I want to understand the use of xargs man in Rampion's code:
screen -t man /bin/sh -c 'xargs man || read'
Thanks to Rampion: we do not need cat!
Why do we need xargs in the command?
I understand the xargs -part as follows
cat nothing to xargs
xargs makes a list of man -commands
I have had an idea that xargs makes a list of commands. For instance,
find . -type f -print0 | xargs -0 grep masi
is the same as a list of commands:
find fileA AND grep masi in it
find fileB AND grep masi in it
and so on for fileC, fileD, ...
No, I don't cat nothing. I cat whatever input I get after I run the command. cat is actually extraneous here, so let's ignore it.
xargs man waits on user input. Which is necessary. Since in the script you grabbed that from, I can't paste in the argument for man until after I create the window. So the command that runs in the window needs to wait for me to give it something, before it tries to run man.
If we just ran screen /bin/sh -d 'man || read', it would always complain "What manual page do you want?" since we never told it.
xargs gathers arguments from stdin and executes the command given with those arguments.
so cat is waiting for something to be typed, and then xargs is running man with that input.
xargs is useful if you have a lot of files to process, I often use it with output from find.
xargs will stuff as many arguments as it can onto the command line.
It's great for doing something like
find . -name '*.o' -print | xargs rm
The cat command does not operate on nothing; it operates on standard input, up until it is told that the input is ended. As Rampion notes, the cat command is not necessary here, but it is operating on its implicit input (standard input), not on nothing.
The xargs command reads the output from cat, and groups the information into arguments to the man command specified as its (only) argument. When it reaches a limit (configurable on the command line), it will execute the man command.
The find ... -print0 | xargs -0 ... idiom deals with file names that contain awkward characters such as blanks, tabs and newlines. The find command prints each filename followed by an ASCII NUL ('\0'); this is one of two characters that cannot appear in a simple file name - the other being '/' (which appears in path names, of course, but not in simple file names). It is not directly equivalent to the sequence you provide; xargs groups collections of file names into a single argument list, up to a size limit. If the names are short enough (they usually are), then there will be fewer executions of grep than there are file names.
Note, too, the grep only prints the file name where the material is found if it has more than one file to search -- or if it supports an option so that it always prints the file names and the option is used: '-H' is a GNU extension to grep that does this. The portable way to ensure that the file names always appear is to list /dev/null as the first file (so 'xargs grep something /dev/null'); it doesn't take long to search /dev/null.
Related
I'm using xargs to try to echo the name of a file, followed by its contents. Here is the command
find output/ -type f | xargs -I {} sh -c "echo {} ; cat {}"
However for some reason, the second replace after cat is not being replaced. Only for some files, so some files do work correctly.
To be clear, I'm not looking for a command that lets me echo the name of a file followed by its contents, I'm trying to understand why this specific command does not work.
Turns out that the command was too long, so it was working with shorter file names and failing for longer ones. From man xargs
-I replstr
Execute utility for each input line, replacing one or more occurrences of replstr in up to replacements (or 5 if no -R flag is specified) arguments to utility with the
entire line of input. The resulting arguments, after replacement is done, will not be allowed to grow beyond 255 bytes; this is implemented by concatenating as much
of the argument containing replstr as possible, to the constructed arguments to utility, up to 255 bytes. The 255 byte limit does not apply to arguments to utility
which do not contain replstr, and furthermore, no replacement will be done on utility itself. Implies -x.
The root cause of the problem is pointed out in Carlos' answer, but without a solution.
After some googling, I couldn't find a way to lift up the 255 characters limit.
So a probable way to workaround it, is to use shell variable as a substitution.
Example:
find . | xargs -I% sh -c 'F="%";iconv -f gb2312 -t utf-8 "$F">"$F.out";mv "$F.out" "$F"'
Remember to use single quotes at the outermost sh -c parameter string, we don't want the $F inside to be replaced by our parent shell.
Is it files with white space in the name that create problems? Try adding \", like this:
find output/ -type f | xargs -I {} sh -c "echo \"{}\" ; cat \"{}\""
This worked for me using Bash.
In Cygwin, I wrote a bashrc function rg, which is basically a recursive grep statement:
rg () {
find . -type f -exec grep -il $1 {} \;
}
This works well, but now I need to run an additional grep on each output line to check for another word. I basically want it to list each file that contains both words (not necessarily on the same line). I tried this command:
rg Word1 -exec grep -il Word2 {} \;
This seems to just output the same files containing "Word1".
I tried this next command, and I thought it'd just output "Ha" on each line, but it still keeps listing the files from the "rg Word1" statement.
rg Word1 -exec echo "Ha" \;
So I'm clearly doing something wrong here. Can anyone clear my confusion? I'm aware there's a way to do this within grep itself, but that seems to work on a per-line basis. I'm guessing what I'm trying to do is pretty common. Also once I get this working, I'd like to put it in another bashrc function for convenience. Not sure if that's makes it more complicated or not.
If you have it or can get it, the xargs command collects arguments for a command from its standard input, then runs the designated command. You could combine that with your rg function to filter the function's output. For example,
rg Word1 | xargs grep -il Word2
The arguments xargs reads from its standard input -- the file names emitted by function rg -- will be appended to the given command (grep -il Word2), and the resulting command run. If xargs's input is long enough, the arguments will be split across multiple invocations of the grep command, which makes no difference to the output (in this case), but avoids command execution failing on account of too many arguments.
Consider also structuring function rg in the same way (i.e. using xargs) to minimize the number of separate grep processes that are executed. Starting a new process is one of the more expensive things you can do.
I am new with shell script.
I need to save the number of files with particular extension(.properties) in a variable using shell script.
I have used
ls |grep .properties$ |wc -l
but this command prints the number of properties files in the folder. How can I assign this value in a variable.
I have tried
count=${ls |grep .properties$ |wc -l}
But it is showing error like:
./replicate.sh: line 57: ${ls |grep .properties$ |wc -l}: bad substitution
What is this type of errors?
Please anyone help me to save the number of particular files in a variable for future use.
You're using the wrong brackets, it should be $() (command output substitution) rather than ${} (variable substitution).
count=$(ls -1 | grep '\.properties$' | wc -l)
You'll also notice I've use ls -1 to force one file per line in case your ls doesn't do this automatically for pipelines, and changed the pattern to match the . correctly.
You can also bypass the grep totally if you use something like:
count=$(ls -1 *.properties 2>/dev/null | wc -l)
Just watch out for "evil" filenames like those with embedded newlines for example, though my ls seems to handle these fine by replacing the newline with a ? character - that's not necessarily a good idea for doing things with files but it works okay for counting them.
There are better tools to use if you have such beasts and you need the actual file name, but they're rare enough that you generally don't have to worry about it.
You could use a loop with globbing:
count=0
for i in *.properties; do
count=$((count+1))
done
If you are using a shell that supports arrays, you can simply capture all such file names
files=( *.properties )
and then determine the number of array elements
count=${#files[#]}
(The above assumes bash; other shells may require slightly different syntax.)
You'd better use find instead of parsing ls. Then, use the var=$(command) syntax to store the value.
var=$(find . -maxdepth 1 -name "*\.properties" | wc -l)
Reference: Why you shouldn't parse the output of ls.
To solve the problem appearing if any file name contains new lines, you can use what chepner suggests in the comments:
var=$(find . -maxdepth 1 -name "*\.properties" -exec 'echo 1' | wc -l)
so that for every match it will print not the name, but any random character (in this case, 1) and then the amount of them will be counted to produce the correct output.
Use:
count=`ls|grep .properties$ | wc -l`
echo $count
You could write your assignment like this:
count=$(ls -q | grep -c '\.properties$')
or
count=$(ls -qA | grep -c '\.properties$')
if you want to include hidden files.
This works with all kind of filenames because we're using ls with q.
Sure it's easier to link to some webpage that tells you to "never parse ls" than to read the ls manual and see there's a q option (and that most implementations default to q if the output is to a terminal device which explains why some people here state their ls seems to handle filenames with newlines just fine by replacing the newline with a ? character).
I have a program that I can run in two ways: single-end or paired-end mode. Here's the syntax:
program <output-directory-name> <input1> [input2]
Where the output directory and at least one input is required. If I wanted to run this on three files, say, sample A, B, and C, I would use something like find with xargs or parallel:
user#host:~/single$ ls
sampleA.txt sampleB.txt sampleC.txt
user#host:~/single$ find . -name "sample*" | xargs -i echo program {}-out {}
program ./sampleA.txt-out ./sampleA.txt
program ./sampleB.txt-out ./sampleB.txt
program ./sampleC.txt-out ./sampleC.txt
user#host:~/single$ find . -name "sample*" | parallel --dry-run program {}-out {}
program ./sampleA.txt-out ./sampleA.txt
program ./sampleB.txt-out ./sampleB.txt
program ./sampleC.txt-out ./sampleC.txt
But when I want to run the program in "paired-end" mode, I need to give it two inputs. These are related files, but they can't simply be concatenated - you have to run the program with both as inputs. Files are named sensibly, e.g., sampleA_1.txt and sampleA_2.txt.
I want to be able to create this easily on the command line with something like xargs (or preferably parallel):
user#host:~/paired$ ls
sampleA_1.txt sampleB_1.txt sampleC_1.txt
sampleA_2.txt sampleB_2.txt sampleC_2.txt
user#host:~/paired$ find . -name "sample*_1.txt" | sed/awk? | parallel ?
program ./sampleA-out ./sampleA_1.txt ./sampleA_2.txt
program ./sampleB-out ./sampleB_1.txt ./sampleB_2.txt
program ./sampleC-out ./sampleC_1.txt ./sampleC_2.txt
Ideally, the command would strip off the _1.txt to create the output directory name (sampleA-out, etc), but I really need to be able to take that argument and change the _1 to a _2 for the second input.
I know this is dead simple with a script - I did this in Perl with a quick regular expression substitution. But I would love to be able to do this with a quick one-liner.
Thanks in advance.
I did this in Perl with a quick regular expression substitution. But I would love to be able to do this with a quick one-liner.
Perl has one-liners, too, just as sed and awk do. You can write:
find . -name "sample*_1.txt" | perl -pe 's/_1\.txt$//' | parallel program {}-out {}_1.txt {}_2.txt
(The -e flag means "the next argument is the program text"; the -p flag means "the program should be run in loop; for each line of input, set $_ to that line, then run the program, then print $_".)
With sed and xargs you could do something like this:
find . -name "sample*_1.txt" | sed -n 's/_1\..*$//;h;s/$/_out/p;g;s/$/_1.txt/p;g;s/$/_2.txt/p' | xargs -L 3 echo program
I.e.: sed creates the three arguments and xargs -L 3 composes commands lines with three arguments.
Assuming you always have exactly 2 files in your directory for each pair and assuming they get sorted the right way by find (this you can ensure by piping results of find through sort), maybe xargs -l 2 would do the job. This tells xargs to place 2 consecutive incoming parameters on each command line it executes.
A shorter version:
parallel --xapply program {1.}.out {1} {2} :::: <(ls *_1.txt) <(ls *_2.txt)
but this only works if every _1.txt has a matching _2.txt and vice versa.
I'm trying to list the files in a directory and do something to them in the Mac OS X prompt.
It should go like this: for f in $(ls -1); do echo $f; done
If I have files without spaces in their names (fileA.txt, fileB.txt), the echo works fine.
If the files include spaces in their names ("file A.txt", "file B.txt"), I get 4 strings (file, A.txt, file, B.txt).
I've tried quoting the listing command, but it only changed the problem.
If I do this: for f in $(ls -1); do echo $f; done
I get: file A.txt\nfile B.txt
(It displays correctly, but it is a single string and I need the 2 lines separated.
Step away from ls if at all possible. Use find from the findutils package.
find /target/path -type f -print0 | xargs -0 your_command_here
-print0 will cause find to output the names separated by NUL characters (ASCII zero). The -0 argument to xargs tells it to expect the arguments separated by NUL characters too, so everything will work just fine.
Replace /target/path with the path under which your files are located.
-type f will only locate files. Use -type d for directories, or omit altogether to get both.
Replace your_command_here with the command you'll use to process the file names. (Note: If you run this from a shell using echo for your_command_here you'll get everything on one line - don't get confused by that shell artifact, xargs will do the expected right thing anyway.)
Edit: Alternatively (or if you don't have xargs), you can use the much less efficient
find /target/path -type f -exec your_command_here \{\} \;
\{\} \; is the escape for {} ; which is the placeholder for the currently processed file. find will then invoke your_command_here with {} ; replaced by the file name, and since your_command_here will be launched by find and not by the shell the spaces won't matter.
The second version will be less efficient since find will launch a new process for each and every file found. xargs is smart enough to pipe the commands to a newly launched process if it can figure it's safe to do so. Prefer the xargs version if you have the choice.
for f in *; do echo "$f"; done
should do what you want. Why are you using ls instead of * ?
In general, dealing with spaces in shell is a PITA. Take a look at the $IFS variable, or better yet at Perl, Ruby, Python, etc.
Here's an answer using $IFS as discussed by derobert
http://www.cyberciti.biz/tips/handling-filenames-with-spaces-in-bash.html
You can pipe the arguments into read. For example, to cat all files in the directory:
ls -1 | while read FILENAME; do cat "$FILENAME"; done
This means you can still use ls, as you have in your question, or any other command that produces $IFS delimited output.
The while loop makes it much easier to do several things to the argument, and makes complex processing more readable in my opinion. A contrived example:
ls -1 | while read FILE
do
echo 1: "$FILE"
echo 2: "$FILE"
done
look --quoting-style option.
for instance, --quoting-style=c would produce :
$ ls --quoting-style=c
"file1" "file2" "dir one"
Check out the manpage for xargs:
it works like this:
ls -1 /tmp/*.jpeg | xargs rm