变量与命令行参数
echo hello; echo "hello" # no different
a="HelloWorld"; echo ${a}
b[1]=7; b[3]=4 # no declaring for list. or:
declare -a vec=(element1 element2 element3)
for i in ${vec[@]}; do echo $i; done
# get values, length
echo ${c[*]} ${#b[*]}
# get length of string
echo ${#a}
d=$(($a + $b)) # + - * /
# command line args
# $#：number of args # $@：each args
# $*：all args as ONE string # $n: No.n args
echo $0 $1 $* $@ $#
# special variable
echo $RANDOM
echo $PWD
重定向 / 读取变量
# > for stdout, 2> for stderr, >> for append stdout to file
echo 'hello_world' > hello.txt 2> error.txt >> add.txt
# discard all
echo &> /dev/null
# file as stdin
echo < read.txt
# print to file AND stdout
command | tee output.txt
command | tee -a output.txt # append
# channel
echo 'hello_world' | less
# from file to var
read var_name < file
# from file to stdin
cat filename
# get stdin
read var_name
read -p 'Please input x:' x
判断 / If
# if is dir / file
[ -d dir_name ] && [ -f filename ]
[ -e name ] # if is file or dir
# compare numbers
[ num1 -eq num2 ]
# -eq like == in clang # -ne like != in clang
# -gt like > in clang # -ge like >= in clang
# -lt like < in clang # -le like <= in clang
# use "" and when $1 is null it won't crash
[ "$1" = "hello" ]
[ $1 = hello ] # crash when $1 is null
# some true expr:
[ -n NullString ] && [ noNullString ]
[ string1 = string1 ] && [ string1 != string2 ]
[ -z variableExist ]
[ -x "$(command -v command_name)" ] # if command exist
[ ! bool_expression ] # not
[ expr1 -a expr2 ] # and
[ expr1 -o expr2 ] # or
# if structure
if [ test ]; then
echo helloworld
else
echo helloworld
fi
控制流 / For / While
# loop
for i in {1..10}; do
echo $i
done
# c-style for
for ((i=0;i<10;i++)); do
echo $i
done
while [ test ]; do
echo helloword
done
函数 / Function
# Function
function _echo { # be careful for the spaces.
echo $1
echo $2
# keyword return can be used in function
# if no return, the value of the last sentence
# will be the return value of this function
res='this var can be access out of this function.'
}
echo res
字符串属性与操作
# all of substring can be regexp
# REF: https://www.centos.bz/2013/08/bash-shell-string-operate-summary/
${#string} # length of string
${string:position} # string[position:]
${string:position:length} # string[position:position+len]
${string#substring} # remove substring from the BEGIN of string.
# if substring is a regexp, remove the SHORTEST match
${string##substring} # remove substring from the BEGIN of string.
# if substring is a regexp, remove the LONGEST match
${string%substring} # remove substring from the END of string.
# if substring is a regexp, remove the SHORTEST match
${string%%substring} # remove substring from the END of string.
# if substring is a regexp, remove the LONGEST match
${string/substring/replacement}
# replace 'substring' with 'replacement' for the FIRST match.
${string//substring/replacement}
# replace 'substring' with 'replacement' for ALL match.
${string/#substring/replacement}
# replace 'substring' with 'replacement' only if substring is PREFIX.
${string/%substring/replacement}
# replace 'substring' with 'replacement' only if substring is SUFFIX.
特殊目录
脚本运行时，其工作目录是调用脚本时所在的工作目录 pwd. $0 参数是脚本文件名，跟 pwd 拼接之后就是脚本绝对路径。
$(pwd) # work dir
$(dirname "$0") # script file path
$(readlink -f "$0") # absolute script file path