5 ms·

It's one more process to run, so `cat single_file | ` can be easily substituted with `<single_file `.

by tzot 5y ago

justin_oaks 5y ago
I was going to point this out too. For those who may not know, you can put the I/O redirection anywhere in the command, not just at the end. So you can write < input_file.txt grep some_pattern | grep another Which has the same effect as grep some_pattern < input_file.txt | grep another but puts the input redirection at the beginning so you can easily follow the data flow from left to right. I use "redirection first" in other cases such as 1>&2 echo "Error: missing file" To make it clear up-front that I'm writing to stderr.
- tzot 5y ago
  Or plain `>&2` since the `1` is implied for '>'
- enriquto 5y ago
  Yep. Writing "<file " is just as good as "cat file |". Still I'm more used to writing cat. The only (marginal) advantage of cat over explicit redirection at the beginning is maybe that you can split the lines more cleanly with cat: cat file |\ program1 |\ program2 |\ ... If you use a redirection, since the command does not start with a pipe the symmetry is broken. Redirections at the start of the line are fun. I have an alias alias null='>/dev/null 2>/dev/null' this allows to run GUI programs without cluttering the terminal with useless GTK/QT warnings: null evince file.pdf
  - fooblat 5y ago
    Pro tip, if a line ends in a | you don't need a trailing backslash. The same is true if a line ends in || or && as those all imply continuation. Edit: cat file | program1 | program2 | ...
    - enriquto 5y ago
      wow, that's great. Now what I miss is to be able to put comments after each such line.
      - fooblat 5y ago
        I just tested it and, in bash at least, it actually works fine. echo foo | # echoing foo tr 'a-z' 'A-Z' FOO
        enriquto 5y ago
        this is incredible, I had already given up on this, and the solution was in front of my eyes the whole time! You have just greatly enhanced the elegance of several scripts in my lab.
- scubbo 5y ago
  Wow. Thank you for explaining this! I'd never understood why `cat` was called useless when (so far as I then knew) there was no other way to "start a left-to-right pipe by reading from a file" (other than the aforementioned `grep <filename> *`). This helps!
probably_wrong 5y ago
There's a natural workflow that tends to get ignored in these discussions: when you're not sure of all the details of the pipeline yet and your file is big enough to slow you down. I typically start with (say) `head file | grep '2020'`. Once it does what I want I move to the next step, and so on until it's done. At that point, and having chained 5-6 commands, replacing `head` with `cat` is faster and less likely to break due to me putting the '<' in the wrong place. The simplicity and extra peace of mind is well worth an extra process IMHO.

It's one more process to run, so `cat single_file | ` can be easily substituted with `<single_file `.

by tzot 5y ago

It's one more process to run, so `cat single_file | ` can be easily substituted with `<single_file `.

I was going to point this out too. For those who may not know, you can put the I/O redirection anywhere in the command, not just at the end. So you can write < input_file.txt grep some_pattern | grep another Which has the same effect as grep some_pattern < input_file.txt | grep another but puts the input redirection at the beginning so you can easily follow the data flow from left to right. I use "redirection first" in other cases such as 1>&2 echo "Error: missing file" To make it clear up-front that I'm writing to stderr.

tzot 5y ago
Or plain `>&2` since the `1` is implied for '>'
enriquto 5y ago
Yep. Writing "<file " is just as good as "cat file |". Still I'm more used to writing cat. The only (marginal) advantage of cat over explicit redirection at the beginning is maybe that you can split the lines more cleanly with cat: cat file |\ program1 |\ program2 |\ ... If you use a redirection, since the command does not start with a pipe the symmetry is broken. Redirections at the start of the line are fun. I have an alias alias null='>/dev/null 2>/dev/null' this allows to run GUI programs without cluttering the terminal with useless GTK/QT warnings: null evince file.pdf
- fooblat 5y ago
  Pro tip, if a line ends in a | you don't need a trailing backslash. The same is true if a line ends in || or && as those all imply continuation. Edit: cat file | program1 | program2 | ...
  - enriquto 5y ago
    wow, that's great. Now what I miss is to be able to put comments after each such line.
    - fooblat 5y ago
      I just tested it and, in bash at least, it actually works fine. echo foo | # echoing foo tr 'a-z' 'A-Z' FOO
      - enriquto 5y ago
        this is incredible, I had already given up on this, and the solution was in front of my eyes the whole time! You have just greatly enhanced the elegance of several scripts in my lab.
scubbo 5y ago
Wow. Thank you for explaining this! I'd never understood why `cat` was called useless when (so far as I then knew) there was no other way to "start a left-to-right pipe by reading from a file" (other than the aforementioned `grep <filename> *`). This helps!

probably_wrong 5y ago

There's a natural workflow that tends to get ignored in these discussions: when you're not sure of all the details of the pipeline yet and your file is big enough to slow you down. I typically start with (say) `head file | grep '2020'`. Once it does what I want I move to the next step, and so on until it's done. At that point, and having chained 5-6 commands, replacing `head` with `cat` is faster and less likely to break due to me putting the '<' in the wrong place. The simplicity and extra peace of mind is well worth an extra process IMHO.