Write to file when different in Unix

Question

我在变量中有字符串，我想把它从变量写入文件，但只有当这个字符串和文件的内容不同时。

Answer 1

You can use the following script:

string="foo"

# Use diff to check if the file's content and the variable differ
# <(...) is called "process substitution"
diff file <(cat <<< "$string")

# diff returns 0 if both inputs are the same
[ $? -ne 0 ] || cat <<< "$string" > file

Answer 2

You will have to store the content of the file in a variable and do a check if the variables are same. Something like:

[[ "$var" != "$checkvar" ]] && echo "$var" >> file || echo "content same"

Test:

$ var=jaypalsingh        # Create a variable
$ cat file               # Content of the file
jaypalsingh
$ checkvar=$(<file)      # Add the content of the file to variable
$ [[ "$var" != "$checkvar" ]] && echo "$var" >> file || echo "content same"
content same

$ var=jaypal
$ [[ "$var" != "$checkvar" ]] && echo "$var" >> file || echo "content same"
$ cat file
jaypalsingh
jaypal

Now this will get messier and complicated if you file is big with many lines. The variables will need to have explicit new lines in them.

$ var=jaypalsingh$'\n'jaypal     # Variable with new line
$ echo "$var"
jaypalsingh
jaypal
$ cat file
jaypalsingh
jaypal
$ checkvar=$(<file)
$ [[ "$var" != "$checkvar" ]] && echo "$var" >> file || echo "content same"
content same

Quotes around [[..]] are not needed but usage here will protect against meta-characters as suggested in the comments by that other guy . He also makes another valid point regarding usage of echo here. printf will be more suited here to generate your variable.

Answer 3

>> echo hello world > file
>> cat file
hello world
>> str="hello world"
>> if [ "$(diff file <(echo $str))" != "" ]; then echo $str >> file; fi
>> cat file
hello world
>> str="hello SO"
>> if [ "$(diff file <(echo $str))" != "" ]; then echo $str >> file; fi
>> cat file
hello world
hello SO

If you want to overwrite file instead of appending to it, use echo $str > file .