5 ms·

My #1 abuse is xargs -n1. A lot of people like writing bash for loops, I will try and avoid that as much as possible, xargs -n1 is the bash equivalent of a cal

My #1 abuse is xargs -n1. A lot of people like writing bash for loops, I will try and avoid that as much as possible, xargs -n1 is the bash equivalent of a call to 'map' in a functional language. For instance, let's say you want to create thumbnails of a bunch of jpegs: find images -name "*.jpg" | xargs -n1 -IF echo F F | sed -e 's/.jpg$/_thumb.jpg/' | xargs -n2 echo convert -geometry 200x Additionally, it's fully parallelizable as xargs supports something akin to pmap.

howeyc 14y ago
If you like xargs, but want jobs to execute in parallel, you may enjoy GNU parallel. Your trivial example: find images -name "*.jpg" | parallel convert -geometry 200x {} {.}_thumb.jpg
- kylek 14y ago
  xargs --max-procs=#
  - icebraining 14y ago
    The nice thing about parallel is that it can actually run some of the instances on remote machines using SSH, transparently (as long as they have the commands). You just need a couple of parameters and it takes care of uploading the files the command needs and then downloading the results. It's quite awesome.
  - keeperofdakeys 14y ago
    However, unlike xargs, GNU Parallel gives you a guarantee on the order of the output. From the man page: GNU parallel makes sure output from the commands is the same output as you would get had you run the commands sequentially. This makes it possible to use output from GNU parallel as input for other programs.
    - etrain 14y ago
      Want to comment just to stress that this feature is EXTREMELY useful and has saved me from all sorts of tricks with file intermediate output with filenames that match process ids, etc.
sillysaurus 14y ago
Would anyone mind doing me a favor by explaining xargs in more detail? I've tried learning it a couple times but I always seem to forget the primary situations in which it's useful. Thank you in advance!
- sjs 14y ago
  It's most common use is to take some stuff on stdin and then use those as arguments to a command. Here is a fancy way to do `ls * `[1] using xargs: find . -print | xargs ls `find` dumps the results to stdout, the pipe shuttles `find`'s stdout to `xargs` stdin, `xargs` uses its stdin as arguments for `ls`. (Don't run this in your home directory) [1] Pedants will realize this is actually equivalent to `ls .* *` as hidden files are found by `find`.
  - BrandonM 14y ago
    Maybe you'll call me a pedant, but you should be aware that `find .` is not equivalent to `ls .* *`. The find command starts at the indicated directories (. in this case) and lists each file and directory within it, recursing into subdirectories. You can use things like -type, -[i]name, and -mtime to filter the results, as well as -mindepth and -maxdepth to constrain the traversal. Note also that "-print" is the default command for find, so you can leave it off. Other commands include -print0 (NUL-delimited instead of newline-delimited) and -exec.
- Dobbs 14y ago
  Xargs takes a newline separated list and maps the list to a command. find ./ -name '*.log' | xargs rm Finds all log files and map them to 'rm' commands. e.g. if it finds system.log and rails.log it will run the command `rm system.log rails.log`. xargs will automatically do things like break up very long lists into multiple command calls so that it doesn't exceed the maximum number of arguments a command can have. Other useful things about xargs are '-P <NUM>' which lets you run the same command in parallel. I use this with curl to do ghetto benchmarks. The next major flag is `-n <NUM>` which changes the number of arguments per command call. e.g. `-n 1` will run the command per argument passed to xargs. And the last flag I commonly use is `-I {}`. This sets `{}` as a variable which contains the arguments. (This also forces `-n 1`). This is useful for things like: find ./ -name '*.log' | xargs -I{} sh -c "if [ -f {}.gz ]; then rm {}; fi"
ninetax 14y ago
This may be of interest to you, http://blog.last.fm/2009/04/06/mapreduce-bash-script http://blog.last.fm/2009/04/06/mapreduce-bash-script
przemoc 14y ago
I have a tendency to use xargs too, but often you can go with find's -exec, especially "command {} +" construct used for spitting many files at once to the given command. E.g. find . -iname '*.pdf' -exec pdfgrep -i keyword {} +
- krzyk 14y ago
  Upvoted just for mentioning that there is such thing as pdfgrep, thanks :)
sonnym 14y ago
I use xargs a lot for refactoring work when I cannot simply use sed, e.g. vim $(grep -lr foo | xargs) and doing what I need to do on a file by file basis. Otherwise, for renaming functions and the like, I do a lot of: find . -name foo_fn exec sed -i s/foo_fn/bar_fn/g '{}' \; I generally love abusing bash. Just today I was asked about how to rename a bunch of files, specifically containing spaces, and came up with either of these two options: find -name foo_bar -exec cp "'{}'"{,.bak} \; and for file in $(find -name foo_bar); do cp "$file"{,.bak}; done Ultimately, the great thing is, if you learn CTRL-R, you can always search for these types of commands and modify them as necessary for the particular task at hand and not necessarily remember them. One I use all the time, to push git branches upstream is the following: CTRL-R --set- which gives me: git push -f --set-upstream origin `git rev-parse --abbrev-ref HEAD` This is entirely unique in my history, a common part of my workflow, and trivially searchable. I also enjoy being able to perform something along the lines of: vim $(bundle show foo-gem)
- ralph 14y ago
  Sorry, what do you think vim $(grep -lr foo | xargs) is doing? Assuming a missing directory after the `foo' why can't it just be vim $(grep -lr foo .) I don't see that xargs's default behaviour is adding anything. For find . -name foo_fn exec sed -i s/foo_fn/bar_fn/g '{}' \; you may find -exec's + of use. The above has the fork/execve overhead for each file found. find -name '*.[ch]' -exec sed -i 's/\<foo\>/bar/g' {} +
  - sonnym 14y ago
    You are correct on both counts regarding the vim and grep example - I guess I just assumed I would have to have all the files on a single line before handing them off to vim. Thanks for the suggestion about -exec +; I will have to remember it in the future.
    - ralph 14y ago
      Backticks, ``, and $(), use IFS to parse the command's output into words which then become the $()'s replacement and IFS normally contains \n.
gnosis 14y ago
You might be interested in "zargs" in zsh, which would save you the call to find. Furthermore, instead of the pipe to sed and extra xargs, it might be clearer and simpler to do something like: zargs -n 1 **/*.jpg -- make-thumb Where "make-thumb" is a short script (or even a zsh function, if you care about saving a fork for each input file) containing: convert -geometry 200x $1 ${1%%.jpg}_thumb.jpg But, in real life, instead of writing such a script or function, what I'd probably do instead is: zargs -n 1 **/*.jpg | vipe > myscript and do some quick editing in vim to modify the zargs output by hand to do whatever I need -- and then I'd run the resulting "myscript". Just fyi, "vipe" is part of the "moreutils" package [1] and lets you use your editor in the middle of a pipe. One final trick is for when you need to do in-place image manipulation. Instead of using "convert", you can use another ImageMagick command: "mogrify". It will overwrite the original file with the modified file. Of course, you should be very careful with it. [1] - http://joeyh.name/code/moreutils/ http://joeyh.name/code/moreutils/
BrandonM 14y ago
Why do that when a loop is clearer, safer, and shorter? find images -name '*.jpg' | while read jpg; do convert -geometry 200x "$jpg" "$(echo "$jpg" | sed 's/.jpg$/_thumb.jpg/')"; done This version works even when there are spaces in a filename, whereas yours will break.
- _ZeD_ 14y ago
  false $ ls -Ql totale 4 -rw-r--r-- 1 zed users 33 set 6 07:30 " spaces " $ $ find -name '*spaces*' | while read text; do cat "$text"; done cat: ./ spaces: File o directory non esistente $ $ find -name '*spaces*' -print0 | xargs -0 cat while read is broken with spaces $
  - mkl 14y ago
    Only for file names starting or ending with spaces, as far as I can tell, which are pretty unusual.
  - ralph 14y ago
    IFS needs some care and attention and read should have -r. $ ls -Q " spaces " $ ls | while IFS="\n" read -r f; do ls "$f"; done spaces $ For lots of grim detail see David A. Wheeler's http://www.dwheeler.com/essays/fixing-unix-linux-filenames.html http://www.dwheeler.com/essays/fixing-unix-linux-filenames.h...
  - BrandonM 14y ago
    That is a good point, but it is a much more degenerate case than the more common internal space.
- shorter 14y ago
  an alternative approach echo images/*.jpg|sed 's/.*/convert -geometry 200x \"&\" \"&_thumb.jpg\"/;s/\.jpg_thumb/_thumb/' > script; sh -v script 2>log you could also use -x instead of -v reasoning: globbing is as fast as find and is faster than find _if_ you do the same glob twice (b/c it's cached); i'm assuming the images dir is not full of subfolders otherwise i would would do echo images/*/* if the dir structure gets any deeper than 2 levels, i'd use find while loops typed from the command line are a p.i.t.a. if you don't bg them initially and want to bg them later; you need to put them inside brackets or into a function using a script and a log, if something goes wrong while it's running, you can restart after looking at the log; you can't do that with a loop
ralph 14y ago
I too like xargs but see no need for these two xargs given sed is already used? find images -name '*.jpg' | sed 's/\.jpg$// s/.*/convert -geometry 200x &.jpg &_thumb.jpg/'
johnchristopher 14y ago
Is there a way to use an alias with xargs like in the command below ? xargs -a cmd_list.txt -I % alias % This has been bothering me for a while.

howeyc 14y ago

If you like xargs, but want jobs to execute in parallel, you may enjoy GNU parallel. Your trivial example: find images -name "*.jpg" | parallel convert -geometry 200x {} {.}_thumb.jpg

kylek 14y ago
xargs --max-procs=#
- icebraining 14y ago
  The nice thing about parallel is that it can actually run some of the instances on remote machines using SSH, transparently (as long as they have the commands). You just need a couple of parameters and it takes care of uploading the files the command needs and then downloading the results. It's quite awesome.
- keeperofdakeys 14y ago
  However, unlike xargs, GNU Parallel gives you a guarantee on the order of the output. From the man page: GNU parallel makes sure output from the commands is the same output as you would get had you run the commands sequentially. This makes it possible to use output from GNU parallel as input for other programs.
  - etrain 14y ago
    Want to comment just to stress that this feature is EXTREMELY useful and has saved me from all sorts of tricks with file intermediate output with filenames that match process ids, etc.

sillysaurus 14y ago

Would anyone mind doing me a favor by explaining xargs in more detail? I've tried learning it a couple times but I always seem to forget the primary situations in which it's useful. Thank you in advance!

sjs 14y ago
It's most common use is to take some stuff on stdin and then use those as arguments to a command. Here is a fancy way to do `ls * `[1] using xargs: find . -print | xargs ls `find` dumps the results to stdout, the pipe shuttles `find`'s stdout to `xargs` stdin, `xargs` uses its stdin as arguments for `ls`. (Don't run this in your home directory) [1] Pedants will realize this is actually equivalent to `ls .* *` as hidden files are found by `find`.
- BrandonM 14y ago
  Maybe you'll call me a pedant, but you should be aware that `find .` is not equivalent to `ls .* *`. The find command starts at the indicated directories (. in this case) and lists each file and directory within it, recursing into subdirectories. You can use things like -type, -[i]name, and -mtime to filter the results, as well as -mindepth and -maxdepth to constrain the traversal. Note also that "-print" is the default command for find, so you can leave it off. Other commands include -print0 (NUL-delimited instead of newline-delimited) and -exec.
Dobbs 14y ago
Xargs takes a newline separated list and maps the list to a command. find ./ -name '*.log' | xargs rm Finds all log files and map them to 'rm' commands. e.g. if it finds system.log and rails.log it will run the command `rm system.log rails.log`. xargs will automatically do things like break up very long lists into multiple command calls so that it doesn't exceed the maximum number of arguments a command can have. Other useful things about xargs are '-P <NUM>' which lets you run the same command in parallel. I use this with curl to do ghetto benchmarks. The next major flag is `-n <NUM>` which changes the number of arguments per command call. e.g. `-n 1` will run the command per argument passed to xargs. And the last flag I commonly use is `-I {}`. This sets `{}` as a variable which contains the arguments. (This also forces `-n 1`). This is useful for things like: find ./ -name '*.log' | xargs -I{} sh -c "if [ -f {}.gz ]; then rm {}; fi"

ninetax 14y ago

This may be of interest to you, http://blog.last.fm/2009/04/06/mapreduce-bash-script http://blog.last.fm/2009/04/06/mapreduce-bash-script

przemoc 14y ago

I have a tendency to use xargs too, but often you can go with find's -exec, especially "command {} +" construct used for spitting many files at once to the given command. E.g. find . -iname '*.pdf' -exec pdfgrep -i keyword {} +

krzyk 14y ago
Upvoted just for mentioning that there is such thing as pdfgrep, thanks :)

sonnym 14y ago

I use xargs a lot for refactoring work when I cannot simply use sed, e.g. vim $(grep -lr foo | xargs) and doing what I need to do on a file by file basis. Otherwise, for renaming functions and the like, I do a lot of: find . -name foo_fn exec sed -i s/foo_fn/bar_fn/g '{}' \; I generally love abusing bash. Just today I was asked about how to rename a bunch of files, specifically containing spaces, and came up with either of these two options: find -name foo_bar -exec cp "'{}'"{,.bak} \; and for file in $(find -name foo_bar); do cp "$file"{,.bak}; done Ultimately, the great thing is, if you learn CTRL-R, you can always search for these types of commands and modify them as necessary for the particular task at hand and not necessarily remember them. One I use all the time, to push git branches upstream is the following: CTRL-R --set- which gives me: git push -f --set-upstream origin `git rev-parse --abbrev-ref HEAD` This is entirely unique in my history, a common part of my workflow, and trivially searchable. I also enjoy being able to perform something along the lines of: vim $(bundle show foo-gem)

ralph 14y ago
Sorry, what do you think vim $(grep -lr foo | xargs) is doing? Assuming a missing directory after the `foo' why can't it just be vim $(grep -lr foo .) I don't see that xargs's default behaviour is adding anything. For find . -name foo_fn exec sed -i s/foo_fn/bar_fn/g '{}' \; you may find -exec's + of use. The above has the fork/execve overhead for each file found. find -name '*.[ch]' -exec sed -i 's/\<foo\>/bar/g' {} +
- sonnym 14y ago
  You are correct on both counts regarding the vim and grep example - I guess I just assumed I would have to have all the files on a single line before handing them off to vim. Thanks for the suggestion about -exec +; I will have to remember it in the future.
  - ralph 14y ago
    Backticks, ``, and $(), use IFS to parse the command's output into words which then become the $()'s replacement and IFS normally contains \n.

gnosis 14y ago

You might be interested in "zargs" in zsh, which would save you the call to find. Furthermore, instead of the pipe to sed and extra xargs, it might be clearer and simpler to do something like: zargs -n 1 **/*.jpg -- make-thumb Where "make-thumb" is a short script (or even a zsh function, if you care about saving a fork for each input file) containing: convert -geometry 200x $1 ${1%%.jpg}_thumb.jpg But, in real life, instead of writing such a script or function, what I'd probably do instead is: zargs -n 1 **/*.jpg | vipe > myscript and do some quick editing in vim to modify the zargs output by hand to do whatever I need -- and then I'd run the resulting "myscript". Just fyi, "vipe" is part of the "moreutils" package [1] and lets you use your editor in the middle of a pipe. One final trick is for when you need to do in-place image manipulation. Instead of using "convert", you can use another ImageMagick command: "mogrify". It will overwrite the original file with the modified file. Of course, you should be very careful with it. [1] - http://joeyh.name/code/moreutils/ http://joeyh.name/code/moreutils/

BrandonM 14y ago

Why do that when a loop is clearer, safer, and shorter? find images -name '*.jpg' | while read jpg; do convert -geometry 200x "$jpg" "$(echo "$jpg" | sed 's/.jpg$/_thumb.jpg/')"; done This version works even when there are spaces in a filename, whereas yours will break.

_ZeD_ 14y ago
false $ ls -Ql totale 4 -rw-r--r-- 1 zed users 33 set 6 07:30 " spaces " $ $ find -name '*spaces*' | while read text; do cat "$text"; done cat: ./ spaces: File o directory non esistente $ $ find -name '*spaces*' -print0 | xargs -0 cat while read is broken with spaces $
- mkl 14y ago
  Only for file names starting or ending with spaces, as far as I can tell, which are pretty unusual.
- ralph 14y ago
  IFS needs some care and attention and read should have -r. $ ls -Q " spaces " $ ls | while IFS="\n" read -r f; do ls "$f"; done spaces $ For lots of grim detail see David A. Wheeler's http://www.dwheeler.com/essays/fixing-unix-linux-filenames.html http://www.dwheeler.com/essays/fixing-unix-linux-filenames.h...
- BrandonM 14y ago
  That is a good point, but it is a much more degenerate case than the more common internal space.
shorter 14y ago
an alternative approach echo images/*.jpg|sed 's/.*/convert -geometry 200x \"&\" \"&_thumb.jpg\"/;s/\.jpg_thumb/_thumb/' > script; sh -v script 2>log you could also use -x instead of -v reasoning: globbing is as fast as find and is faster than find _if_ you do the same glob twice (b/c it's cached); i'm assuming the images dir is not full of subfolders otherwise i would would do echo images/*/* if the dir structure gets any deeper than 2 levels, i'd use find while loops typed from the command line are a p.i.t.a. if you don't bg them initially and want to bg them later; you need to put them inside brackets or into a function using a script and a log, if something goes wrong while it's running, you can restart after looking at the log; you can't do that with a loop

ralph 14y ago

I too like xargs but see no need for these two xargs given sed is already used? find images -name '*.jpg' | sed 's/\.jpg$// s/.*/convert -geometry 200x &.jpg &_thumb.jpg/'

johnchristopher 14y ago

Is there a way to use an alias with xargs like in the command below ? xargs -a cmd_list.txt -I % alias % This has been bothering me for a while.