[bash/en] Provide example outputs for #549

This commit is contained in:
Keith Miyake 2017-09-29 09:09:39 -07:00
parent e27533f7d1
commit 96f62560ba

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@ -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"}
# => 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