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diff --git a/bash.html.markdown b/bash.html.markdown new file mode 100644 index 00000000..191f916a --- /dev/null +++ b/bash.html.markdown @@ -0,0 +1,282 @@ +--- +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"] +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 can't be found. + +# Or like this: +Variable= 'Some string' +# 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' +# 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! + +# String substitution in variables +echo ${Variable/Some/A} +# This will substitute the first occurrence of "Some" with "A" + +# Substring from a variable +Length=7 +echo ${Variable:0:Length} +# This will return only the first 7 characters of the value + +# Default value for variable +echo ${Foo:-"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. + +# 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..." + +# 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 isn't your username" +else + echo "Your name is 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" + +# 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 $(( 10 + 5 )) + +# Unlike other programming languages, bash is a shell so it works in the context +# of a current directory. You can list files and directories in the current +# directory with the ls command: +ls + +# These commands have options that control their execution: +ls -l # Lists every file and directory on a separate line + +# Results of the previous command can be passed to the next command as input. +# grep command filters the input with provided patterns. That's how we can list +# .txt files in the current directory: +ls -l | grep "\.txt" + +# You can redirect command input and output (stdin, stdout, and stderr). +# Read from stdin until ^EOF$ 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 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 +# 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) +rm -v output.out error.err output-and-error.log + +# 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 + +# Or write it the "traditional for loop" way: +for ((a=1; a <= 3; a++)) +do + echo $a +done + +# They can also be used to act on files.. +# This will run the command 'cat' on file1 and file2 +for Variable in file1 file2 +do + cat "$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 + +# 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" $Name + +# There are a lot of useful commands you should learn: +# prints last 10 lines of file.txt +tail -n 10 file.txt +# prints first 10 lines of file.txt +head -n 10 file.txt +# sort file.txt's lines +sort file.txt +# report or omit repeated lines, with -d it reports them +uniq -d file.txt +# prints only the first column before the ',' character +cut -d ',' -f 1 file.txt +# replaces every occurrence of 'okay' with 'great' in file.txt, (regex compatible) +sed -i 's/okay/great/g' file.txt +# print to stdout all lines of file.txt which match some regex +# The example prints lines which begin with "foo" and end in "bar" +grep "^foo.*bar$" file.txt +# pass the option "-c" to instead print the number of lines matching the regex +grep -c "^foo.*bar$" file.txt +# if you literally want to search for the string, +# and not the regex, use fgrep (or grep -F) +fgrep "^foo.*bar$" file.txt + + +# 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 +``` |