53 lines
1.7 KiB
Markdown
53 lines
1.7 KiB
Markdown
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# Shell pipes and fifo
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Most bash "blocks" accept a redirection parameter at the end of their
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declaration. This can replace where the output and errors are going
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```bash
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fn () {
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echo "error" >&3
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} 3>&2 2>file
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# Prints "error" to the standard error. The real stderr is stored in ./file
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```
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- Functions, as above
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- Sub-shells: `(echo "one"; echo "two") > file`
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- Loops: `while (1); do echo "loop"; done > file`
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Remember that redirection syntax looks like `[fd][< | >][&fd | file_path]`,
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where `fd` is the index in the process' file descriptors. This works for `<` as
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well. `|&` is identical to using `2>&1 |`
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The order of IO redirection matters. Redirection will go to wherever the
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redirected destination currently goes. Redeclaring a redirection doesn't work
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```
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┌3 writes to stdout
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│ ┌stdout writes to stderr
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│ │ ┌stderr writes to where 3 writes, so stdout
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│ │ │
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echo "one" 3>&1 1>&2 2>&3
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```
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To do this for the entire shell or script, use `exec`
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```bash
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exec 4>&1 # Holds onto reference to stdout
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exec 1>/dev/null # Silences stdout and loses track of it
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exec 1>&4 4>&1 # Restore stdout through reference in 4 and close 4
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# Everything back to normal
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```
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`mkfifo` creates a named pipe, which can be easier to interact with between
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scripts. Make sure it's being held open at use
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```bash
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mkfifo fifo
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cat fifo >ofif & # Holds open fifo pipe and copies anything in there to ofif
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exec 3>fifo # fd 3 reffers to pipe fifo
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echo "double" >&3 # Writes to fifo. >fifo would close the pipe right after
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exec 3>&- # Manually close the open pipe
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```
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The example above works similar to `tee` in logging everything in the pipe
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