bash.html.markdown

---
category: tool
tool: bash
contributors:
    - ["Max Yankov", "https://github.com/golergka"]
    - ["Darren Lin", "https://github.com/CogBear"]
    - ["Alexandre Medeiros", "http://alemedeiros.sdf.org"]
    - ["Denis Arh", "https://github.com/darh"]
    - ["akirahirose", "https://twitter.com/akirahirose"]
    - ["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
---

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.

[Read more here.](http://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)
# 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 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" # => 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 $Variable # => Some string
echo "$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} # => 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} # => 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

# 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} # => 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'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..."

# 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!

# We have the usual if structure:
# 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 [ != $USER ]
# which is invalid syntax
# so the "safe" way to use potentially empty variables in bash is:
if [ "$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 ]
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

# 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

# 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":
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

# 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.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

# 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.";;
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

# 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"
    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 ()
{
    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

# 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)
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"

# 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
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
```