编程Linux Shell 入门:学习笔记 – 第二部分:Shell 脚本编程
什么是shell脚本
当命令或者程序语句写在文件中,我们执行文件,读取其中的代码,这个程序就称之为shell脚本。
有了shell脚本肯定是要有对应的解释器了,常见的shell脚本解释器有sh、python、perl、tcl、php、ruby等。一般这种使用文件方式来执行sh命令的方式被称为非交互方式。
shell脚本规则
在Linux系统中,shell脚本或者称之为bash shell程序,通常都是vim编辑,有Linux命令、bash shell指令、逻辑控制语句和注释信息组成。
google bash编程规范链接
sudo
。STDERR
打印错误信息。err() {
echo "[$(date +'%Y-%m-%dT%H:%M:%S%z')]: $@" >&2
}
if ! do_something; then
err "Unable to do_something"
exit "${E_DID_NOTHING}"
fi
#!/bin/bash
#
# Perform hot backups of Oracle databases.
例如:
#!/bin/bash
#
# Perform hot backups of Oracle databases.
export PATH='/usr/xpg4/bin:/usr/bin:/opt/csw/bin:/opt/goog/bin'
#######################################
# Cleanup files from the backup dir
# Globals:
# BACKUP_DIR
# ORACLE_SID
# Arguments:
# None
# Returns:
# None
#######################################
cleanup() {
...
}
# TODO(mrmonkey): Handle the unlikely edge cases (bug ####)
80
个字符。使用下述两种方式规避长字符串:
# DO use 'here document's
cat <<END;
I am an exceptionally long
string.
END
# Embedded newlines are ok too
long_string="I am an exceptionally
long string."
2
个空格。这适用于使用管道符’|’的合并命令链以及使用’||’和’&&’的逻辑运算链。# All fits on one line
command1 | command2
# Long commands
command1 \
| command2 \
| command3 \
| command4
; do
, ; then
应该和 if/for/while
放在同一行。 else
应该单独一行,结束语句应该单独一行并且跟开始语句垂直对齐。例如:
for dir in ${dirs_to_cleanup}; do
if [[ -d "${dir}/${ORACLE_SID}" ]]; then
log_date "Cleaning up old files in ${dir}/${ORACLE_SID}"
rm "${dir}/${ORACLE_SID}/"*
if [[ "$?" -ne 0 ]]; then
error_message
fi
else
mkdir -p "${dir}/${ORACLE_SID}"
if [[ "$?" -ne 0 ]]; then
error_message
fi
fi
done
;&
和 ;;&
符号。关键规则如下:* 通过
2
个空格缩进可选项。* 在同一行可选项的模式右圆括号之后和结束符
;;
之前各需要一个空格。* 长可选项或者多命令可选项应该被拆分成多行,模式、操作和结束符
;;
在不同的行。例子1:
case "${expression}" in
a)
variable="..."
some_command "${variable}" "${other_expr}" ...
;;
absolute)
actions="relative"
another_command "${actions}" "${other_expr}" ...
;;
*)
error "Unexpected expression '${expression}'"
;;
esac
例子2:
verbose='false'
aflag=''
bflag=''
files=''
while getopts 'abf:v' flag; do
case "${flag}" in
a) aflag='true' ;;
b) bflag='true' ;;
f) files="${OPTARG}" ;;
v) verbose='true' ;;
*) error "Unexpected option ${flag}" ;;
esac
done
${var}
而不是 $var
例如:
# Section of recommended cases.
# Preferred style for 'special' variables:
echo "Positional: $1" "$5" "$3"
echo "Specials: !=$!, -=$-, _=$_. ?=$?, #=$# *=$* @=$@ \$=$$ ..."
# Braces necessary:
echo "many parameters: ${10}"
# Braces avoiding confusion:
# Output is "a0b0c0"
set -- a b c
echo "${1}0${2}0${3}0"
# Preferred style for other variables:
echo "PATH=${PATH}, PWD=${PWD}, mine=${some_var}"
while read f; do
echo "file=${f}"
done < <(ls -l /tmp)
# Section of discouraged cases
# Unquoted vars, unbraced vars, brace-quoted single letter
# shell specials.
echo a=$avar "b=$bvar" "PID=${$}" "${1}"
# Confusing use: this is expanded as "${1}0${2}0${3}0",
# not "${10}${20}${30}
set -- a b c
echo "$10$20$30"
[[
中模式匹配的引用规则$@
除非你有特殊原因需要使用 $*
例如:
# 'Single' quotes indicate that no substitution is desired.
# "Double" quotes indicate that substitution is required/tolerated.
# Simple examples
# "quote command substitutions"
flag="$(some_command and its args "$@" 'quoted separately')"
# "quote variables"
echo "${flag}"
# "never quote literal integers"
value=32
# "quote command substitutions", even when you expect integers
number="$(generate_number)"
# "prefer quoting words", not compulsory
readonly USE_INTEGER='true'
# "quote shell meta characters"
echo 'Hello stranger, and well met. Earn lots of $$$'
echo "Process $$: Done making \$\$\$."
# "command options or path names"
# ($1 is assumed to contain a value here)
grep -li Hugo /dev/null "$1"
# Less simple examples
# "quote variables, unless proven false": ccs might be empty
git send-email --to "${reviewers}" ${ccs:+"--cc" "${ccs}"}
# Positional parameter precautions: $1 might be unset
# Single quotes leave regex as-is.
grep -cP '([Ss]pecial|\|?characters*)$' ${1:+"$1"}
# For passing on arguments,
# "$@" is right almost everytime, and
# $* is wrong almost everytime:
#
# * $* and $@ will split on spaces, clobbering up arguments
# that contain spaces and dropping empty strings;
# * "$@" will retain arguments as-is, so no args
# provided will result in no args being passed on;
# This is in most cases what you want to use for passing
# on arguments.
# * "$*" expands to one argument, with all args joined
# by (usually) spaces,
# so no args provided will result in one empty string
# being passed on.
# (Consult 'man bash' for the nit-grits ;-)
set -- 1 "2 two" "3 three tres"; echo $# ; set -- "$*"; echo "$#, $@")
set -- 1 "2 two" "3 three tres"; echo $# ; set -- "$@"; echo "$#, $@")
$(command)
而不是反引号# This is preferred:
var="$(command "$(command1)")"
# This is not:
var="`command \`command1\``"
[[ ... ]]
,而不是 [
, test
, 和 /usr/bin/[
# This ensures the string on the left is made up of characters in the
# alnum character class followed by the string name.
# Note that the RHS should not be quoted here.
# For the gory details, see
# E14 at http://tiswww.case.edu/php/chet/bash/FAQ
if [[ "filename" =~ [^[:digit:]]+name ]]; then
echo "Match"
fi
# This matches the exact pattern "f*" (Does not match in this case)
if [[ "filename" == "f*" ]]; then
echo "Match"
fi
# This gives a "too many arguments" error as f* is expanded to the
# contents of the current directory
if [ "filename" == f* ]; then
echo "Match"
fi
例子1
# Do this:
if [[ "${my_var}" = "some_string" ]]; then
do_something
fi
# -z (string length is zero) and -n (string length is not zero) are
# preferred over testing for an empty string
if [[ -z "${my_var}" ]]; then
do_something
fi
# This is OK (ensure quotes on the empty side), but not preferred:
if [[ "${my_var}" = "" ]]; then
do_something
fi
# Not this:
if [[ "${my_var}X" = "some_stringX" ]]; then
do_something
fi
例子2
# Use this
if [[ -n "${my_var}" ]]; then
do_something
fi
# Instead of this as errors can occur if ${my_var} expands to a test
# flag
if [[ "${my_var}" ]]; then
do_something
fi
# Here's the contents of the directory:
# -f -r somedir somefile
# This deletes almost everything in the directory by force
psa@bilby$ rm -v *
removed directory: `somedir'
removed `somefile'
# As opposed to:
psa@bilby$ rm -v ./*
removed `./-f'
removed `./-r'
rm: cannot remove `./somedir': Is a directory
removed `./somefile'
eval简单例子
# What does this set?
# Did it succeed? In part or whole?
eval $(set_my_variables)
# What happens if one of the returned values has a space in it?
variable="$(eval some_function)"
# Single function
my_func() {
...
}
# Part of a package
mypackage::my_func() {
...
}
当函数名后存在 () 时,关键词 function 是多余的。但是其促进了函数的快速辨识。
for zone in ${zones}; do
something_with "${zone}"
done
# Constant
readonly PATH_TO_FILES='/some/path'
# Both constant and environment
declare -xr ORACLE_SID='PROD'
动态生成的常量,可以使用readonly
和 export
来进行设置,在函数中declare
不会对全局变量进行操作。
VERBOSE='false'
while getopts 'v' flag; do
case "${flag}" in
v) VERBOSE='true' ;;
esac
done
readonly VERBOSE
readonly
或者 declare -r
来确保变量只读zip_version="$(dpkg --status zip | grep Version: | cut -d ' ' -f 2)"
if [[ -z "${zip_version}" ]]; then
error_message
else
readonly zip_version
fi
local
声明特定功能的变量。声明和赋值应该在不同行。local
作用域为函数内部和子函数中可见。my_func2() {
local name="$1"
# Separate lines for declaration and assignment:
local my_var
my_var="$(my_func)" || return
# DO NOT do this: $? contains the exit code of 'local', not my_func
local my_var="$(my_func)"
[[ $? -eq 0 ]] || return
...
}
例子1
if ! mv "${file_list}" "${dest_dir}/" ; then
echo "Unable to move ${file_list} to ${dest_dir}" >&2
exit "${E_BAD_MOVE}"
fi
# Or
mv "${file_list}" "${dest_dir}/"
if [[ "$?" -ne 0 ]]; then
echo "Unable to move ${file_list} to ${dest_dir}" >&2
exit "${E_BAD_MOVE}"
fi
例子2(Bash内部PIPESTATUS
变量)
-作用
tar -cf - ./* | ( cd "${dir}" && tar -xf - )
if [[ "${PIPESTATUS[0]}" -ne 0 || "${PIPESTATUS[1]}" -ne 0 ]]; then
echo "Unable to tar files to ${dir}" >&2
fi
例子3(PIPESTATUS
优化)
tar -cf - ./* | ( cd "${DIR}" && tar -xf - )
return_codes=(${PIPESTATUS[*]})
if [[ "${return_codes[0]}" -ne 0 ]]; then
do_something
fi
if [[ "${return_codes[1]}" -ne 0 ]]; then
do_something_else
fi
# Prefer this:
addition=$((${X} + ${Y}))
substitution="${string/#foo/bar}"
# Instead of this:
addition="$(expr ${X} + ${Y})"
substitution="$(echo "${string}" | sed -e 's/^foo/bar/')"