Merge branch 'master' of https://github.com/torgeek/learnxinyminutes-docs

1 files changed, 172 insertions, 53 deletions

diff --git a/bash.html.markdown b/bash.html.markdown
index 271ef62c..11ce4e74 100644
--- a/bash.html.markdown
+++ b/bash.html.markdown
@@ -11,82 +11,121 @@ contributors:
     - ["Rahil Momin", "https://github.com/iamrahil"]
     - ["Gregrory Kielian", "https://github.com/gskielian"]
     - ["Etan Reisner", "https://github.com/deryni"]
-    - ["Jonathan Wang", "https://github.com/Jonathansw"]   
+    - ["Jonathan Wang", "https://github.com/Jonathansw"]
     - ["Leo Rudberg", "https://github.com/LOZORD"]
     - ["Betsy Lorton", "https://github.com/schbetsy"]
     - ["John Detter", "https://github.com/jdetter"]
+    - ["Harry Mumford-Turner", "https://github.com/harrymt"]
+    - ["Martin Nicholson", "https://github.com/mn113"]
 filename: LearnBash.sh
+translators:
+    - ["Dimitri Kokkonis", "https://github.com/kokkonisd"]
 ---
 
-Bash is a name of the unix shell, which was also distributed as the shell for the GNU operating system and as default shell on Linux and Mac OS X.
-Nearly all examples below can be a part of a shell script or executed directly in the shell.
+Bash is a name of the unix shell, which was also distributed as the shell
+for the GNU operating system and as the default shell on most Linux distros.
+Nearly all examples below can be a part of a shell script
+or executed directly in the shell.
 
-[Read more here.](http://www.gnu.org/software/bash/manual/bashref.html)
+[Read more here.](https://www.gnu.org/software/bash/manual/bashref.html)
 
 ```bash
-#!/bin/bash
-# First line of the script is shebang which tells the system how to execute
-# the script: http://en.wikipedia.org/wiki/Shebang_(Unix)
+#!/usr/bin/env bash
+# First line of the script is the shebang which tells the system how to execute
+# the script: https://en.wikipedia.org/wiki/Shebang_(Unix)
 # As you already figured, comments start with #. Shebang is also a comment.
 
 # Simple hello world example:
-echo Hello world!
+echo Hello world! # => Hello world!
 
-# Each command starts on a new line, or after semicolon:
+# Each command starts on a new line, or after a semicolon:
 echo 'This is the first line'; echo 'This is the second line'
+# => This is the first line
+# => This is the second line
 
 # Declaring a variable looks like this:
 Variable="Some string"
 
 # But not like this:
-Variable = "Some string"
+Variable = "Some string" # => returns error "Variable: command not found"
 # Bash will decide that Variable is a command it must execute and give an error
 # because it can't be found.
 
-# Or like this:
-Variable= 'Some string'
+# Nor like this:
+Variable= 'Some string' # => returns error: "Some string: command not found"
 # Bash will decide that 'Some string' is a command it must execute and give an
 # error because it can't be found. (In this case the 'Variable=' part is seen
 # as a variable assignment valid only for the scope of the 'Some string'
 # command.)
 
 # Using the variable:
-echo $Variable
-echo "$Variable"
-echo '$Variable'
+echo $Variable # => Some string
+echo "$Variable" # => Some string
+echo '$Variable' # => $Variable
 # When you use the variable itself — assign it, export it, or else — you write
 # its name without $. If you want to use the variable's value, you should use $.
 # Note that ' (single quote) won't expand the variables!
 
 # Parameter expansion ${ }:
-echo ${Variable}
+echo ${Variable} # => Some string
 # This is a simple usage of parameter expansion
-# Parameter Expansion gets a value from a variable.  It "expands" or prints the value
-# During the expansion time the value or parameter are able to be modified
+# Parameter Expansion gets a value from a variable.
+# It "expands" or prints the value
+# During the expansion time the value or parameter can be modified
 # Below are other modifications that add onto this expansion
 
 # String substitution in variables
-echo ${Variable/Some/A}
+echo ${Variable/Some/A} # => A string
 # This will substitute the first occurrence of "Some" with "A"
 
 # Substring from a variable
 Length=7
-echo ${Variable:0:Length}
+echo ${Variable:0:Length} # => Some st
 # This will return only the first 7 characters of the value
+echo ${Variable: -5} # => tring
+# This will return the last 5 characters (note the space before -5)
+
+# String length
+echo ${#Variable} # => 11
+
+# Indirect expansion
+OtherVariable="Variable"
+echo ${!OtherVariable} # => Some String
+# This will expand the value of OtherVariable
 
 # Default value for variable
 echo ${Foo:-"DefaultValueIfFooIsMissingOrEmpty"}
+# => DefaultValueIfFooIsMissingOrEmpty
 # This works for null (Foo=) and empty string (Foo=""); zero (Foo=0) returns 0.
 # Note that it only returns default value and doesn't change variable value.
 
+# Declare an array with 6 elements
+array0=(one two three four five six)
+# Print first element
+echo $array0 # => "one"
+# Print first element
+echo ${array0[0]} # => "one"
+# Print all elements
+echo ${array0[@]} # => "one two three four five six"
+# Print number of elements
+echo ${#array0[@]} # => "6"
+# Print number of characters in third element
+echo ${#array0[2]} # => "5"
+# Print 2 elements starting from forth
+echo ${array0[@]:3:2} # => "four five"
+# Print all elements. Each of them on new line.
+for i in "${array0[@]}"; do
+    echo "$i"
+done
+
 # Brace Expansion { }
 # Used to generate arbitrary strings
-echo {1..10}
-echo {a..z}
+echo {1..10} # => 1 2 3 4 5 6 7 8 9 10
+echo {a..z} # => a b c d e f g h i j k l m n o p q r s t u v w x y z
 # This will output the range from the start value to the end value
 
-# Builtin variables:
-# There are some useful builtin variables, like
+# Built-in variables:
+# There are some useful built-in variables, like
 echo "Last program's return value: $?"
 echo "Script's PID: $$"
 echo "Number of arguments passed to script: $#"
@@ -98,7 +137,7 @@ echo "Script's arguments separated into different variables: $1 $2..."
 
 # Our current directory is available through the command `pwd`.
 # `pwd` stands for "print working directory".
-# We can also use the builtin variable `$PWD`.
+# We can also use the built-in variable `$PWD`.
 # Observe that the following are equivalent:
 echo "I'm in $(pwd)" # execs `pwd` and interpolates output
 echo "I'm in $PWD" # interpolates the variable
@@ -114,13 +153,14 @@ read Name # Note that we didn't need to declare a new variable
 echo Hello, $Name!
 
 # We have the usual if structure:
-# use 'man test' for more info about conditionals
+# use `man test` for more info about conditionals
 if [ $Name != $USER ]
 then
     echo "Your name isn't your username"
 else
     echo "Your name is your username"
 fi
+# True if the value of $Name is not equal to the current user's login username
 
 # NOTE: if $Name is empty, bash sees the above condition as:
 if [ != $USER ]
@@ -133,7 +173,11 @@ if [ "" != $USER ] ...
 
 # There is also conditional execution
 echo "Always executed" || echo "Only executed if first command fails"
+# => Always executed
 echo "Always executed" && echo "Only executed if first command does NOT fail"
+# => Always executed
+# => Only executed if first command does NOT fail
+
 
 # To use && and || with if statements, you need multiple pairs of square brackets:
 if [ "$Name" == "Steve" ] && [ "$Age" -eq 15 ]
@@ -146,22 +190,39 @@ then
     echo "This will run if $Name is Daniya OR Zach."
 fi
 
+# There is also the `=~` operator, which tests a string against a Regex pattern:
+Email=me@example.com
+if [[ "$Email" =~ [a-z]+@[a-z]{2,}\.(com|net|org) ]]
+then
+    echo "Valid email!"
+fi
+# Note that =~ only works within double [[ ]] square brackets,
+# which are subtly different from single [ ].
+# See https://www.gnu.org/software/bash/manual/bashref.html#Conditional-Constructs for more on this.
+
+# Redefine command `ping` as alias to send only 5 packets
+alias ping='ping -c 5'
+# Escape the alias and use command with this name instead
+\ping 192.168.1.1
+# Print all aliases
+alias -p
+
 # Expressions are denoted with the following format:
-echo $(( 10 + 5 ))
+echo $(( 10 + 5 )) # => 15
 
 # Unlike other programming languages, bash is a shell so it works in the context
 # of a current directory. You can list files and directories in the current
 # directory with the ls command:
-ls
+ls # Lists the files and subdirectories contained in the current directory
 
-# These commands have options that control their execution:
+# This command has options that control its execution:
 ls -l # Lists every file and directory on a separate line
-ls -t # Sort the directory contents by last-modified date (descending)
+ls -t # Sorts the directory contents by last-modified date (descending)
 ls -R # Recursively `ls` this directory and all of its subdirectories
 
 # Results of the previous command can be passed to the next command as input.
-# grep command filters the input with provided patterns. That's how we can list
-# .txt files in the current directory:
+# The `grep` command filters the input with provided patterns.
+# That's how we can list .txt files in the current directory:
 ls -l | grep "\.txt"
 
 # Use `cat` to print files to stdout:
@@ -169,7 +230,12 @@ cat file.txt
 
 # We can also read the file using `cat`:
 Contents=$(cat file.txt)
-echo "START OF FILE\n$Contents\nEND OF FILE"
+# "\n" prints a new line character
+# "-e" to interpret the newline escape characters as escape characters
+echo -e "START OF FILE\n$Contents\nEND OF FILE"
+# => START OF FILE
+# => [contents of file.txt]
+# => END OF FILE
 
 # Use `cp` to copy files or directories from one place to another.
 # `cp` creates NEW versions of the sources,
@@ -187,13 +253,16 @@ cp -r srcDirectory/ dst/ # recursively copy
 # `mv` is also useful for renaming files!
 mv s0urc3.txt dst.txt # sorry, l33t hackers...
 
-# Since bash works in the context of a current directory, you might want to 
+# Since bash works in the context of a current directory, you might want to
 # run your command in some other directory. We have cd for changing location:
 cd ~    # change to home directory
+cd      # also goes to home directory
 cd ..   # go up one directory
         # (^^say, from /home/username/Downloads to /home/username)
 cd /home/username/Documents   # change to specified directory
 cd ~/Documents/..    # still in home directory..isn't it??
+cd -    # change to last directory
+# => /home/username/Documents
 
 # Use subshells to work across directories
 (echo "First, I'm here: $PWD") && (cd someDir; echo "Then, I'm here: $PWD")
@@ -203,6 +272,8 @@ pwd # still in first directory
 mkdir myNewDir
 # The `-p` flag causes new intermediate directories to be created as necessary.
 mkdir -p myNewDir/with/intermediate/directories
+# if the intermediate directories didn't already exist, running the above
+# command without the `-p` flag would return an error
 
 # You can redirect command input and output (stdin, stdout, and stderr).
 # Read from stdin until ^EOF$ and overwrite hello.py with the lines
@@ -216,13 +287,22 @@ print("#stderr", file=sys.stderr)
 for line in sys.stdin:
     print(line, file=sys.stdout)
 EOF
+# Variables will be expanded if the first "EOF" is not quoted
+
+# Run the hello.py Python script with various stdin, stdout, and
+# stderr redirections:
+python hello.py < "input.in" # pass input.in as input to the script
+
+python hello.py > "output.out" # redirect output from the script to output.out
+
+python hello.py 2> "error.err" # redirect error output to error.err
 
-# Run hello.py with various stdin, stdout, and stderr redirections:
-python hello.py < "input.in"
-python hello.py > "output.out"
-python hello.py 2> "error.err"
 python hello.py > "output-and-error.log" 2>&1
+# redirect both output and errors to output-and-error.log
+
 python hello.py > /dev/null 2>&1
+# redirect all output and errors to the black hole, /dev/null, i.e., no output
+
 # The output error will overwrite the file if it exists,
 # if you want to append instead, use ">>":
 python hello.py >> "output.out" 2>> "error.err"
@@ -245,57 +325,75 @@ echo "#helloworld" | tee output.out >/dev/null
 # WARNING: `rm` commands cannot be undone
 rm -v output.out error.err output-and-error.log
 rm -r tempDir/ # recursively delete
+# You can install the `trash-cli` Python package to have `trash`
+# which puts files in the system trash and doesn't delete them directly
+# see https://pypi.org/project/trash-cli/ if you want to be careful
 
 # Commands can be substituted within other commands using $( ):
 # The following command displays the number of files and directories in the
 # current directory.
 echo "There are $(ls | wc -l) items here."
 
-# The same can be done using backticks `` but they can't be nested - the preferred way
-# is to use $( ).
+# The same can be done using backticks `` but they can't be nested -
+# the preferred way is to use $( ).
 echo "There are `ls | wc -l` items here."
 
-# Bash uses a case statement that works similarly to switch in Java and C++:
+# Bash uses a `case` statement that works similarly to switch in Java and C++:
 case "$Variable" in
-    #List patterns for the conditions you want to meet
+    # List patterns for the conditions you want to meet
     0) echo "There is a zero.";;
     1) echo "There is a one.";;
-    *) echo "It is not null.";;
+    *) echo "It is not null.";;  # match everything
 esac
 
-# for loops iterate for as many arguments given:
+# `for` loops iterate for as many arguments given:
 # The contents of $Variable is printed three times.
 for Variable in {1..3}
 do
     echo "$Variable"
 done
+# => 1
+# => 2
+# => 3
+
 
 # Or write it the "traditional for loop" way:
 for ((a=1; a <= 3; a++))
 do
     echo $a
 done
+# => 1
+# => 2
+# => 3
 
 # They can also be used to act on files..
-# This will run the command 'cat' on file1 and file2
+# This will run the command `cat` on file1 and file2
 for Variable in file1 file2
 do
     cat "$Variable"
 done
 
 # ..or the output from a command
-# This will cat the output from ls.
+# This will `cat` the output from `ls`.
 for Output in $(ls)
 do
     cat "$Output"
 done
 
+# Bash can also accept patterns, like this to `cat`
+# all the Markdown files in current directory
+for Output in ./*.markdown
+do
+    cat "$Output"
+done
+
 # while loop:
 while [ true ]
 do
     echo "loop body here..."
     break
 done
+# => loop body here...
 
 # You can also define functions
 # Definition:
@@ -306,6 +404,11 @@ function foo ()
     echo "This is a function"
     return 0
 }
+# Call the function `foo` with two arguments, arg1 and arg2:
+foo arg1 arg2
+# => Arguments work just like script arguments: arg1 arg2
+# => And: arg1 arg2...
+# => This is a function
 
 # or simply
 bar ()
@@ -313,6 +416,8 @@ bar ()
     echo "Another way to declare functions!"
     return 0
 }
+# Call the function `bar` with no arguments:
+bar # => Another way to declare functions!
 
 # Calling your function
 foo "My name is" $Name
@@ -320,42 +425,56 @@ foo "My name is" $Name
 # There are a lot of useful commands you should learn:
 # prints last 10 lines of file.txt
 tail -n 10 file.txt
+
 # prints first 10 lines of file.txt
 head -n 10 file.txt
+
 # sort file.txt's lines
 sort file.txt
+
 # report or omit repeated lines, with -d it reports them
 uniq -d file.txt
+
 # prints only the first column before the ',' character
 cut -d ',' -f 1 file.txt
-# replaces every occurrence of 'okay' with 'great' in file.txt, (regex compatible)
+
+# replaces every occurrence of 'okay' with 'great' in file.txt
+# (regex compatible)
 sed -i 's/okay/great/g' file.txt
+# be aware that this -i flag means that file.txt will be changed
+# -i or --in-place erase the input file (use --in-place=.backup to keep a back-up)
+
 # print to stdout all lines of file.txt which match some regex
 # The example prints lines which begin with "foo" and end in "bar"
 grep "^foo.*bar$" file.txt
+
 # pass the option "-c" to instead print the number of lines matching the regex
 grep -c "^foo.*bar$" file.txt
+
 # Other useful options are:
 grep -r "^foo.*bar$" someDir/ # recursively `grep`
 grep -n "^foo.*bar$" file.txt # give line numbers
 grep -rI "^foo.*bar$" someDir/ # recursively `grep`, but ignore binary files
+
 # perform the same initial search, but filter out the lines containing "baz"
 grep "^foo.*bar$" file.txt | grep -v "baz"
 
 # if you literally want to search for the string,
-# and not the regex, use fgrep (or grep -F)
+# and not the regex, use `fgrep` (or `grep -F`)
 fgrep "foobar" file.txt
 
-# trap command allows you to execute a command when a signal is received by your script.
-# Here trap command will execute rm if any one of the three listed signals is received.
+# The `trap` command allows you to execute a command whenever your script
+# receives a signal. Here, `trap` will execute `rm` if it receives any of the
+# three listed signals.
 trap "rm $TEMP_FILE; exit" SIGHUP SIGINT SIGTERM
 
 # `sudo` is used to perform commands as the superuser
+# usually it will ask interactively the password of superuser
 NAME1=$(whoami)
 NAME2=$(sudo whoami)
 echo "Was $NAME1, then became more powerful $NAME2"
 
-# Read Bash shell builtins documentation with the bash 'help' builtin:
+# Read Bash shell built-ins documentation with the bash `help` built-in:
 help
 help help
 help for
@@ -363,12 +482,12 @@ help return
 help source
 help .
 
-# Read Bash manpage documentation with man
+# Read Bash manpage documentation with `man`
 apropos bash
 man 1 bash
 man bash
 
-# Read info documentation with info (? for help)
+# Read info documentation with `info` (`?` for help)
 apropos info | grep '^info.*('
 man info
 info info