LearnBashQuickly
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在27分钟内学习bash脚本
这是受到5分钟学习的学习启发，在30分钟内学习生锈和曲折的半小时。
bash （再次是Bourne Shell）是在1989年开发的，它比C年轻，但可能比您习惯于开发的任何东西都要年龄。这并不使其过时或过时。 BASH几乎无处不在：在UNIX/Linux ， MacOS和Windows （WSL）上，它用于各种“现代”软件（Docker，Deployment/build脚本，CI/CD），并且您将在不使用bash的情况下拥有丰富的开发人员职业的机会是准零。那为什么不擅长呢？
每当您在终端/控制台中打字时，您都会在交互式bash壳中（或更精致的表亲zsh或fish ）。您可以在此处输入的任何命令，例如ls ， whoami ， echo "hello"都可以作为Bash命令，并且可以在脚本中使用。
在您的终端中，输入以下命令（不要复制>，它在那里表示命令行的开始）：
> touch myscript.sh # create the file 'myscript.sh' as an empty file使用您喜欢的文本编辑器（Sublime/vcode/jetbrains/...）编辑此新文件，并添加以下2行：
#! /usr/bin/env bash
echo " Hello world! "现在回到您的终端并输入
> chmod +x myscript.sh # make the file executable, so you can call it directly as ./myscript.sh
> ./myscript.sh # execute your new script
Hello world ! bash变量未构图。变量的值和/或上下文确定它是否将其解释为整数，字符串或数组。
width=800 # variables are assigned with '='
name= " James Bond " # strings with spaces should be delimited by " or '
user_name1=jbond # variable names can contain letters, digits and '_', but cannot start with a digit
echo " Welcome $name ! " # variable are referenced with a $ prefix
file= " ${user} _ ${uniq} " # ${var} can be used to clearly delimit the name of the variable
echo " ${width :- 400} " # if variable $width is not set, the default 400 will be used
echo " ${text / etc / &} " # replace the 1st text 'etc' by '&' in the variable before printing it
echo " ${text // etc / &} " # replace all texts 'etc' by '&' in the variable before printing it
w= $(( width + 80 )) # $(()) allows arithmetic: + - / * %
input= $1 # $1, $2 ... are the 1st, 2nd ... parameters specified on the command line
input= " $1 " # put quotes around any variable that might contain " " (space), "t" (tab), "n" (new line)
script= $0 # $0 refers to the actual script's name, as called (so /full/path/script or ../src/script)
temp= " /tmp/ $$ .txt " # $$ is the id of this process and will be different each time the script runs
echo " $SECONDS secs " # there are preset variables that your shell sets automatically: $SECONDS, $HOME, $HOSTNAME, $PWD
LANG=C do_something # start the subcommand do_something but first set LANG to "C" only for that subcommand
script= $( basename " $0 " ) # execute what is between $( ) and use the output as the value Bash具有最常见的“测试”程序（例如test -f output.txt ），该程序最常用于[[ -f output.txt ]] 。还有[ -f output.txt ]的较旧语法，但是双方括号是优选的。该程序测试特定条件，如果满足条件，则以0（“确定”）返回。此目的将在下一章中变得明确。
[[ -f file.txt ]] # file exists
[[ ! -f file.txt ]] # file does not exist -- ! means 'not'
[[ -f a.txt && -f b.txt ]] # both files exist -- && means AND , || means OR
[[ -d ~ /.home ]] # folder exists
[[ -x program ]] # program exists and is executable
[[ -s file.txt ]] # file exists and has a size > 0
[[ -n " $text " ]] # variable $text is not empty
[[ -z " $text " ]] # variable $text is empty
[[ " $text " == " yes " ]] # variable $text == "yes"
[[ $width -eq 800 ]] # $width is the number 800
[[ $width -gt 800 ]] # $width is greater than 800
[[ file1 -nt file2 ]] # file1 is newer (more recently modified) than file2 if [[ ! -f " out.txt " ]] ; then # if ... then ... else ... fi
echo " OK "
else
echo " STOP "
fi
[[ -f " out.txt " ]] && echo " OK " || echo " STOP " # can also be written as 1 line if the 'then' part is 1 line only
while [[ ! -f output.txt ]] ; do # while ... do ... done
(...) # wait for output.txt
continue # return and do next iteration
done
for file in * .txt ; do # for ... in ... do ... done
echo " Process file $file "
(...)
done
for (( i = 0 ; i < n; i ++ )) ; do # for (...) do ... done
(...)
done
case $option in # case ... in ...) ;; esac
export) do_export " $1 " " $2 " # you might know this as a 'switch' statement
;;
refresh) do_refresh " $2 "
;;
* ) echo " Unknown option [ $option ] "
esac myfunction (){ # bash functions are defined with <name>(){...} and have to be defined before they are used
# $1 = input # functions can be called with their own parameters, and they are also referenced as $1 $2
# $2 = output # this means that the main program's $1,$2...parameters are no longer available inside the function
local error # a variable can be defined with local scope. if not, variables are always global
(...)
return 0 # this explicitly exits the function with a status code (0 = OK, > 0 = with an error)
}numbers=(1 2 3) # define array with all values
numbers[0]= " one " # replace 1st element (indexes start at 0) by "one"
echo ${numbers[@]} # [@] represents the whole array
numbers=( ${numbers[@]} 4) # add new element to array
${ # numbers[@]} # nb of elements in the array
for element in ${numbers[@]} ; do
...
done 每个脚本，函数，程序都有3个默认流（文件描述符）：stdin，stdout和stderr。例如，“排序”是一个程序，该程序在stdin上读取文本行，并在Stdout上输出它们，并显示其在STDERR中遇到的任何错误。
# by default, `stdin` is your interactive terminal, `stdout` and `stderr` are both your terminal
sort # stdin = read from terminal (just type and end with CTRL-D), stdout = terminal
< input.txt sort # stdin = read from input.txt, stdout = terminal
< input.txt sort > sorted.txt # stdin = read from input.txt, stdout = written to sorted.txt
< input.txt sort >> sorted.txt # stdin = read from input.txt, stdout = append to sorted.txt
< input.txt sort > /dev/null # stdin = read from input.txt, stdout = just ignore it, throw it away
echo " Output " # writes "Output" to stdout = terminal
echo " Output " >&2 # writes "Output" to stderr = terminal
program 2> /dev/null > output.txt # write stdout to output.txt, and ignore stderr
program & > /output.txt # redirect both stdout and stderr to output.txt
find . -name " *.txt "
| while read -r file ; do # while read is a good way to run some code for each line
output= " $( basename " $file " .txt ) .out "
[[ ! -f " $output " ]] && < " $file " sort > " $output "
done | （管道）角色是Bash的超级大国。它使您可以构建程序的复杂链，每个程序都将其stdout传递到了下一个程序的stdin 。如果Unix哲学规定了“编写一件事情并做得很好的程序”，那么Bash是将所有这些专业程序粘合在一起的理想工具。排序，使用sort ，搜索，使用grep ，替换字符，使用tr ;要将所有这些链接在一起，请使用bash及其管道。
ls | sort | head -1 # ls lists filenames to its stdout, which is 'piped' (connected) to sort's stdin.
# sort sends the sorted names to its stdout, which is piped to stdin of 'head -1'.
# head will just copy the first line from stdin to stdout and then stop
# the following chain will return the 5 most occurring non-comment lines in all .txt files
cat * .txt | grep -v ' ^# ' | sort | uniq -c | sort -nr | head -5
# this line gives a lowercase name for the current script (MyScript.sh -> myscript)
script_name= $( basename " $0 " .sh | tr " [:upper:] " " [:lower:] " ) ( # ( ... ) starts a new subshell with its own stdin/stdout
cat * .txt
curl -s https://host/archive.txt
) | sort
start_in_background & # start the program and return immediately, don't wait for it to end
git commit -a && git push # 'git push' will only execute if 'git commit -a' finished without errors set -uo pipefail ：发生错误时停止脚本
