Merge pull request #1 from adambard/master

update

1 files changed, 203 insertions, 50 deletions

diff --git a/bash.html.markdown b/bash.html.markdown
index 3b163638..0c097c27 100644
--- a/bash.html.markdown
+++ b/bash.html.markdown
@@ -10,6 +10,12 @@ contributors:
     - ["Anton Strömkvist", "http://lutic.org/"]
     - ["Rahil Momin", "https://github.com/iamrahil"]
     - ["Gregrory Kielian", "https://github.com/gskielian"]
+    - ["Etan Reisner", "https://github.com/deryni"]
+    - ["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"]
 filename: LearnBash.sh
 ---
 
@@ -25,92 +31,193 @@ Nearly all examples below can be a part of a shell script or executed directly i
 # 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:
 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"
+Variable="Some string"
 
 # But not like this:
-VARIABLE = "Some string"
-# Bash will decide that VARIABLE is a command it must execute and give an error
-# because it couldn't be found.
+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' # => 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 variable's value, you should use $.
+# 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} # => 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
+# Below are other modifications that add onto this expansion
+
 # String substitution in variables
-echo ${VARIABLE/Some/A}
-# This will substitute the first occurance of "Some" with "A"
+echo ${Variable/Some/A} # => A string
+# This will substitute the first occurrence of "Some" with "A"
 
 # Substring from a variable
-echo ${VARIABLE:0:7}
+Length=7
+echo ${Variable:0:Length} # => Some st
 # This will return only the first 7 characters of the value
 
 # Default value for variable
-echo ${FOO:-"DefaultValueIfFOOIsMissingOrEmpty"}
-# This works for null (FOO=), empty string (FOO=""), zero (FOO=0) returns 0
+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.
+
+# Brace Expansion { }
+# Used to generate arbitrary strings
+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
-echo "Last program return value: $?"
+echo "Last program's return value: $?"
 echo "Script's PID: $$"
-echo "Number of arguments: $#"
-echo "Scripts arguments: $@"
-echo "Scripts arguments seperated in different variables: $1 $2..."
+echo "Number of arguments passed to script: $#"
+echo "All arguments passed to script: $@"
+echo "Script's arguments separated into different variables: $1 $2..."
+
+# Now that we know how to echo and use variables,
+# let's learn some of the other basics of bash!
+
+# Our current directory is available through the command `pwd`.
+# `pwd` stands for "print working directory".
+# We can also use the builtin variable `$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
+
+# If you get too much output in your terminal, or from a script, the command
+# `clear` clears your screen
+clear
+# Ctrl-L also works for clearing output
 
 # Reading a value from input:
 echo "What's your name?"
-read NAME # Note that we didn't need to declare a new variable
-echo Hello, $NAME!
+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
-if [ $NAME -ne $USER ]
+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 [ != $USER ]
+# which is invalid syntax
+# so the "safe" way to use potentially empty variables in bash is:
+if [ "$Name" != $USER ] ...
+# which, when $Name is empty, is seen by bash as:
+if [ "" != $USER ] ...
+# which works as expected
 
 # 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" ] && [ $AGE -eq 15 ]
+if [ "$Name" == "Steve" ] && [ "$Age" -eq 15 ]
 then
-    echo "This will run if $NAME is Steve AND $AGE is 15."
+    echo "This will run if $Name is Steve AND $Age is 15."
 fi
 
-if [ $NAME == "Daniya" ] || [ $NAME == "Zach" ]
+if [ "$Name" == "Daniya" ] || [ "$Name" == "Zach" ]
 then
-    echo "This will run if $NAME is Daniya OR Zach."
+    echo "This will run if $Name is Daniya OR Zach."
 fi
 
 # 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 a context
-# of current directory. You can list files and directories in the current
+# 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:
 ls -l # Lists every file and directory on a separate line
+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:
 ls -l | grep "\.txt"
 
+# Use `cat` to print files to stdout:
+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
+# => 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,
+# so editing the copy won't affect the original (and vice versa).
+# Note that it will overwrite the destination if it already exists.
+cp srcFile.txt clone.txt
+cp -r srcDirectory/ dst/ # recursively copy
+
+# Look into `scp` or `sftp` if you plan on exchanging files between computers.
+# `scp` behaves very similarly to `cp`.
+# `sftp` is more interactive.
+
+# Use `mv` to move files or directories from one place to another.
+# `mv` is similar to `cp`, but it deletes the source.
+# `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 
+# run your command in some other directory. We have cd for changing location:
+cd ~    # change 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??
+
+# Use subshells to work across directories
+(echo "First, I'm here: $PWD") && (cd someDir; echo "Then, I'm here: $PWD")
+pwd # still in first directory
+
+# Use `mkdir` to create new directories.
+mkdir myNewDir
+# The `-p` flag causes new intermediate directories to be created as necessary.
+mkdir -p myNewDir/with/intermediate/directories
+# if the intermediate directories didn't already exist, running the above
+# command without the `-p` flag would return an error
+
 # You can redirect command input and output (stdin, stdout, and stderr).
 # Read from stdin until ^EOF$ and overwrite hello.py with the lines
 # between "EOF":
@@ -124,17 +231,18 @@ for line in sys.stdin:
     print(line, file=sys.stdout)
 EOF
 
-# 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
-python hello.py > /dev/null 2>&1
+# 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
+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"
 
-# Overwrite output.txt, append to error.err, and count lines:
+# Overwrite output.out, append to error.err, and count lines:
 info bash 'Basic Shell Features' 'Redirections' > output.out 2>> error.err
 wc -l output.out error.err
 
@@ -142,14 +250,16 @@ wc -l output.out error.err
 # see: man fd
 echo <(echo "#helloworld")
 
-# Overwrite output.txt with "#helloworld":
+# Overwrite output.out with "#helloworld":
 cat > output.out <(echo "#helloworld")
 echo "#helloworld" > output.out
 echo "#helloworld" | cat > output.out
 echo "#helloworld" | tee output.out >/dev/null
 
 # Cleanup temporary files verbosely (add '-i' for interactive)
+# WARNING: `rm` commands cannot be undone
 rm -v output.out error.err output-and-error.log
+rm -r tempDir/ # recursively delete
 
 # Commands can be substituted within other commands using $( ):
 # The following command displays the number of files and directories in the
@@ -161,7 +271,7 @@ echo "There are $(ls | wc -l) items here."
 echo "There are `ls | wc -l` items here."
 
 # Bash uses a case statement that works similarly to switch in Java and C++:
-case "$VARIABLE" in 
+case "$Variable" in
     #List patterns for the conditions you want to meet
     0) echo "There is a zero.";;
     1) echo "There is a one.";;
@@ -169,30 +279,37 @@ case "$VARIABLE" in
 esac
 
 # for loops iterate for as many arguments given:
-# The contents of $VARIABLE is printed three times.
-for VARIABLE in {1..3}
+# The contents of $Variable is printed three times.
+for Variable in {1..3}
 do
-    echo "$VARIABLE"
+    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
-for VARIABLE in file1 file2
+for Variable in file1 file2
 do
-    cat "$VARIABLE"
+    cat "$Variable"
 done
 
 # ..or the output from a command
 # This will cat the output from ls.
-for OUTPUT in $(ls)
+for Output in $(ls)
 do
-    cat "$OUTPUT"
+    cat "$Output"
 done
 
 # while loop:
@@ -201,6 +318,7 @@ do
     echo "loop body here..."
     break
 done
+# => loop body here...
 
 # You can also define functions
 # Definition:
@@ -211,6 +329,11 @@ 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 ()
@@ -218,30 +341,60 @@ 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
+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
-# print to stdout all lines of file.txt which match some regex, the example prints lines which begin with "foo" and end in "bar"
+
+# 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
-# if you literally want to search for the string, and not the regex, use fgrep (or grep -F)
-fgrep "^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)
+fgrep "foobar" file.txt
+
+# 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 $TEMP_FILE; exit" SIGHUP SIGINT SIGTERM
+
+# `sudo` is used to perform commands as the superuser
+NAME1=$(whoami)
+NAME2=$(sudo whoami)
+echo "Was $NAME1, then became more powerful $NAME2"
 
 # Read Bash shell builtins documentation with the bash 'help' builtin:
 help