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diff --git a/bash.html.markdown b/bash.html.markdown new file mode 100644 index 00000000..d5d08e9d --- /dev/null +++ b/bash.html.markdown @@ -0,0 +1,168 @@ +--- +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"] +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! + +# Each command starts on a new line, or after semicolon: +echo 'This is the first line'; echo 'This is the second line' + +# Declaring a variable looks like this: +VARIABLE="Some string" + +# But not like this: +VARIABLE = "Some string" +# Bash will decide that VARIABLE is a command it must execute and give an error +# because it couldn't be found. + +# Using the variable: +echo $VARIABLE +echo "$VARIABLE" +echo '$VARIABLE' +# When you use the variable itself — assign it, export it, or else — you write +# its name without $. If you want to use variable's value, you should use $. +# Note that ' (single quote) won't expand the variables! + +# String substitution in variables +echo ${VARIABLE/Some/A} +# This will substitute the first occurance of "Some" with "A" + +# Substring from a variable +echo ${VARIABLE:0:7} +# This will return only the first 7 characters of the value + +# Default value for variable +echo ${FOO:-"DefaultValueIfFOOIsMissingOrEmpty"} +# This works for null (FOO=), empty string (FOO=""), zero (FOO=0) returns 0 + +# Builtin variables: +# There are some useful builtin variables, like +echo "Last program return value: $?" +echo "Script's PID: $$" +echo "Number of arguments: $#" +echo "Scripts arguments: $@" +echo "Scripts arguments seperated in different variables: $1 $2..." + +# 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 -ne $USER ] +then + echo "Your name is your username" +else + echo "Your name isn't your username" +fi + +# There is also conditional execution +echo "Always executed" || echo "Only executed if first command fails" +echo "Always executed" && echo "Only executed if first command does NOT fail" + +# Expressions are denoted with the following format: +echo $(( 10 + 5 )) + +# Unlike other programming languages, bash is a shell — so it works in a context +# of current directory. You can list files and directories in the current +# directory with the ls command: +ls + +# These commands have options that control their execution: +ls -l # Lists every file and directory on a separate line + +# 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" + +# You can also redirect a command, input and error output. +python2 hello.py < "input.in" +python2 hello.py > "output.out" +python2 hello.py 2> "error.err" +# The output error will overwrite the file if it exists, if you want to +# concatenate them, use ">>" instead. + +# 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 var $VARIABLE is printed three times. +for VARIABLE in {1..3} +do + echo "$VARIABLE" +done + +# while loop: +while [true] +do + echo "loop body here..." + break +done + +# 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 +} + +# or simply +bar () +{ + echo "Another way to declare functions!" + return 0 +} + +# Calling your function +foo "My name is" $NAME + +# There are a lot of useful commands you should learn: +tail -n 10 file.txt +# prints last 10 lines of file.txt +head -n 10 file.txt +# prints first 10 lines of file.txt +sort file.txt +# sort file.txt's lines +uniq -d file.txt +# report or omit repeated lines, with -d it reports them +cut -d ',' -f 1 file.txt +# prints only the first column before the ',' character +``` |