4 ms·

What does history -a do? Seems to write the history to disc and not produce something to pipe into logger. How does this work?

by atesti 15y ago

zooz 15y ago
It appends the "new" history lines to a $HISTFILE by default, unless other file is specified.
- atesti 15y ago
  Thanks, that's what I was missing: -a can have a file. Cool stuff and thanks to all for explaining it. http://unixhelp.ed.ac.uk/CGI/man-cgi?history http://unixhelp.ed.ac.uk/CGI/man-cgi?history does not mention the command line argument, http://linux.die.net/man/1/bash http://linux.die.net/man/1/bash does.
  - zooz 15y ago
    You're welcome, it took me a bit of time to come up with this idea, of course, it's not perfect, but does the job pretty well.
despite2 15y ago
With bash putting a command in <() uses the output of the command as though it were a file. I use it to diff the output of two commands: diff <(command 1) <(command 2) I've never used it, but I assume >() uses a command as though it were a writable file. So instead of history -a file writing to file, he does history -a >(logger) to write to the logger command. Seems pretty clever to me.
- zooz 15y ago
  Yup, that's what it does. Shortly speaking <() or >() in bash that creates a temporary pipe.
- sciurus 15y ago
  It's called process substitution. In this case it runs the logger command, connects its input to a file in /dev/fd, and places the name of that file in the argument list of history. I've included a small demonstration below. This is useful since the history command won't write to standard out so you can't use a normal pipe. It's more common to see process substitution used to gather output from multiple commands, see http://www.linuxjournal.com/content/shell-process-redirection http://www.linuxjournal.com/content/shell-process-redirectio... $ cat writer.sh #!/bin/bash if [[ "$#" == 1 ]]; then echo "Wrote to file named $1" > "$1" else echo "Wrote to stdout" fi $ ./writer.sh Wrote to stdout $ ./writer.sh | cat Wrote to stdout $ ./writer.sh >(cat) Wrote to file named /dev/fd/63

What does history -a do? Seems to write the history to disc and not produce something to pipe into logger. How does this work?

by atesti 15y ago

What does history -a do? Seems to write the history to disc and not produce something to pipe into logger. How does this work?

zooz 15y ago

It appends the "new" history lines to a $HISTFILE by default, unless other file is specified.

atesti 15y ago
Thanks, that's what I was missing: -a can have a file. Cool stuff and thanks to all for explaining it. http://unixhelp.ed.ac.uk/CGI/man-cgi?history http://unixhelp.ed.ac.uk/CGI/man-cgi?history does not mention the command line argument, http://linux.die.net/man/1/bash http://linux.die.net/man/1/bash does.
- zooz 15y ago
  You're welcome, it took me a bit of time to come up with this idea, of course, it's not perfect, but does the job pretty well.

despite2 15y ago

With bash putting a command in <() uses the output of the command as though it were a file. I use it to diff the output of two commands: diff <(command 1) <(command 2) I've never used it, but I assume >() uses a command as though it were a writable file. So instead of history -a file writing to file, he does history -a >(logger) to write to the logger command. Seems pretty clever to me.

zooz 15y ago
Yup, that's what it does. Shortly speaking <() or >() in bash that creates a temporary pipe.
sciurus 15y ago
It's called process substitution. In this case it runs the logger command, connects its input to a file in /dev/fd, and places the name of that file in the argument list of history. I've included a small demonstration below. This is useful since the history command won't write to standard out so you can't use a normal pipe. It's more common to see process substitution used to gather output from multiple commands, see http://www.linuxjournal.com/content/shell-process-redirection http://www.linuxjournal.com/content/shell-process-redirectio... $ cat writer.sh #!/bin/bash if [[ "$#" == 1 ]]; then echo "Wrote to file named $1" > "$1" else echo "Wrote to stdout" fi $ ./writer.sh Wrote to stdout $ ./writer.sh | cat Wrote to stdout $ ./writer.sh >(cat) Wrote to file named /dev/fd/63