bash-3.2$ ls | grep Makefile > a.txt | cat a.txt
why this don't work?? "Makefile" is exist.
There is no output from the grep command since you are redirecting it to a file. Therefore the pipe gets closed before the cat a.txt actually gets called. As per my comment, use && instead of that last |.
Do do what you want which is to save the output of a command to a file but still print to stdout use tee. (also I like to do ls -1 to make sure only one item per line is printed this is not necessary but force of habit)
ls -1 | grep Makefile | tee a.txt
Related
I write down some commands row by row in a file, and I want to execute the commands through grep and pipe;
for example:
1.there is a file a.txt,which content is like below:
echo "hello world"
ls -l
2.then I want execute the first line in my terminal, so I want it like this:
cat a.txt | grep echo | execute the output of previous commands
so that, I can finally execute the command, which is the first line of a.txt.
(can not find any answer of this, so I come here to find some help.)
You can either pipe the command to bash (or any other shell) to execute it:
sed -n 1p a.txt | bash
or you can use eval with command substitution:
eval $(head -n1 a.txt)
BTW, I showed you another two ways how to extract the line from the file.
What I'm trying to do is:
Start a process that outputs text continously
Pipe that output to two commands:
A logger script
'head' command, so I can save the first lines the initial process output.
What I tried so far (unsuccessfuly) is:
./myProgram | tee > (myLogger log.txt) | head > firstLines.txt
The problem is that the myProgram exits as soon as head is finished.
Even if I use -i in tee command, I can't get myProgram to keep running.
Since the logger may append the incoming text to an existing file, executing
head log.txt > firstLines.txt
won't work in this case.
You can use awk as an alternative to do both:
./myProgram |
awk 'NR<=10{print > "firstLines.txt"} NR>10{close("firstLines.txt")} 1' > log.txt
Like this maybe:
yes | awk 'FNR<4 {print >>"file"; close("file")} 1' | more
where yes is your program, file is where you send the output of head to, and more is your logger.
I have practiced in using command line in CentOS6.
I tried to create file, which content would be the output of command man grep. Also I used command man with col -b option to save file as Text-Only. All of this must be in one command.
I tried to do like this:
grep man grep | col -b > output.txt
But it didn't work.
What is the proper way to save output of command man grep as Text-Only file with using option col -b?
Don't you really need this:
man grep | col -b > output.txt
Why do you need to call grep in the first place?
Other, hacky way using grep:
man grep | grep -v somephrasethatwontoccur | col -b > output.txt
But, truly, it makes no sense. grep -v looks for lines without the specified phrase.
Basically I'm wondering why this doesn't output anything:
tail --follow=name file.txt | grep something | grep something_else
You can assume that it should produce output I have run another line to confirm
cat file.txt | grep something | grep something_else
It seems like you can't pipe the output of tail more than once!? Anyone know what the deal is and is there a solution?
EDIT:
To answer the questions so far, the file definitely has contents that should be displayed by the grep. As evidence if the grep is done like so:
tail --follow=name file.txt | grep something
Output shows up correctly, but if this is used instead:
tail --follow=name file.txt | grep something | grep something
No output is shown.
If at all helpful I am running ubuntu 10.04
You might also run into a problem with grep buffering when inside a pipe.
ie, you don't see the output from
tail --follow=name file.txt | grep something > output.txt
since grep will buffer its own output.
Use the --line-buffered switch for grep to work around this:
tail --follow=name file.txt | grep --line-buffered something > output.txt
This is useful if you want to get the results of the follow into the output.txt file as rapidly as possible.
Figured out what was going on here. It turns out that the command is working it's just that the output takes a long time to reach the console (approx 120 seconds in my case). This is because the buffer on the standard out is not written each line but rather each block. So instead of getting every line from the file as it was being written I would get a giant block every 2 minutes or so.
It should be noted that this works correctly:
tail file.txt | grep something | grep something
It is the following of the file with --follow=name that is problematic.
For my purposes I found a way around it, what I was intending to do was capture the output of the first grep to a file, so the command would be:
tail --follow=name file.txt | grep something > output.txt
A way around this is to use the script command like so:
script -c 'tail --follow=name file.txt | grep something' output.txt
Script captures the output of the command and writes it to file, thus avoiding the second pipe.
This has effectively worked around the issue for me, and I have explained why the command wasn't working as I expected, problem solved.
FYI, These other stackoverflow questions are related:
Trick an application into thinking its stdin is interactive, not a pipe
Force another program's standard output to be unbuffered using Python
You do know that tail starts by default with the last ten lines of the file? My guess is everything the cat version found is well into the past. Try tail -n+1 --follow=name file.txt to start from the beginning of the file.
works for me on Mac without --follow=name
bash-3.2$ tail delme.txt | grep po
position.bin
position.lrn
bash-3.2$ tail delme.txt | grep po | grep lr
position.lrn
grep pattern filename | grep pattern | grep pattern | grep pattern ......
I have two files: file1 and file2. How do I append the contents of file2 to file1 so that contents of file1 persist the process?
Use bash builtin redirection (tldp):
cat file2 >> file1
cat file2 >> file1
The >> operator appends the output to the named file or creates the named file if it does not exist.
cat file1 file2 > file3
This concatenates two or more files to one. You can have as many source files as you need. For example,
cat *.txt >> newfile.txt
Update 20130902
In the comments eumiro suggests "don't try cat file1 file2 > file1." The reason this might not result in the expected outcome is that the file receiving the redirect is prepared before the command to the left of the > is executed. In this case, first file1 is truncated to zero length and opened for output, then the cat command attempts to concatenate the now zero-length file plus the contents of file2 into file1. The result is that the original contents of file1 are lost and in its place is a copy of file2 which probably isn't what was expected.
Update 20160919
In the comments tpartee suggests linking to backing information/sources. For an authoritative reference, I direct the kind reader to the sh man page at linuxcommand.org which states:
Before a command is executed, its input and output may be redirected
using a special notation interpreted by the shell.
While that does tell the reader what they need to know it is easy to miss if you aren't looking for it and parsing the statement word by word. The most important word here being 'before'. The redirection is completed (or fails) before the command is executed.
In the example case of cat file1 file2 > file1 the shell performs the redirection first so that the I/O handles are in place in the environment in which the command will be executed before it is executed.
A friendlier version in which the redirection precedence is covered at length can be found at Ian Allen's web site in the form of Linux courseware. His I/O Redirection Notes page has much to say on the topic, including the observation that redirection works even without a command. Passing this to the shell:
$ >out
...creates an empty file named out. The shell first sets up the I/O redirection, then looks for a command, finds none, and completes the operation.
Note: if you need to use sudo, do this:
sudo bash -c 'cat file2 >> file1'
The usual method of simply prepending sudo to the command will fail, since the privilege escalation doesn't carry over into the output redirection.
Try this command:
cat file2 >> file1
Just for reference, using ddrescue provides an interruptible way of achieving the task if, for example, you have large files and the need to pause and then carry on at some later point:
ddrescue -o $(wc --bytes file1 | awk '{ print $1 }') file2 file1 logfile
The logfile is the important bit. You can interrupt the process with Ctrl-C and resume it by specifying the exact same command again and ddrescue will read logfile and resume from where it left off. The -o A flag tells ddrescue to start from byte A in the output file (file1). So wc --bytes file1 | awk '{ print $1 }' just extracts the size of file1 in bytes (you can just paste in the output from ls if you like).
As pointed out by ngks in the comments, the downside is that ddrescue will probably not be installed by default, so you will have to install it manually. The other complication is that there are two versions of ddrescue which might be in your repositories: see this askubuntu question for more info. The version you want is the GNU ddrescue, and on Debian-based systems is the package named gddrescue:
sudo apt install gddrescue
For other distros check your package management system for the GNU version of ddrescue.
Another solution:
tee < file1 -a file2
tee has the benefit that you can append to as many files as you like, for example:
tee < file1 -a file2 file3 file3
will append the contents of file1 to file2, file3 and file4.
From the man page:
-a, --append
append to the given FILEs, do not overwrite
Zsh specific: You can also do this without cat, though honestly cat is more readable:
>> file1 < file2
The >> appends STDIN to file1 and the < dumps file2 to STDIN.
cat can be the easy solution but that become very slow when we concat large files, find -print is to rescue you, though you have to use cat once.
amey#xps ~/work/python/tmp $ ls -lhtr
total 969M
-rw-r--r-- 1 amey amey 485M May 24 23:54 bigFile2.txt
-rw-r--r-- 1 amey amey 485M May 24 23:55 bigFile1.txt
amey#xps ~/work/python/tmp $ time cat bigFile1.txt bigFile2.txt >> out.txt
real 0m3.084s
user 0m0.012s
sys 0m2.308s
amey#xps ~/work/python/tmp $ time find . -maxdepth 1 -type f -name 'bigFile*' -print0 | xargs -0 cat -- > outFile1
real 0m2.516s
user 0m0.028s
sys 0m2.204s