Shell脚本编程30分钟入门

1. $开头shell变量的含义：

• $1, $2, $3, ... are the positional parameters.
• "$@" is an array-like construct of all positional parameters, {$1, $2, $3 ...}.
• "$*" is the IFS expansion of all positional parameters, $1 $2 $3 ....
• $# is the number of positional parameters.
• $- current options set for the shell.
• $$ pid of the current shell (not subshell).
• $_ most recent parameter (or the abs path of the command to start the current shell immediately after startup).
• $IFS is the (input) field separator.
• $? is the most recent foreground pipeline exit status.
• $! is the PID of the most recent background command.
• $0 is the name of the shell or shell script.

What are the special dollar sign shell variables?

2. 重定向：

• 标准输入（stdin）：代码为0，使用<或<<
• 标准输出（stdout）：代码为1，使用>或>>
• 标准错误输出（stderr）：代码为2，使用2>或2>>
• /dev/null：垃圾桶

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3. shift：

脚本位置参数左移

4. pushd和popd：

5. awk：

6. source：

使用source命令可以加载其他脚本文件中定义的函数：
https://bash.cyberciti.biz/guide/Source_command

# helper method for providing error messages for a command
run_or_fail() {
  local explanation=$1
  shift 1
  "$@"
  if [ $? != 0 ]; then
    echo $explanation 1>&2
    exit 1
  fi
}

#!/bin/bash

source run_or_fail.sh

# delete previous id
rm -f .commit_id

# go to repo and update it to given commit
run_or_fail "Repository folder not found!" pushd $1 1> /dev/null
run_or_fail "Could not reset git" git reset --hard HEAD

# get the most recent commit
COMMIT=$(run_or_fail "Could not call 'git log' on repository" git log -n1)
if [ $? != 0 ]; then
  echo "Could not call 'git log' on repository"
  exit 1
fi
# get its id
COMMIT_ID=`echo $COMMIT | awk '{ print $2 }'`

# update the repo
run_or_fail "Could not pull from repository" git pull

# get the most recent commit
COMMIT=$(run_or_fail "Could not call 'git log' on repository" git log -n1)
if [ $? != 0 ]; then
  echo "Could not call 'git log' on repository"
  exit 1
fi
# get its id
NEW_COMMIT_ID=`echo $COMMIT | awk '{ print $2 }'`

# if the id changed, then write it to a file
if [ $NEW_COMMIT_ID != $COMMIT_ID ]; then
  popd 1> /dev/null
  echo $NEW_COMMIT_ID > .commit_id
fi