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| -rw-r--r-- | bash.html.markdown | 90 |
1 files changed, 66 insertions, 24 deletions
diff --git a/bash.html.markdown b/bash.html.markdown index 14366e4c..76710aa8 100644 --- a/bash.html.markdown +++ b/bash.html.markdown @@ -30,59 +30,62 @@ 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" # 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' +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' # => Some string # 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 # 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 # Default value for variable -echo ${Foo:-"DefaultValueIfFooIsMissingOrEmpty"} +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} -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: @@ -121,6 +124,7 @@ then 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 +137,11 @@ 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 ] @@ -147,12 +155,12 @@ then fi # Expressions are denoted with the following format: -echo $(( 10 + 5 )) +echo $(( 10 + 5 )) # => 15 # Unlike other programming languages, bash is a shell so it works in the context # of a current directory. You can list files and directories in the current # directory with the ls command: -ls +ls # Lists the files and subdirectories contained in the current directory # These commands have options that control their execution: ls -l # Lists every file and directory on a separate line @@ -169,7 +177,10 @@ cat file.txt # We can also read the file using `cat`: Contents=$(cat file.txt) -echo "START OF FILE\n$Contents\nEND OF FILE" +echo "START OF FILE\n$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, @@ -203,6 +214,8 @@ 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 @@ -217,12 +230,15 @@ 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" @@ -269,12 +285,19 @@ 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 @@ -296,6 +319,7 @@ do echo "loop body here..." break done +# => loop body here... # You can also define functions # Definition: @@ -306,6 +330,11 @@ 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 +342,8 @@ 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 @@ -320,25 +351,35 @@ foo "My name is" $Name # There are a lot of useful commands you should learn: # prints last 10 lines of file.txt tail -n 10 file.txt + # prints first 10 lines of file.txt head -n 10 file.txt + # sort file.txt's lines sort file.txt + # report or omit repeated lines, with -d it reports them uniq -d file.txt + # prints only the first column before the ',' character cut -d ',' -f 1 file.txt -# replaces every occurrence of 'okay' with 'great' in file.txt, (regex compatible) + +# replaces every occurrence of 'okay' with 'great' in file.txt +# (regex compatible) sed -i 's/okay/great/g' file.txt + # print to stdout all lines of file.txt which match some regex # The example prints lines which begin with "foo" and end in "bar" grep "^foo.*bar$" 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$" file.txt | grep -v "baz" @@ -346,8 +387,9 @@ grep "^foo.*bar$" file.txt | grep -v "baz" # 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 |