Detailed explanation of the output of the "time" command under shell (bash)

Preface

I believe everyone knows that time is a very useful command in bash. It can time the execution of a script or a program. This is usually very convenient when roughly comparing program execution efficiency. But you will find that the time text output by the time command cannot be simply redirected, for example to a text file, and can only be displayed on the screen, which is very inconvenient for non-interactive timing.

For example:

$ time find . -name "mysql.sh" >1.txt
 
real 0m0.081s
user 0m0.060s
sys  0m0.020s
 
$ time find . -name "mysql.sh" 2>2.txt
./work186/sms/bin/mysql.sh
./work186/sms/src/scripts/mysql.sh
./work186/sms/src/scripts1/mysql.sh
./work186/sms1/bin/mysql.sh
./work186/sms1/src/scripts/mysql.sh
./temp/sms/bin/mysql.sh
./temp/sms/src/scripts/mysql.sh
 
real 0m0.068s
user 0m0.040s
sys  0m0.030s

Through the above attempts, it was found that the output information of time cannot be redirected to a file. Why? Because time is a keyword of the shell, the shell has done special processing. It will process the command line following the time command as a whole. When redirecting, it is actually for the following commands. The output of the time command itself and will not be redirected.

The keyword time sets a mark. The timing information is not printed to stderr until the command(find) command is executed. The time keyword requires the entire command and pipeline, as well as related redirections, to be advanced. That's why a simple redirect doesn't work for time. This is defined by Bash syntax. The redirection after command is part of command for time.

Note: The output of the time command is sent to standard error (stderr)

When the time command is executed, command runs at the next level of the current shell (that is, the shell where the time command is executed) shell, and the output of time itself is located on stderr of the current shell. Redirecting as shown above will only cause the command's stdout to be redirected to a text file, but will not output the output content of time itself.

The first solution is to put the time command and the command line to be executed in a shell code block, that is, within a pair of curly brackets. Pay attention to the use of spaces and semicolons.

$ { time find . -name "mysql.sh"; } 2>2.txt
./work186/sms/bin/mysql.sh
./work186/sms/src/scripts/mysql.sh
./work186/sms/src/scripts1/mysql.sh
./work186/sms1/bin/mysql.sh
./work186/sms1/src/scripts/mysql.sh
./temp/sms/bin/mysql.sh
./temp/sms/src/scripts/mysql.sh
$ cat 2.txt
 
real 0m0.068s
user 0m0.030s
sys  0m0.040s

The first method was successful. To sum up, { time command-line; } 2>file Pay attention to the use of separators.

Another way is to use a sub-Shell

As shown below:

$ (time find . -name "mysql.sh") 2>2.txt
./work186/sms/bin/mysql.sh
./work186/sms/src/scripts/mysql.sh
./work186/sms/src/scripts1/mysql.sh
./work186/sms1/bin/mysql.sh
./work186/sms1/src/scripts/mysql.sh
./temp/sms/bin/mysql.sh
./temp/sms/src/scripts/mysql.sh
$ cat 2.txt
 
real 0m0.083s
user 0m0.040s
sys  0m0.020s
[root@web186 root]#

The second method was also successful, summary It looks like (time command-line) 2>file where time is immediately followed by parentheses (it’s also ok, and there is no need for a semicolon at the end of the command line. Of course, it’s best to use the first method, after all, it takes a lot to start a subshell It takes up some resources.

Summary

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