc =~ $regex ]] # use WITHOUT quotes$ && echo matchedmatched$ # Capturing group assigned to BASH_REMATCH variable$ echo $BASH_REMATCHabc$ echo ${BASH_REMATCH[1]} / ${BASH_REMATCH[2]}b / c
  50. 50. Brace Expansion• Brace expansion is used to generate lists of words.• Globs only expand to actual filenames, but brace expansion will extpand to any possible permutaion of their contents.
  51. 51. Brace Expansion$ echo {a,b,c} # NO spaces between words.a b c$ echo a{b,c}dabd acd$ echo {1..5} # Creating ranges1 2 3 4 5$ # Generate all possible combinations$ echo {A..C}{1..3}A1 A2 A3 B1 B2 B3 C1 C2 C3
  52. 52. Exit status• Whenever a command ends it notifies its parent of its exit status. This is represented by a number ranging from 0 to 255. This code is a hint as to the success of the commands execution• Convention dictates that we use 0 to denote success, and any other number to denote failure of some sort.$ # Make sure that your scripts always return a$ # non-zero exit code if something wrong in execution.$ rm file || { echo Something wrong! >&2; exit 1; }$ echo $? # Exit code of the last foreground process
  53. 53. Control Operators (&& and ||)• && and || are used to link commands together. They check the exit code of the previous command to determine whether or not execute the next command. This concept is called conditional execution.$ mkdir d && cd d$ rm foo.txt || echo I couldnt remove the file
  54. 54. Grouping statements• You can group commands using culry brace.$ # Use a semicolon to separate the commands$ { echo "A"; echo "B"; echo "C"; }ABC$ # Using multi-line$ cd not_exist || {$ echo "Directory does not exist >&2$ exit 1$}
  55. 55. Conditional Blocks• if COMMAND; then COMMAND elif COMMAND; then COMMAND else COMMAND fi$ if [[ $name = "foo" ]]; then$ echo "Hello, $name"$ elif [[ $name = "bar" ]]; then$ echo "Bonjour, $name"$ else$ echo "Who are you? $name"$ fi
  56. 56. Test commands• [ or test is a normal command that reads its arguments and does some checks with them.• [[ is much like [, but i offers far more versatility.• Whenever youre making a BASH script, you should always use [[ rather than [.
  57. 57. Test commands$ [[ -e FILE ]] # True if file exists$ [[ -f FILE ]] # True if file is a regular file$ [[ -d FILE ]] # True if file is a directory$ [[ -h FILE ]] # True if file is a symbolic link$ [[ -w FILE ]] # True if a writable by you$ [[ -r FILE ]] # True if redable by you$ [[ FILE -ot FILE ]] # True if the first file is$ # older than second$ [[ FILE -nt FILE ]] # newer than second$ [[ INT -eq INT ]] # True if both integers are equal$ [[ INT -lt INT ]] # less than$ [[ INT -gt INT ]] # greater than$ [[ INT -le INT ]] # less than or equal$ [[ INT -ge INT ]] # greater than or equal
  58. 58. Test commands$ [[ STRING = STRING ]] # string comparison$ [[ STRING != STRING ]]$ [[ STRING = PATTERN ]] # glob pattern matching$ [[ STRING =~ REGEX ]] # regex pattern matching$ [[ EXPR -a EXPR ]] # logical AND$ [[ EXPR && EXPR ]]$ [[ EXPR -o EXPR ]] # logical OR$ [[ EXPR || EXPR ]]$ [[ ! EXPR ]] # logical NOT
  59. 59. Conditional Loops• while COMMAND; do COMMAND• done• until COMMAND; do COMMAND done• for VARIABLE in WORDS; do COMMAND• done• for (( EXPR; EXPR; EXPR )); do COMMAND done
  60. 60. While Loops$ # inifinite Loop$ while true; do$ echo "inifinite loop"$ done$ # Check your email every five minutes$ while sleep 300; do$ kmain --check$ done$ # Wait for a host to come back online.$ until ping -c 1 -W 1 "$host"; do$ echo "$host is still unavailable"$ done
  61. 61. For Loops$ # For loop with arithmetic expression$ for (( i=0; i<3; i++ )); do$ echo $i$ done$ # For loop with brace expansion$ for i in {1..10}; do$ echo $i$ done$ # For loop with glob pattern$ for file in *.mp3; do$ rm "$file" # wrap with quotes!$ done
  62. 62. Choices• case VARIABLE in CASE) COMMAND ;; CASE) COMMAND ;; *) COMMAND ;; esac$ case $LANG in$ en*) echo Hello! ;;$ fr*) echo Salut! ;;$ de*) echo Guten Tag! ;;$ *) echo I cannot speak your language. ;;$ esac
  63. 63. Arrays• An array is a numbered list of strings$ # =() syntax:$ names=("A" "B" "C" "D")$ # You can also specify explicit indexes$ names=([0]="A" [1]="B" [20]="C")$ # You can also get/set a item with index$ echo ${names[0]}$ names[3]="E"$ # If you want to fill an array with filenames,$ # you can use globs in there.$ photos=(~/"my photos"/*.jpg)
  64. 64. Arrays$ # Get filenames of current directory$ files=(*)$ # Using arrays in for loop.$ # ${myarray[@]} will expand all elements$ for file in "${myfiles[@]}"; do$ cp "$file" /backup/$ done$ # you can merge items with delimiter$ # using IFS with "${arrayname[*]}"$ names=("A" "B" "C" "D")$ ( IFS=,; echo "Names: ${names[*]}" )Names: A,B,C,D
  65. 65. Input and Output• Input refers to any information that your program receives or reads.• Bash script can come from: - Command-line arguments (positional parameters) - Environment variables, inherited from parents - Files - Anything else a File Descriptor can point to
  66. 66. Input and Output• Output refres to any information that your program produces or writes.• Output from a Bash script: - Files - Anything else a File Descriptor can point to - Command-line arguments to some other program - Environment variable passe to some other program
  67. 67. Command-line Arguments• There are some special parameters available to every script which contain these arguments. These are called Positional Parameters.• Positional Parameters: "$1", "$2", ... , "${10}", ... "$@" : The entire set of position parameters
  68. 68. The Environment• Every progmra inherits information, resources, privileges and restrictions from its parent process. One of thoes resources is a set of variables called Environment Variables.$ echo $PATH # All-capital letters$ # If you want to put information into the evironment$ # for your CHILD processes to inherits.$ # Use the export command:$ export MYVAR=something
  69. 69. File Descriptos• File Descriptors (in short:FDs) are the way programs refer to files, or to other resources that work like files (such as pipes, devices, sockets, or terminals).• FDs are kind of pointers to sources of data, or places data can be written.• When something reads from writers to that FD, the data is read from or written to that FDs resources.• By default, every new process starts with open FDs: - Standard Input (stdin): File Descriptor 0 - Standard Output (stdou): File Descriptor 1 - Standard Error (stderr): File Descriptor 2
  70. 70. File Descriptos• In an interactive shell, or in a script running on a termnial: FD 0 : the characters type on keyboard FD 1 : normal informations to terminal FD 2 : error informations to terminal
  71. 71. Redirection• Redirection is used to change the data source or destination of a programs FDs. That way, you can send output to a file insted of the terminal, or have an application read from a file instead of from the keyboard.$ echo "Hello" > file$ cat fileHello command > file : Send stdout of command to file command 1> file : Same with above command < file : Use contents of file when command reads from stdin. command 0< file : Same with above
  72. 72. Redirection$ # Redirect stderr to errors file$ rm not_exist 2> errors$ # Append new data to end of file.$ rm not_exist 2>> errors$ # Ignore error message.$ # /dev/null is a virtual divice that is always empty.$ rm not_exist 2> /dev/null$ # >& syntax:$ # Duplicate FD1 and put this duplicate in FD2$ grep keyword file not_exist_file > log 2>&1
  73. 73. Heredocs And Herestrings• Heredocs are useful if youre trying to embed short blocks of multi-line data inside your script.$ grep proud <<END # END = terminator string$ I am a proud sentence.$ END• The most common use of Heredocs is dumping documentation to the user:$ usage() {$ cat <<EOF # Terminator string can be any word$ usage: foobar [-xyz] [file ...]$ EOF$ }
  74. 74. Heredocs And Herestrings• Herestrings are shorter, less instrusive and overall more convenient that Heredoc.$ grep proud <<<"I am a proud sentence"
  75. 75. Pipes• Pipe connects the stdout of one process to the stdin of another. (I/O Redirecting)$ echo "I am proud sentence" | grep proud• The pipe operator creates a subshell environment for each command.$ foo=bar$ # Pipe creates a subshell and modifications in$ # subshell are lost.$ echo Foo changed | read foo$ echo $foobar
  76. 76. Arithmetic Evaluation$ (( a = (5+2)*3 ))$ if (($a == 21 )); then echo Blackjack!; fiBlackjack!$ # Finding error log in input file$ flag=0$ while read line; do$ if [[ $line = *err* ]]; then flag=1; fi$ done < inputfile$ if ((flag)); then echo "Find error!"; fi
  77. 77. Functions$ sum() {$ echo "$1 + $2 = $(($1 + $2))"$ }$ sum 2 52 + 5 = 7$ count() {$ local i # local variable$ for ((i=1;i<=$1;i++)); do echo $i; done$}$ count 212
  78. 78. Aliases• Aliases do not work in scripts, at all. They only work in interactive shell.• Aliases cannot take arguments.• Aliases will not invoke themselves recursively.• Aliases cannot have local variables.$ alias ls=ls --color=auto$ ls /tmp