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91 lines
2.7 KiB
Markdown
91 lines
2.7 KiB
Markdown
---
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category: tool
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tool: bash
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contributors:
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- ["Max Yankov", "https://github.com/golergka"]
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- ["Darren Lin", "https://github.com/CogBear"]
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filename: LearnBash.sh
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---
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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.
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Nearly all examples below can be a part of a shell script or executed directly in the shell.
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[Read more here.](http://www.gnu.org/software/bash/manual/bashref.html)
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```bash
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#!/bin/sh
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# First line of the script is shebang which tells the system how to execute
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# the script: http://en.wikipedia.org/wiki/Shebang_(Unix)
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# As you already figured, comments start with #. Shebang is also a comment.
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# Simple hello world example:
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echo Hello, world!
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# Each command starts on a new line, or after semicolon:
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echo 'This is the first line'; echo 'This is the second line'
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# Declaring a variable looks like this:
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VARIABLE="Some string"
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# But not like this:
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VARIABLE = "Some string"
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# Bash will decide that VARIABLE is a command it must execute and give an error
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# because it couldn't be found.
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# Using the variable:
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echo $VARIABLE
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echo "$VARIABLE"
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# When you use the variable itself — assign it, export it, or else — you write
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# its name without $. If you want to use variable's value, you should use $.
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# Reading a value from input:
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echo "What's your name?"
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read NAME # Note that we didn't need to declare new variable
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echo Hello, $NAME!
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# We have the usual if structure:
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if true
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then
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echo "This is expected"
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else
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echo "And this is not"
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fi
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# Expressions are denoted with the following format:
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echo $(( 10 + 5 ))
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# Unlike other programming languages, bash is a shell — so it works in a context
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# of current directory. You can list files and directories in the current
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# directories with ls command:
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ls
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# These commands have options that control their execution:
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ls -l # Lists every file and directory on a separate line
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# Results of the previous command can be passed to the next command as input.
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# grep command filters the input with provided patterns. That's how we can list
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# txt files in the current directory:
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ls -l | grep "\.txt"
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# Commands can be substitued within other commands using $( ):
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# The following command displays the number of files and directories in the
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# current directory.
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echo "There are $(ls | wc -l) items here."
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# Bash uses a case statement that works similarily to switch in Java and C++:
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case "$VARIABLE" in
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#List patterns for the conditions you want to meet
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0) echo "There is a zero.";;
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1) echo "There is a one.";;
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*) echo "It is not null.";;
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esac
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#For loops iterate for as many arguments given:
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#The contents of var $VARIABLE is printed three times.
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for VARIABLE in x y z
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do
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echo "$VARIABLE"
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done
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```
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