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EmilySeville7cfg 42040d6d7d Update bash help:

- refresh read command sample
- fix if examples and && with ||

2022-08-19 11:09:16 +10:00

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#!/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!" # => Hello world!

# Each command starts on a new line, or after a semicolon:
echo "This is the first command"; echo "This is the second command"
# => This is the first command
# => This is the second command

# Declaring a variable looks like this:
variable="Some string"

# But not like this:
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.

# 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 "$variable" # => Some string
echo '$variable' # => $variable
# When you use a 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!
# You can write variable without surrounding quotes but it's not recommended.

# Parameter expansion ${...}:
echo "${variable}" # => Some string
# This is a simple usage of parameter expansion such as two examples above.
# 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}" # => A string
# This will substitute the first occurrence of "Some" with "A".

# Substring from a variable:
length=7
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).
# The space before minus is mandatory here.

# String length:
echo "${#variable}" # => 11

# Indirect expansion:
other_variable="variable"
echo ${!other_variable} # => Some string
# This will expand the value of `other_variable`.

# The 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:
array=(one two three four five six)
# Print the first element:
echo "${array[0]}" # => "one"
# Print all elements:
echo "${array[@]}" # => "one two three four five six"
# Print the number of elements:
echo "${#array[@]}" # => "6"
# Print the number of characters in third element
echo "${#array[2]}" # => "5"
# Print 2 elements starting from fourth:
echo "${array[@]:3:2}" # => "four five"
# Print all elements each of them on new line.
for item in "${array[@]}"; do
    echo "$item"
done

# 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: $#"
echo "All arguments passed to script: $@"
echo "Script's arguments separated into different variables: $1 $2..."

# 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.
# Note that you can't use variables here:
from=1
to=10
echo {$from..$to} # => {$from..$to}

# 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 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

# 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!"

# We have the usual if structure.
# Condition is true if the value of $name is not equal to the current user's login username:
if [[ "$name" != "$USER" ]]; then
    echo "Your name isn't your username"
else
    echo "Your name is your username"
fi

# To use && and || with if statements, you need multiple pairs of square brackets:
read age
if [[ "$name" == "Steve" ]] && [[ "$age" -eq 15 ]]; then
    echo "This will run if $name is Steve AND $age is 15."
fi

if [[ "$name" == "Daniya" ]] || [[ "$name" == "Zach" ]]; then
    echo "This will run if $name is Daniya OR Zach."
fi
# There are other comparison operators for numbers listed below:
# -ne - not equal
# -lt - less than
# -gt - greater than
# -le - less than or equal to
# -ge - greater than or equal to

# There is also the `=~` operator, which tests a string against the Regex pattern:
email=me@example.com
if [[ "$email" =~ [a-z]+@[a-z]{2,}\.(com|net|org) ]]
then
    echo "Valid email!"
fi

# 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

# A single ampersand & after a command runs it in the background. A background command's
# output is printed to the terminal, but it cannot read from the input.
sleep 30 &
# List background jobs
jobs # => [1]+  Running                 sleep 30 &
# Bring the background job to the foreground
fg
# Ctrl-C to kill the process, or Ctrl-Z to pause it
# Resume a background process after it has been paused with Ctrl-Z
bg
# Kill job number 2
kill %2
# %1, %2, etc. can be used for fg and bg as well

# 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 )) # => 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 # Lists the files and subdirectories contained in the current directory

# This command has options that control its 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 (stdout) of the previous command can be passed as input (stdin) to the next command
# using a pipe |. Commands chained in this way are called a "pipeline", and are run concurrently.
# 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:
cat file.txt

# We can also read the file using `cat`:
Contents=$(cat file.txt)
# "\n" prints a new line character
# "-e" to interpret the newline escape characters as escape characters
echo -e "START OF FILE\n$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,
# 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      # 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/..    # now in home directory (if ~/Documents exists)
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")
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)
# using "redirection operators". Unlike a pipe, which passes output to a command,
# a redirection operator has a command's input come from a file or stream, or
# sends its output to a file or stream.

# Read from stdin until ^EOF$ and overwrite hello.py with the lines
# between "EOF" (which are called a "here document"):
cat > hello.py << EOF
#!/usr/bin/env python
from __future__ import print_function
import sys
print("#stdout", file=sys.stdout)
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

python hello.py > "output-and-error.log" 2>&1
# redirect both output and errors to output-and-error.log
# &1 means file descriptor 1 (stdout), so 2>&1 redirects stderr (2) to the current
# destination of stdout (1), which has been redirected 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.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

# Run a command and print its file descriptor (e.g. /dev/fd/123)
# see: man fd
echo <(echo "#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
# 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 $( ).
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
    # 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.";;  # match everything
esac

# `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
for Variable in file1 file2
do
    cat "$Variable"
done

# ..or the output from a command
# 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:
function foo ()
{
    echo "Arguments work just like script arguments: $@"
    echo "And: $1 $2..."
    echo "This is a function"
    returnValue=0    # Variable values can be returned
    return $returnValue
}
# 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
# Return values can be obtained with $?
resultValue=$?
# More than 9 arguments are also possible by using braces, e.g. ${10}, ${11}, ...

# or simply
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

# 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

# print file.txt's lines in sorted order
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)
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`)
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
# 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 built-ins documentation with the bash `help` built-in:
help
help help
help for
help return
help source
help .

# Read Bash manpage documentation with `man`
apropos bash
man 1 bash
man bash

# Read info documentation with `info` (`?` for help)
apropos info | grep '^info.*('
man info
info info
info 5 info

# Read bash info documentation:
info bash
info bash 'Bash Features'
info bash 6
info --apropos bash

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