变量和值

variables are referenced by $var or ${var}. Global variables are visible to all sub bash sessions, and are often called environment variables, local variables are only visible to local session, not sub-sessions.

Global variables can be viewed as env, and can be created by export.

TEST=testing; export $TEST # or
export TEST=testing # NOTE: no $

Useful variables

HOME    Same to ~
IFS
PATH    Search path
EUID    User id
GROUPS    Groups for current user
HOSTNAME    Hostname
LANG
LC_ALL
OLDPWD
PWD

定义和使用变量

定义变量，注意因为 shell 中的语法使用空格作为命令分割的限制，等于号前后不能加空格。

FOO=bar

使用变量，需要添加上 $ 符号。

echo $FOO

字符串在双引号中可以直接插入，这时候要加上大括号来指示变量名的起始位置。

echo "${FOO}xxx"

变量默认实在当前的回话中可见的，而不会作为环境变量传递给调用的命令。可以使用 export 导出变量，或者在命令前加上指定的环境变量。

-> % cat env.py
import os
print("FOO env variable is: ", os.environ.get("FOO"))

-> % python3 env.py
FOO env variable is:  None

-> % FOO=bar python3 env.py
FOO env variable is:  bar

使用 export

-> % export FOO=bar
-> % python3 env.py
FOO env variable is:  bar

一些有用的内置变量

$HOME     家目录，比如 /home/kongyifei
$IFS      默认的分隔符，和 for 循环紧密相关
$PATH     搜索路径，当你执行 ls 的时候，shell 会在这个变量中查找 ls 命令
$EUID     当前有效用户 ID
$LANG
$LC_ALL
$OLDPWD   上一个工作目录
$PWD      当前工作目录

数组

使用小括号来定义一个数组，关于 for 循环随后会讲

A=(1 2 3)

for el in ${A[@]}; do
    echo $el
done

字符串操作

大括号里面的字符串会被展开成独立的字符串

% echo {1,2,3,4}
1 2 3 4
% mkdir -p test/{a,b,c,d}{1,2,3,4}
% ls test/
a1  a2  a3  a4  b1  b2  b3  b4  c1  c2  c3  c4  d1  d2  d3  d4
% mv test/{a,c}.conf  # 这个命令的意思是：mv test/a.conf test/c.conf

切片：${string:start:length}

默认值 ${var:-default}

设定值 ${var:=default}

长度 ${#var}

字符串 Expansion and slice

[zorro@zorrozou-pc0 bash]$ mkdir -p test/zorro/{a,b,c,d}{1,2,3,4}
[zorro@zorrozou-pc0 bash]$ ls test/zorro/
a1  a2  a3  a4  b1  b2  b3  b4  c1  c2  c3  c4  d1  d2  d3  d4

[zorro@zorrozou-pc0 bash]$ mv test/{a,c}.conf
这个命令的意思是：mv test/a.conf test/c.conf

${string:start :length} string slice

default value: ${var:-default}
set value: ${var:=default}

${#var} get variable length

Redirection

input: <, output >, append >>

cat > file << EOF
this
line
will
be redirected to
file
EOF

pipe

pipe commands will be run simultaneously, but the second command will wait for the input

Sub shell

use $(expression)

条件语句

if 语句成立的条件是 expr 返回值为 0。

if expr; then
    statement;
elif expr; then
    statement;
else
    statement;
fi

test command

虽然可以使用任意的语句作为判断条件，不过我们一般情况下都是用 [ 这个命令来作为判断条件的，需要注意的是 [ 并不是一个语法，而是一个命令。不过由于 [ 这个上古命令实在功能太少，现在一般采用 [[ 来作为判断条件。

if [[ "a" == "b" ]]; then
    echo "wtf"
else
    echo "meh"
fi

[[支持的条件有

1 数值比较，仅限整数，注意不能使用 > < 等符号。

n1 -eq n2    equal
n1 -ge n2    greater or equal
n1 -gt n2    greater
n1 -le n2       less or equal
n1 -lt n2    less
n1 -ne n2    not equal

2 字符串比较

Note: Variables may contain space, so the best way to comparison is to add quotes: "$var1" = "$var2"

str1 == str2    equal
str1 != str2    not equal
str1 < str2    less
str1 > str2    greater
-z str    zero
-n str    not zero length

3 file comparison

-d    is directory?
-e    exist?
-f    is regular file?
-r    exist and readable?
-s    exist and has content
-w     exist and writable
-x     exist and executealbe
-O    exist and owned
-G    exist and in same group
file -nt file2    newer than
file1 -ot file2     older than

case

case var in
parttern | pattern2) commands;;
pattern3) commands2;
*) default commnads;;
esac

Loops

foreach 语句

for var in list; do
     echo $var
done

其中 list 可以是一个数组，也可以是一个被 $IFS 分割的字符串。默认情况下，$IFS 是 " \n\t"。其中包含了空格。

如果要覆盖 IFS，一般这样使用：

OLDIFS=$IFS
IFS="\n" # new seperator
# do things
IFS=$OLDIFS

while-loop

until/while expr; do
    # commands
done

pipe

the result of a for loop is pipe-able to other command

for city in beijing shanghai; do
    echo $city is big
done > cities.txt
# will save the result in cities.txt

输入输出

命令行参数

Parameters to a script can be obtained as $1, $2, $3...。 $0 is the script name, remember to check whether the parameter is empty. $# is the number of parameters(without script name).

$0    script name / function name
$1...$x    command line arguments / parameters
$#    number of arguments(without $0)
$*    all parameters as a string
$@    all parameters as a string list

shift

processing parameters using shift,

while [ -n "$1" ]; do
    case "$1" in
        -a) echo "option -a" ;;
        --) shift
            break;;
        *) echo "$1" is not a option ;;
    esac
    shift
done

read

read OPTIONS VARNAME read input to variables

• read -p Prompt
• read -t timeout
• read -s hide input

we can use read to read file or stdin

redirection

2> redirect STDERR m>&n redirect fd m to fd n's associated file

Note: you have to use command >> command.log 2>&1 (put 2>&1 at the end), since this means redirect 2 to 1's in a scirpt exec 2> filename # reopen stdout to filename

Signal

Trap commnad signal is used to handle signals in shell

Functions

有两种定义函数的方式

function name {
    # function body
}

foo() {
    # function body
}

要调用上面这个函数，直接就输入

$ foo

就好了

return

Shell functions behave like a small script, and it does NOT return a computed value... It returns a exit code, which is between 0 and 255. If no return is specified, the exit code of last command will be returned

You can read the return value by $? like any normal commands

The right way to to return a value from function, you will have to echo out the value, and put the function is sub-shell

function foo {
    # do some compute
    echo $result;
}

retval=$(foo)

Note: any thing that echos in the function body will be captured, so please keep that from happening

parameters

like a shell script, $0 holds the function name, $1 ... $9 holds the parameters, $# is the number of parameters

local variables

use local to declare local variables

alias

alias new_name="command string"
$ \command  # bypass alias

debugging

DEBUG macro

multiprocess

PID_ARRAY=()
for file in filelist; do
    md5sum file &;
    PID_ARRAY+=("$!")
done
wait ${PID_ARRAY[@]}

References

1. https://www.mulle-kybernetik.com/modern-bash-scripting/intro.html