Bourne-again shell master in 15 minutes

What is bash?

Bash is Unix shell and command language, among many other shell scripting languages is most widespread.

How to use it

Usually use console or terminal or ssh connection. You can directly put in commands one line at a time. For example, echo command is used to print text:

user@unix_pc:~$ echo "hello there"
hello there

To execute batch of comands, save it into a file (e.g. something.sh containing exact formula above) and execute it, for example:

user@unix_pc:~$ bash something.sh
hello there

Comment

to ensure your script will be readable, you should describe what are you doing in comments. In bash they are separated with #, fr example:

user@unix_pc:~$ echo "only this will be printed" # echo "this will not"
only this will be printed

Variables

to avoid typing in everything manually you can use variables, example:

user@unix_pc:~$ NAME="John"
user@unix_pc:~$ echo "Hello $NAME!"
Hello John!

to assign output of different command or program to variable, use $(command) or `command`syntax, for example saving current directory, pwd into variable and using it:

user@unix_pc:~$ CUR_DIR=$(pwd)
user@unix_pc:~$ echo "Hello I am in $CUR_DIR"
Hello I am in /home/user

Expansions

There are multiple expansions which help you manipulate texts, strings, attributes…

Brace expansions

The bash posses possibility to easily generate set of strings via braces, example:

user@unix_pc:~$ echo A{b,c,d}E
AbE AcE AdE
user@unix_pc:~$ echo "#"{1,2,3}{A,B,C}
#1A #1B #1C #2A #2B #2C #3A #3B #3C
user@unix_pc:~$ echo file_{1..9}.txt
file_1.txt file_2.txt file_3.txt file_4.txt file_5.txt file_6.txt file_7.txt file_8.txt file_9.txt

Parameter Expansions and text manipulation

Cutting/slicing

You can easily manipulate existing string via slicing, substitution, cutting. To do so you may use letter index which is letter’s number representation from the beginning of the text:

Text Hello !	H	e	l	l	o		!
ID from the beginning	0	1	2	3	4	5	6
ID from the end	-7	-6	-5	-4	-3	-2	-1

Example worth thousand words:

user@unix_pc:~$ text="Hello there, I am John Doe" # assign variable
user@unix_pc:~$ echo ${text::5} # print first 5 letters
Hello
user@unix_pc:~$ echo ${text::-3} # print everything but last 3 letters
Hello there, I am John
user@unix_pc:~$ echo ${text:(-3)} # print everything after letter 3 letters before end
Doe
user@unix_pc:~$ echo ${text:12:-3} # print everything 12 letters from beginning up to 3 from letters end
I am John

Text manipulation and letter replacement

Sometimes you want to have text only in upper/lower case or you want to replace sme letters, this example becomes handy:

# Manipulation
user@unix_pc:~$ echo ${text^^} # all Uppercase
HELLO THERE, I AM JOHN DOE
user@unix_pc:~$ echo ${text,,} # all Lowercase
hello there, i am john doe

# Replacement
user@unix_pc:~$ echo ${text/J/j} # replace first "J" with "j"
Hello there, I am john Doe
user@unix_pc:~$ echo ${text//o/O} # replace all "o"s with "O"
HellO there, I am JOhn DOe
user@unix_pc:~$ text2="...Hello again..."
user@unix_pc:~$ echo ${text2#...} # remove ... from beginning only
Hello again...
user@unix_pc:~$ echo ${text2%..} # remove .. from end only
...Hello again.
user@unix_pc:~$ echo ${text2//./} # replace all . with nothing, thus remove them
Hello again
user@unix_pc:~$ echo ${text2//./}" Dude!" # and append something to it
Hello again Dude!

Default values and errors

If you want to avoid mistakes or prepare your script for inexperienced user, default values are good way to go:

user@unix_pc:~$ echo ${FOO:?"Variable is not set..."}
bash: FOO2: Variable is not set...
user@unix_pc:~$ FOO=$RANDOM
user@unix_pc:~$ echo ${FOO:?"Variable is not set..."}
23732
user@unix_pc:~$ echo ${BAR-"default"} # print "default" if BAR is not set
default
user@unix_pc:~$ BAR=$RANDOM
user@unix_pc:~$ echo ${BAR-"default"} # print BAR's value if exists
15399
user@unix_pc:~$ echo ${BAR+"conditional"} # print "conditional" only if BAR exists, nothing otherwise
conditional
user@unix_pc:~$ unset BAR # unset variable
user@unix_pc:~$ echo ${BAR="default"} # print default value and assign it to BAR
default

Miscelaneous

various handy tools how to get additional information of variables, files….

user@unix_pc:~$ echo ${#text} # get the length of text
30

user@unix_pc:~$ FILE=$(pwd)"/ping.log" # get full path to file ping.log
user@unix_pc:~$ echo $FILE
/home/user/ping.log
user@unix_pc:~$ BASE=${FILE##*/} # remove everything before last "/" (including)
user@unix_pc:~$ echo $BASE
ping.log
user@unix_pc:~$ FILENAME=${FILE%$BASE} # remove BASE and leave only filename
user@unix_pc:~$ echo $FILENAME
/home/user/
user@unix_pc:~$ FILENAME2=${FILE%/*} # which is equivalent to this
user@unix_pc:~$ echo $FILENAME2
/home/user

Conditions

as any other scripting language, bash implements conditional program flow, syntax is basically:

if command1; then # if this condition is met
    echo "execute some command"
elif command3; then # if this condition is met
    echo "execute something else"
else # if no condition is met
    echo "execute something different"
fi

for example, testing internet connection can be done via command ping the following way:

if ping -c1 google.com; then # try to ping google.com
    echo "Connected to internet"
elif ping -c1 8.8.8.8; then # if failed, try it using direct ip
    echo "Connected to internet but without valid DNS"
else # everything failed, user is disconnected
    echo "Disconnected from internet"
fi

Conditional execution

you can easily chain commands, one depending on another with and or or statements, && or || respectively:

# returns True only if both commands execute successfully
git commit && git push
# returns True if one of commands execute successfully
# if first is executed successfully not proceed to other one
# because its not necessary
git commit || echo "Commit failed"

[[ expression ]]

to check some basic properties or compare variables it is handy to use [[ … ]] syntax. It is executed as any other program, better to see the example:

# String
if [[ -z "$string" ]]; then
  echo "String is empty"
elif [[ -n "$string" ]]; then
  echo "String is not empty"
fi

list of most useful expressions:

# string conditions
[[ -z STRING ]] # Empty string
[[ -n STRING ]] # Not empty string
[[ STRING == STRING ]] # Equal
[[ STRING != STRING ]] # Not Equal
[[ STRING =~ STRING ]] # Regexp

# number comparison
[[ NUM -eq NUM ]] # Equal
[[ NUM -ne NUM ]] # Not equal
[[ NUM -lt NUM ]] # Less than
[[ NUM -le NUM ]] # Less than or equal
[[ NUM -gt NUM ]] # Greater than
[[ NUM -ge NUM ]] # Greater than or equal
(( NUM < NUM )) # Numeric conditions
[[ -o noclobber ]] # If OPTIONNAME is enabled

# adv. conditions
[[ ! EXPR ]] # Not
[[ X ]] && [[ Y ]] # And
[[ X ]] || [[ Y ]] # Or

# File conditions
[[ -e FILE ]] # Exists
[[ -r FILE ]] # Readable
[[ -h FILE ]] # Symlink
[[ -d FILE ]] # Directory
[[ -w FILE ]] # Writable
[[ -s FILE ]] # Size is > 0 bytes
[[ -f FILE ]] # File
[[ -x FILE ]] # Executable
[[ FILE1 -nt FILE2 ]] # 1 is more recent than 2
[[ FILE1 -ot FILE2 ]] # 2 is more recent than 1
[[ FILE1 -ef FILE2 ]] # Same files

examples

# Combinations
if [[ X ]] && [[ Y ]]; then
  ...
fi

if (( $a < $b )); then
   echo "$a is smaller than $b"
fi

if [[ -e "file.txt" ]]; then
  echo "file exists"
fi

Redirection

Bash uses 3 standard streams of informations.

stream	description
STDOUT	standard output of application
STDERR	errors from program run
STDIN	standard input for application

To efficiently work with modular structure of unix you may redirect STDOUT of one program and use it as input of another or save it as a file instead of printing it out with following syntax:

command1 | command2 # execute command1 and it's output use as input for command2
command1 > file.txt # execute command1 and save output to file.txt, rewrite the file
command1 >> file.txt # execute command1 and save output to file.txt, append the text to end of file

For example, cat program reads STDIN of multiple files and prints them to STDOUT, grep program finds text in file/stream

user@unix_pc:~$ ping -c1 google.com 1>ping.log# ping google 1x, save stats to "ping.log"
user@unix_pc:~$ ping -c4 google.com 1>>ping.log# ping google 4x, append stats to "ping.log"
user@unix_pc:~$ cat ping.log|grep "packet loss" # search for packet loss inside log
1 packets transmitted, 1 received, 0% packet loss, time 0ms
4 packets transmitted, 4 received, 0% packet loss, time 8ms

lets extend conditional expression from previous chapter:

if ping -c1 google.com 1>/dev/null; then # try to ping google.com, redirect STDOUT to /dev/null "bottomless pit"
    echo "Connected to internet"
else # everything failed, user is disconnected
    echo "Disconnected from internet"
fi

now you should see only relevant information without statistics

To do

• functions
• loops
• case/switch
• history
• arrays
• dictionaries

Sources

based on:

1. learnXinYminutes
2. bash-hackers
3. devhints/bash

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Sorry for mistakes, didn’t read much after myself