shell脚本编程基础--文本比较

1.概述

允许测试Linux文件系统上文件的目录和状态。

2.详解

2.1 检查目录

-d测试会检查指定的目录是否存在于系统中。当我们打算将文件写入目录或是准备切换到该目录时，先进行测试是比较好的做法。

#!/bin/bash
#Look before you leap
#
jump_dir=/home/songzhen
#
if [ -d $jump_dir ]
then
        echo "The $jump_dir directory exists"
        cd $jump_dir
        ls
else
        echo "The $junp_dir directory does not exist"
fi

执行该脚本，结果如下：

2.2 检查对象是否存在（文件或目录）

-e比较允许你的脚本代码在使用文件或者目录前先检查它们是否存在。

#!/bin/bash
#Check if either a directory or file exists
#
location=$HOME
file_name="sentinel"
#
if [ -e $location ]
then
        echo "OK on the $location directory."
        echo "Now checking on the file, $file_name."
        #
        if [ -e $location/$file_name ]
        then
                echo "OK on the filename"
                echo "Updating Current Date..."
                date >> $location/$file_name
        #
        else
                echo "File not exist"
                echo "touch the File"
                touch $file_name
        fi
#
else
        echo "The $location directory does not exist."
        echo "Nothing to update"
fi

执行该脚本，结果如下：

2.3 检查文件

如果确定了对象为文件，必须用-f比较。

#!/bin/bash
#Check if either a directory or file exists
#
item_name=$HOME
echo
echo "The item being checked: $item_name"
echo
#
if [ -e $item_name ]
then
        echo "The item, $item_name, does exist."
        echo "But is it a file?"
        echo
        #
        if [ -f $item_name ]
        then
                echo "Yes, $item_name is a file."
        else
                echo "No, $item_name is not a file."
        fi
#
else
        echo "The item, $item_name, does not exist."
        echo "Nothing to update"
fi

执行该脚本，结果如下：

2.4 检查是否可读

在尝试从文件中读取数据之前，最好先测试一下文件是否可读，可以使用-r比较测试。

#!/bin/bash
#testing if you can read a file
pwfile=/etc/shadow
#
# first, test if the file exists, and is a file
if [ -f $pwfile ]
then
        #now test if you can read it
        if [ -r $pwfile ]
        then
                tail $pwfile
        else
                echo "Sorry, I am unable to read the $pwfile file"
        fi
else
        echo "Sorry, the file $pwfile does not exist"
fi

执行该脚本，结果如下：

2.5 检查是否可写

-w比较会判断你对文件是否有可写权限。

#!/bin/bash
#Check if either a directory or file exists
#
item_name=$HOME/sentinel
echo
echo "The item being checked: $item_name"
echo
#
if [ -e $item_name ]
then
        echo "The item, $item_name, does exist."
        echo "But is it a file?"
        echo
        #
        if [ -f $item_name ]
        then
                echo "Yes, $item_name is a file."
                echo "But is it writable?"
                echo
                #
                if [ -w $item_name ]
                then
                        echo "Writing current time to $item_name"
                        date +%H%M >> $item_name
                #
                else
                        echo "Unable to write to $item_name"
                fi
        #
        else
                echo "No, $item_name is not a file."
        fi
#
else
        echo "The item, $item_name, does not exist."
        echo "Nothing to update"
fi
#

执行该脚本，结果如下：

2.6 检查空文件

用-s比较来检查文件是否为空，尤其是在不想删除非空文件时经常使用，-s比较成功则说明文件中有数据。

#!/bin/bash
#Testing if a file is empty
#
file_name=$HOME/sentinel
#
if [ -f $file_name ]
then
        if [ -s $file_name ]
        then
                echo "The $file_name file exists and has data in it."
                echo "will not remove this file."
        else
                echo "The $file_name file exists but is empty."
                echo "Deleting empty file..."
                rm $file_name
        fi
else
        echo "The $file_name does not exist."
fi

执行该脚本，结果如下：

2.7 检查文件是否可以执行

-x比较是判断特定文件是否有执行权限的一个简单办法。

#!/bin/bash
#testing file execution
#
if [ -x test9.sh ]
then
        echo "You can run the script:"
else
        echo "Sorry,You are unable to execute the script"]
fi

执行该脚本，结果如下：

2.8 检查所属关系

-O比较可以测试出你是否是文件的属主

#!/bin/bash
#check file ownership
#
if [ -O /etc/passwd ]
then
        echo "You are the owner of the /etc/passwd file"
else
        echo "Sorry, you are not the owner of the /etc/passwd file"
fi

执行该脚本，结果如下：

2.9 检查默认属组关系

-G比较会检查文件的默认组，如果它匹配了用户的默认组，则测试成功，-G比较只会检查默认组而非用户所属的所有组。

#!/bin/bash
#check file group test
#
if [ -G $HOME/testing ]
then
        echo "You are in the same group as the file"
else
        echo "The file is not owned by your group"
fi

执行该脚本，结果如下：

2.10 检查文件日期

-nt和-ot用来对两个文件的创建日期进行比较，在编写软件安装脚本时非常有用，但是要注意的是要先确认文件是存在的。

#!/bin/bash
#testing file dates
if [ test9.sh -nt test8.sh ]
then
        echo "The test9.sh file is newer than test8.sh."
else
        echo "The test8.sh file is newer than test9.sh."
fi

执行该脚本，结果如下：