# 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."
for dir in ${dir_to_cleanup}; do
if [[ -d "${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
# Section of recommended cases.
# Preferred style for 'special' variables:
echo "Positional: $1" "$5" "$3"
echo "Specials: !=$!, -=$-, _=$_. ?=$?, #=$# *=$* @=$@ \$=$$ ..."
# Braces necessary:
echo "many parameters: ${10}"
# Braces avoiding confusion:
# Output is "a0b0c0"
set -- a b c
echo "${1}0${2}0${3}0"
# Preferred style for other variables:
echo "PATH=${PATH}, PWD=${PWD}, mine=${some_var}"
while read f; do
echo "file=${f}"
done < <(ls -l /tmp)
# Section of discouraged cases
# Unquoted vars, unbraced vars, brace-quoted single letter
# shell specials.
echo a=$avar "b=$bvar" "PID=${$}" "${1}"
# Confusing use: this is expanded as "${1}0${2}0${3}0",
# not "${10}${20}${30}
set -- a b c
echo "$10$20$30"
5.7 引用
除非需要小心不带引用的扩展，否则总是引用包含变量、命令替换符、空格或shell元字符的字符串。
推荐引用是单词的字符串（而不是命令选项或者路径名）。
千万不要引用整数。
注意 [[ 中模式匹配的引用规则。
请使用 $@ 除非你有特殊原因需要使用 $* 。
# 'Single' quotes indicate that no substitution is desired.
# "Double" quotes indicate that substitution is required/tolerated.
# Simple examples
# "quote command substitutions"
flag="$(some_command and its args "$@" 'quoted separately')"
# "quote variables"
echo "${flag}"
# "never quote literal integers"
value=32
# "quote command substitutions", even when you expect integers
number="$(generate_number)"
# "prefer quoting words", not compulsory
readonly USE_INTEGER='true'
# "quote shell meta characters"
echo 'Hello stranger, and well met. Earn lots of $$$'
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 "$#, $@")
# This ensures the string on the left is made up of characters in the
# alnum character class followed by the string name.
# Note that the RHS should not be quoted here.
# For the gory details, see
# E14 at http://tiswww.case.edu/php/chet/bash/FAQ
if [[ "filename" =~ ^[[:alnum:]]+name ]]; then
echo "Match"
fi
# This matches the exact pattern "f*" (Does not match in this case)
if [[ "filename" == "f*" ]]; then
echo "Match"
fi
# This gives a "too many arguments" error as f* is expanded to the
# contents of the current directory
if [ "filename" == f* ]; then
echo "Match"
fi
# Do this:
if [[ "${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 [[ "${my_var}X" = "some_stringX" ]]; then
do_something
fi
为了避免对你测试的目的产生困惑，请明确使用-z或者-n
# Use this
if [[ -n "${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
6.4 文件名的通配符扩展
当进行文件名的通配符扩展时，请使用明确的路径。
为文件名可能以 - 开头，所以使用扩展通配符 ./* 比* 来得安全得多。
# 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'
6.5 Eval
应该避免使用eval。
当用于给变量赋值时，Eval解析输入，并且能够设置变量，但无法检查这些变量是什么。
# 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)"
total=0
last_file=
while read count filename; do
total+="${count}"
last_file="${filename}"
done < <(your_command | uniq -c)
# This will output the second field of the last line of output from
# the command.
echo "Total = ${total}"
echo "Last one = ${last_file}"
# Trivial implementation of awk expression:
# awk '$3 == "nfs" { print $2 " maps to " $1 }' /proc/mounts
cat /proc/mounts | while read src dest type opts rest; do
if [[ ${type} == "nfs" ]]; then
echo "NFS ${dest} maps to ${src}"
fi
done
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)"
[[ $? -eq 0 ]] || return
...
}
if ! mv "${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
tar -cf - ./* | ( cd "${dir}" && tar -xf - )
if [[ "${PIPESTATUS[0]}" -ne 0 || "${PIPESTATUS[1]}" -ne 0 ]]; then
echo "Unable to tar files to ${dir}" >&2
fi
tar -cf - ./* | ( cd "${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
8.2 内建命令和外部命令
可以在调用shell内建命令和调用另外的程序之间选择，请选择内建命令。
我们更喜欢使用内建命令，如在 bash(1) 中参数扩展函数。因为它更强健和便携（尤其是跟像 sed 这样的命令比较）
