Some Tricks About Sh
Some Tricks About Bash/Zsh/Fish…
Yestorday, I learned some tricks about *sh when I try to run my program. The tricks I used are as follows:
1. wc which means Word Count:
wc -l which gives the number of lines in the standard output:
some_command_that_can_be_output_by_line | wc -l
For example:
- Input:
ls -l | wc -l - Ouput:
5the total number of the files in the current directory
P.S. When using upper case…em…I mean using wc -L which you can get the number of bytes in the longest row. It’s true, I’ve tried.😂
2.
tail -n 14 nohup.out | grep -A 7 "iter:"
Looks a bit complicated. Don’t worry. I will explain one by one:
tail FILE
You can use man tail to see:
print the last 10 lines of each FILE to standard output.
Ugh… it seems that I didn’t explain anything except copying the manual. I remain indifferent to it. just for fun. Don’t say something useless, continue.
tail -n NUM FILE
output the last NUM lines.
Of course, you can use it like this ls -l | tail -n 1 which print the last line of this command ls -l output.
P.S. The usage of head is the same, except that the output is the first few lines of the file. for instance:
- Input:
ls -l | tail -n 1 - Output:
drwxr-xr-x 7 tipsy 224 5 7 14:43 LabelVOC
grep -A NUM PATTERN
A means After Context, so you can also use --after-context=NUM instead.
Print NUM lines of trailing context after matching lines.
grep -B NUM PATTERN
B means Before Context. It can instead of --before-context. Yep, similar to above.
Print NUM lines of leading context before matching lines.
grep -C NUM PATTERN
C means Context. I don’t wanna say more. It just makes the above two commands work together.
Print NUM lines of output context.
3.
nohup unzip /dir/zipfile.zip -o -d /dir_you_want > unzip.info.txt 2>&1 &
hah, more complicated than last one.
nohup
one of the shell buildin commands which maybe means no hungup. it sets the signal SIGHUP to be ignored. As a result, it will not be terminated when you logout from ssh. In short:
Run COMMAND, ignoring hangup signals.
If standard output is a terminal, append output to ‘nohup.out’ if possible, ‘$HOME/nohup.out’ otherwise. If standard error is a terminal, redirect it to standard output.
unzip /dir/zipfile.zip -o
can creates ZIP archives. -o means overwrite existing files without promping. -d /dir_you_want: An optional directory to which to extract files. By default, will be created in the current directory. ```
> redirected_standard_output_file
It redirects standard output to this file, overwriting the file.
P.S. >> will not overwrite instead of appending the redirected output at the end.
2>&1
seems mystery, but not at all. 2 and 1 just are two of file descriptor. A file descriptor is a non-negative integer. 2 represents standard error, while 1 represents standard output. the last number 0 represents standard input. the ‘&’ tells system ‘1’ represents standard output rather than file named ‘1’.
Writing just
2>1would redirect the standard error to a file called “1”, not to standard output.[2]
P.S. 2>&1 call the dup2(1,2), I could not tell you more, search it if interest.
&
the last symbol which can start the program as a background job.
P.S. fg foreground. It can bring a background job back to the foreground. (but if you’ve redirected output you won’t see much.)[3]
TO BE CONTINUE
These days, I was exhausted. wanan.
References:
Have Question?
Trun to here.