find out current location in disk (stands for “print working directory”)
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pwd
list contents of current directory
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ls
list contents of current directory with more information, and in list format
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ls -l
make a directory
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mkdir new_dir
change into a directory
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cd new_dir
leave the directory (go back up one level)
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cd ..
remove the directory
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rmdir new_dir
create blank file
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touch new_file
write contents to the file: Note > will overwrite whatever was in that file before, >> does not overwrite contents of file but rather appends it to the end
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echo hello world > new_file echo hello world2 >> new_file
print contents of file to terminal
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cat new_file
browse contents of file
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less new_file
while browsing using less you can use the following keyboard shortcuts: * q to quit * j,k to navigate up and down * u,d to navigate pages up and down
search for the string “hello” in file
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grep "hello" new_file
redirect output of a command to another new file
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grep "hello" new_file > newer_file
overwrite contents of file
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echo bye world > new_file # > does overwrite contents of the file
create a bash file using a text editor
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touch test.sh #touch creates a file if one doesn't exist
check read (r), write (w) and execute (x) permissions of the test.sh file
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ls -al
type man ls on the command line to examine what the flags ‘-a’ and ‘-l’ do
add a bash command to the file
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echo "echo 'hello world'" > test.sh
The > redirects the output of the command to a file
examine the contents of this file
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less test.sh
execute the file
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bash test.sh
add another line to the file
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echo "echo 'bye bye world'" >> test.sh
The >> tell the shell to concatenate the output of the command to the file test.sh but doesn’t overwrite it
add executable permission to it
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chmod a+x test.sh
now we can execute this file using dot notation
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./test.sh
add ‘sha-bang’ (i.e., #!) to the file so that it’s interpreted as a bash script
use a text editor to open the file and add #!/bin/bash to the very top or use “vim test.sh” or “nano test.sh” to edit it
how to create a variable
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var1='hello world' var2=helloworld var3=999 var4=$var3 #can assign a variable to the contents of another variable var5=${var4}appendedtext #can use curly brackets to clarify variable names
Now have a look at what the values we’ve assigned to these variables
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echo var1 is $var1 echo var2 is $var2 echo var3 is $var3 echo var4 is $var4 echo var5 is $var5
Some examples of for loops over files
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touch 1.txt 2.txt 3.txt #make a few dummy files for demonstration
In a bash script, you can write a for loop over multiple lines like so:
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for filename in ./*; do #loop over files in current directory echo $filename #* is a wildcard that matches anything done
You can also write it all in one line if you want to run it on the command line (note the use of semicolons ;):
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for filename in ./* ; do echo $filename ; done
Some more example for loops:
loop over files with given extension
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for filename in *.txt; do echo $filename done
loop over files in the parent directory, .. is an alias for the parent directory)
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for filename in ../*; do echo $filename done
how to loop over numbers: a C++ style for loop which iterates over variable i from 0 to 4 (stop at 5)
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for ((i=0; i<5; i++)); do echo $i done
Another example: use the seq command
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for j in $(seq 5 10); do echo $j done
how to call a script from within a for loop (e.g. in another script)
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for ((i=0; i<5; i++)); do ./test.sh done
how to call a script with arguments: First let’s create a script called test2.sh that writes out “printing number” and then the first and second command line arguments (i.e., whitespace separated strings that are written immediately after the command) $1 refers to the first command line argument, $2 refers to the second.
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touch test2.sh chmod a+x test2.sh # make it executable echo "echo 'printing number' \$1 \$2" > test2.sh # $1 refers to first command line argument
Now put the script into a for loop.
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#note: we use \$ to escape the dollar sign for ((i=0; i<5; i++)); do ./test2.sh $i wow done
It should print out:
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printing number 0 wow printing number 1 wow printing number 2 wow printing number 3 wow printing number 4 wow
how to iterate over the lines contained within a text file
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touch test3.txt chmod a+x test3.txt echo 'line1' >> test3.txt echo 'line2' >> test3.txt echo 'line3' >> test3.txt echo 'line4' >> test3.txt echo 'line5' >> test3.txt for line in `cat test3.txt`; do #note: we are using `! not ' or "... echo $line done
be careful using single and double quotes, they have different behaviours when expanding variables
login shell加载环境变量的顺序是：① /etc/profile ② ~/.bash_profile ③ ~/.bashrc ④ /etc/bashrc
而non-login shell加载环境变量的顺序是： ① ~/.bashrc ② /etc/bashrc
也就是nog-login少了前面两步，我们先看后面两步。
下面是一个 .bashrc 的内容：
$ cat .bashrc
# .bashrc
# Source global definitions
if [ -f /etc/bashrc ]; then
. /etc/bashrc
fi
基本没有什么内容，它主要是去加载 /etc/bashrc 而他里面也没有看到sbin相关的东西
那我们再看non-login少的两步： ① /etc/profile ② ~/.bash_profile
cat /etc/profile : if [ “$EUID” = “0” ]; then pathmunge /usr/sbin pathmunge /usr/local/sbin else pathmunge /usr/local/sbin after pathmunge /usr/sbin after fi
Read and execute commands from the filename argument in the current shell context. If filename does not contain a slash, the PATH variable is used to find filename. When Bash is not in POSIX mode, the current directory is searched if filename is not found in $PATH. If any arguments are supplied, they become the positional parameters when filename is executed. Otherwise the positional parameters are unchanged. If the -T option is enabled, source inherits any trap on DEBUG; if it is not, any DEBUG trap string is saved and restored around the call to source, and source unsets the DEBUG trap while it executes. If -T is not set, and the sourced file changes the DEBUG trap, the new value is retained when source completes. The return status is the exit status of the last command executed, or zero if no commands are executed. If filename is not found, or cannot be read, the return status is non-zero. This builtin is equivalent to source.
set +o histexpand （！ 是history展开符号， histexpand 可以打开或者关闭这个展开符） alias 之后，想要用原来的命令：+alias （命令前加)
bash程序执行，当“$0”是“sh”的时候，则要求下面的代码遵循一定的规范，当不符合规范的语法存在时，则会报错，所以可以这样理解，“sh”并不是一个程序，而是一种标准（POSIX），这种标准，在一定程度上（具体区别见下面的“Things bash has that sh does not”）保证了脚本的跨系统性（跨UNIX系统）
Linux 分 shell变量(set)，用户变量(env)， shell变量包含用户变量，export是一种命令工具，是显式那些通过export命令把shell变量中包含的用户变量导入给用户变量的那些变量.
set -euxo pipefail //-u unset -e 异常退出 http://www.ruanyifeng.com/blog/2017/11/bash-set.html
比如： su admin 会重新加载 ~/.bashrc ，但是不会切换到admin 的home目录。 但是 su - admin 不会重新加载 ~/.bashrc ，但是会切换admin的home目录。
The su command is used to become another user during a login session. Invoked without a username, su defaults to becoming the superuser. The optional argument - may be used to provide an environment similar to what the user would expect had the user logged in directly.
