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292 lines
8.5 KiB
Markdown
292 lines
8.5 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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- ["Alexandre Medeiros", "http://alemedeiros.sdf.org"]
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- ["Denis Arh", "https://github.com/darh"]
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- ["akirahirose", "https://twitter.com/akirahirose"]
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- ["Anton Strömkvist", "http://lutic.org/"]
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- ["Rahil Momin", "https://github.com/iamrahil"]
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- ["Gregrory Kielian", "https://github.com/gskielian"]
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- ["Etan Reisner", "https://github.com/deryni"]
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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/bash
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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 can't be found.
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# Or like this:
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Variable= 'Some string'
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# Bash will decide that 'Some string' is a command it must execute and give an
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# error because it can't be found. (In this case the 'Variable=' part is seen
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# as a variable assignment valid only for the scope of the 'Some string'
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# command.)
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# Using the variable:
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echo $Variable
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echo "$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 the variable's value, you should use $.
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# Note that ' (single quote) won't expand the variables!
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# String substitution in variables
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echo ${Variable/Some/A}
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# This will substitute the first occurrence of "Some" with "A"
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# Substring from a variable
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Length=7
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echo ${Variable:0:Length}
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# This will return only the first 7 characters of the value
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# Default value for variable
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echo ${Foo:-"DefaultValueIfFooIsMissingOrEmpty"}
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# This works for null (Foo=) and empty string (Foo=""); zero (Foo=0) returns 0.
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# Note that it only returns default value and doesn't change variable value.
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# Builtin variables:
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# There are some useful builtin variables, like
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echo "Last program's return value: $?"
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echo "Script's PID: $$"
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echo "Number of arguments passed to script: $#"
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echo "All arguments passed to script: $@"
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echo "Script's arguments separated into different variables: $1 $2..."
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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 a new variable
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echo Hello, $Name!
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# We have the usual if structure:
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# use 'man test' for more info about conditionals
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if [ $Name -ne $USER ]
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then
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echo "Your name isn't your username"
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else
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echo "Your name is your username"
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fi
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# NOTE: if $Name is empty, bash sees the above condition as:
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if [ -ne $USER ]
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# which is invalid syntax
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# so the "safe" way to use potentially empty variables in bash is:
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if [ "$Name" -ne $USER ] ...
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# which, when $Name is empty, is seen by bash as:
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if [ "" -ne $USER ] ...
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# which works as expected
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# There is also conditional execution
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echo "Always executed" || echo "Only executed if first command fails"
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echo "Always executed" && echo "Only executed if first command does NOT fail"
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# To use && and || with if statements, you need multiple pairs of square brackets:
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if [ "$Name" == "Steve" ] && [ "$Age" -eq 15 ]
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then
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echo "This will run if $Name is Steve AND $Age is 15."
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fi
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if [ "$Name" == "Daniya" ] || [ "$Name" == "Zach" ]
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then
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echo "This will run if $Name is Daniya OR Zach."
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fi
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# Expressions are denoted with the following format:
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echo $(( 10 + 5 ))
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# Unlike other programming languages, bash is a shell so it works in the context
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# of a current directory. You can list files and directories in the current
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# directory with the ls command:
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ls
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# These commands have options that control their execution:
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ls -l # Lists every file and directory on a separate line
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# Results of the previous command can be passed to the next command as input.
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# grep command filters the input with provided patterns. That's how we can list
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# .txt files in the current directory:
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ls -l | grep "\.txt"
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# You can redirect command input and output (stdin, stdout, and stderr).
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# Read from stdin until ^EOF$ and overwrite hello.py with the lines
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# between "EOF":
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cat > hello.py << EOF
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#!/usr/bin/env python
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from __future__ import print_function
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import sys
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print("#stdout", file=sys.stdout)
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print("#stderr", file=sys.stderr)
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for line in sys.stdin:
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print(line, file=sys.stdout)
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EOF
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# Run hello.py with various stdin, stdout, and stderr redirections:
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python hello.py < "input.in"
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python hello.py > "output.out"
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python hello.py 2> "error.err"
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python hello.py > "output-and-error.log" 2>&1
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python hello.py > /dev/null 2>&1
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# The output error will overwrite the file if it exists,
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# if you want to append instead, use ">>":
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python hello.py >> "output.out" 2>> "error.err"
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# Overwrite output.out, append to error.err, and count lines:
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info bash 'Basic Shell Features' 'Redirections' > output.out 2>> error.err
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wc -l output.out error.err
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# Run a command and print its file descriptor (e.g. /dev/fd/123)
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# see: man fd
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echo <(echo "#helloworld")
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# Overwrite output.out with "#helloworld":
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cat > output.out <(echo "#helloworld")
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echo "#helloworld" > output.out
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echo "#helloworld" | cat > output.out
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echo "#helloworld" | tee output.out >/dev/null
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# Cleanup temporary files verbosely (add '-i' for interactive)
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rm -v output.out error.err output-and-error.log
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# Commands can be substituted 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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# The same can be done using backticks `` but they can't be nested - the preferred way
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# is to use $( ).
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echo "There are `ls | wc -l` items here."
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# Bash uses a case statement that works similarly to switch in Java and C++:
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case "$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 $Variable is printed three times.
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for Variable in {1..3}
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do
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echo "$Variable"
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done
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# Or write it the "traditional for loop" way:
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for ((a=1; a <= 3; a++))
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do
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echo $a
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done
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# They can also be used to act on files..
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# This will run the command 'cat' on file1 and file2
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for Variable in file1 file2
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do
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cat "$Variable"
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done
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# ..or the output from a command
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# This will cat the output from ls.
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for Output in $(ls)
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do
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cat "$Output"
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done
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# while loop:
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while [ true ]
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do
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echo "loop body here..."
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break
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done
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# You can also define functions
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# Definition:
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function foo ()
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{
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echo "Arguments work just like script arguments: $@"
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echo "And: $1 $2..."
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echo "This is a function"
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return 0
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}
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# or simply
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bar ()
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{
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echo "Another way to declare functions!"
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return 0
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}
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# Calling your function
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foo "My name is" $Name
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# There are a lot of useful commands you should learn:
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# prints last 10 lines of file.txt
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tail -n 10 file.txt
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# prints first 10 lines of file.txt
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head -n 10 file.txt
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# sort file.txt's lines
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sort file.txt
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# report or omit repeated lines, with -d it reports them
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uniq -d file.txt
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# prints only the first column before the ',' character
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cut -d ',' -f 1 file.txt
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# replaces every occurrence of 'okay' with 'great' in file.txt, (regex compatible)
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sed -i 's/okay/great/g' file.txt
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# print to stdout all lines of file.txt which match some regex
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# The example prints lines which begin with "foo" and end in "bar"
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grep "^foo.*bar$" file.txt
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# pass the option "-c" to instead print the number of lines matching the regex
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grep -c "^foo.*bar$" file.txt
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# if you literally want to search for the string,
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# and not the regex, use fgrep (or grep -F)
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fgrep "^foo.*bar$" file.txt
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# Read Bash shell builtins documentation with the bash 'help' builtin:
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help
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help help
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help for
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help return
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help source
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help .
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# Read Bash manpage documentation with man
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apropos bash
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man 1 bash
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man bash
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# Read info documentation with info (? for help)
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apropos info | grep '^info.*('
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man info
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info info
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info 5 info
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# Read bash info documentation:
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info bash
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info bash 'Bash Features'
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info bash 6
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info --apropos bash
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```
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