In a open terminal how could I see all the new content added to a file whenever a process writes data into it?
I've tried combinations with cat and tee but no success
Use tail with -f
tail -f filename
Taken from the man pages for tail:
-f, --follow[={name|descriptor}]
output appended data as the file grows;
an absent option argument means 'descriptor'
You can not do it with cat, you should use tail -f <filename> or less <filename> and push F in order to wait data.
$ man less
...
F Scroll forward, and keep trying to read when the end of file is reached. Normally this
command would be used when already at the end of the file. It is a way to monitor the
tail of a file which is growing while it is being viewed. (The behavior is similar to
the "tail -f" command.)
...
Related
I have a printf command that will write a file but won't print to stdout. I would like to have both so I can let the user see what's happening, and at the same time, write a record to a log file.
printf "%s\n" "This is some text" "That will be written to a file" "There will be several lines" | tee -a bin/logfile.log > bin/newfile.conf
That command appends to the log file and writes to the new file, but writes no output to the screen :(
OS: Centos 7
It's because you're redirecting the screen output with > bin/newfile.conf in addition to what you're doing with tee. Just drop the > and everything after it. If you want to output to both of those files at once in addition to the screen, you can use tee twice, e.g.:
printf ... | tee -a bin/logfile.log | tee bin/newfile.conf
That appends to logfile.log and overwrites newfile.conf, and also writes out to the screen. Use or omit the -a option as needed.
As John1024 points out you can also use tee once since it accepts multiple filenames, although in that case -a applies to all filenames, but it can be useful in the case where you want the append vs. overwrite behavior to be the same for all files.
Is there a way to modify so that the tail -f lists the line number of the current file as well.
Something similar to grep -n <Strings> *.
Try less
Instead of using tail to follow data and less or nl for numbering, I suggest using a single tool for both:
less -N +F <filename>
This will make less print line numbers and follow the file. From man less:
F
Scroll forward, and keep trying to read when the end of file is reached. Normally this command would be used when already at the end of the file. It is a way to monitor the tail of a file which is growing while it is being viewed. (The behavior is similar to the tail -f command.)
You could do a Ctrl+C to stop following when inside less; to start following again, you could press F again. With this method, you get the additional goodies that less offers like regex-based search, tags, etc.
This command takes into account the number of lines above also
tail -f -n +1 yourfile.txt | nl
You can use less command,
tail -f <filename> | less -N
According to man page of less
-N or --LINE-NUMBERS
Causes a line number to be displayed at the beginning of each
line in the display.
In terminal, sometimes I would like to display the standard output and also save it as a backup. but if I use redirection ( > &> etc), it does not display the output in the terminal anymore.
I think I can do for example ls > localbackup.txt | cat localbackup.txt. But it just doesn't feel right. Is there any shortcut to achieve this?
Thank you!
tee is the command you are looking for:
ls | tee localbackup.txt
In addition to using tee to duplicate the output (and it's worth mentioning that tee is able to append to the file instead of overwriting it, by using tee -a, so that you can run several commands in sequence and retain all of the output), you can also use tail -f to "follow" the output file from a parallel process (e.g. a separate terminal):
command1 >localbackup.txt # create output file
command2 >>localbackup.txt # append to output
and from a separate terminal, at the same time:
tail -f localbackup.txt # this will keep outputting as text is appended to the file
I am working on a Java EE application where its logs will be generated inside a Linux server .
I have used the command tail -f -n -10000 MyLog
It displayed last 1000 lines from that log file .
Now I pressed Ctrl + c in Putty to disconnect the logs updation ( as i am feared it may be updated with new requests and I will loose my data )
In the displayed result, how can I search for a particular keyword ?? (Used / String name to search but it's not working)
Pipe your output to PAGER.
tail -f -n LINE_CNT LOG_FILE | less
then you can use
/SEARCH_STRING
Two ways:
tail -n 10000 MyLog| grep -i "search phrase"
tail -f -n 10000 MyLog | less
The 2nd method will allow you to search with /. It will only search down but you can press g to go back to the top.
Edit: On testing it seems method 2 doesn't work all that well... if you hit the end of the file it will freeze till you ctrl+c the tail command.
You need to redirect the output from tail into a search utility (e.g. grep). You could do this in two steps: save the output to a file, then search in the file; or in one go: pipe the ouput to the search utility
To see what goes into the file (so you can hit Ctlr+c) you can use the tee command, which duplicates the output to the screen and to a file:
tail -f -n -10000 MyLog | tee <filename>
Then search within the file.
If you want to pipe the result into the search utility, you can use the same trick as above, but use your search program instead of tee
Controlling terminal output on the fly
While running any command in a terminal such as Putty you can use CTRL-S and CTRL-Q to stop and start output to the Putty terminal.
Excluding lines using grep
If you want to exclude lines that contain a specific pattern use grep -v the following would remove all line that contain the string INFO
tail -f logfile | grep -v INFO
Show lines that do not contain the words INFO or DEBUG
tail -f logfile | grep -v -E 'INFO|DEBUG'
Finally, the MOTHER AND FATHER of all tailing tools is xtail.pl
If you have perl on your host xtail.pl is a very nice tool to learn and in a nutshell you can use it to tail multiple files. Very handy.
You can just open it with less command
less logfile_name
when you open the file you can use this guide here
Tip: I suggest, first to use G to go to the end of the file and then to you use Backward Search
When I am trying to sort a file and save the sorted output in itself, like this
sort file1 > file1;
the contents of the file1 is getting erased altogether, whereas when i am trying to do the same with 'tee' command like this
sort file1 | tee file1;
it works fine [ed: "works fine" only for small files with lucky timing, will cause lost data on large ones or with unhelpful process scheduling], i.e it is overwriting the sorted output of file1 in itself and also showing it on standard output.
Can someone explain why the first case is not working?
As other people explained, the problem is that the I/O redirection is done before the sort command is executed, so the file is truncated before sort gets a chance to read it. If you think for a bit, the reason why is obvious - the shell handles the I/O redirection, and must do that before running the command.
The sort command has 'always' (since at least Version 7 UNIX) supported a -o option to make it safe to output to one of the input files:
sort -o file1 file1 file2 file3
The trick with tee depends on timing and luck (and probably a small data file). If you had a megabyte or larger file, I expect it would be clobbered, at least in part, by the tee command. That is, if the file is large enough, the tee command would open the file for output and truncate it before sort finished reading it.
It doesn't work because '>' redirection implies truncation, and to avoid keeping the whole output of sort in the memory before re-directing to the file, bash truncates and redirects output before running sort. Thus, contents of the file1 file will be truncated before sort will have a chance to read it.
It's unwise to depend on either of these command to work the way you expect.
The way to modify a file in place is to write the modified version to a new file, then rename the new file to the original name:
sort file1 > file1.tmp && mv file1.tmp file1
This avoids the problem of reading the file after it's been partially modified, which is likely to mess up the results. It also makes it possible to deal gracefully with errors; if the file is N bytes long, and you only have N/2 bytes of space available on the file system, you can detect the failure creating the temporary file and not do the rename.
Or you can rename the original file, then read it and write to a new file with the same name:
mv file1 file1.bak && sort file1.bak > file1
Some commands have options to modify files in place (for example, perl and sed both have -i options (note that the syntax of sed's -i option can vary). But these options work by creating temporary files; it's just done internally.
Redirection has higher precedence. So in the first case, > file1 executes first and empties the file.
The first command doesn't work (sort file1 > file1), because when using the redirection operator (> or >>) shell creates/truncates file before the sort command is even invoked, since it has higher precedence.
The second command works (sort file1 | tee file1), because sort reads lines from the file first, then writes sorted data to standard output.
So when using any other similar command, you should avoid using redirection operator when reading and writing into the same file, but you should use relevant in-place editors for that (e.g. ex, ed, sed), for example:
ex '+%!sort' -cwq file1
or use other utils such as sponge.
Luckily for sort there is the -o parameter which write results to the file (as suggested by #Jonathan), so the solution is straight forward: sort -o file1 file1.
Bash open a new empty file when reads the pipe, and then calls to sort.
In the second case, tee opens the file after sort has already read the contents.
You can use this method
sort file1 -o file1
This will sort and store back to the original file. Also, you can use this command to remove duplicated line:
sort -u file1 -o file1