diff options
| -rw-r--r-- | Doc/Zsh/redirect.yo | 61 |
1 files changed, 55 insertions, 6 deletions
diff --git a/Doc/Zsh/redirect.yo b/Doc/Zsh/redirect.yo index d6d662d4a..f192470bc 100644 --- a/Doc/Zsh/redirect.yo +++ b/Doc/Zsh/redirect.yo @@ -70,12 +70,22 @@ occurs, `tt(\)' followed by a newline is removed, and `tt(\)' must be used to quote the characters `tt(\)', `tt($)', `tt(`)' and the first character of var(word). +Note that var(word) itself does not undergo shell expansion. Backquotes +in var(word) do not have their usual effect; instead they behave +similarly to double quotes, except that the backquotes themselves are +passed through unchanged. (This information is given for completeness +and it is not recommended that backquotes be used.) Quotes in the form +tt($')var(...)tt(') have their standard effect of expanding backslashed +references to special characters. + If tt(<<-) is used, then all leading tabs are stripped from var(word) and from the document. ) item(tt(<<<) var(word))( Perform shell expansion on var(word) and pass the result to standard input. This is known as a em(here-string). +Compare the use of var(word) in here-documents above, where var(word) +does not undergo shell expansion. ) xitem(tt(<&) var(number)) item(tt(>&) var(number))( @@ -138,12 +148,29 @@ If the order of redirections were reversed, file descriptor 2 would be associated with the terminal (assuming file descriptor 1 had been) and then file descriptor 1 would be associated with file var(fname). + +The `tt(|&)' command separator described in +ifzman(em(Simple Commands & Pipelines) in zmanref(zshmisc))\ +ifnzman(noderef(Simple Commands & Pipelines)) +is a shorthand for `tt(2>&1 |)'. + +The various forms of process substitution, `tt(<LPAR())var(list)tt(RPAR())', +and `tt(=LPAR())var(list)(RPAR())' for input and +`tt(>LPAR())var(list)tt(RPAR())' for output, are often used together with +redirection. For example, if var(word) in an output redirection is of the +form `tt(>LPAR())var(list)tt(RPAR())' then the output is piped to the +command represented by var(list). See +ifzman(\ +em(Process Substitution) in zmanref(zshexpn))\ +ifnzman(\ +noderef(Process Substitution)). sect(Multios) +cindex(multios) pindex(MULTIOS, use of) If the user tries to open a file descriptor for writing more than once, the shell opens the file descriptor as a pipe to a process that copies its input to all the specified outputs, similar to bf(tee), -provided the tt(MULTIOS) option is set. Thus: +provided the tt(MULTIOS) option is set, as it is by default. Thus: example(date >foo >bar) @@ -195,23 +222,45 @@ example(echo foo > bar > baz) when tt(MULTIOS) is unset will truncate bar, and write `tt(foo)' into baz. +There is a problem when an output multio is attached to an external +program. A simple example shows this: + +example(cat file >file1 >file2 +cat file1 file2) + +Here, it is possible that the second `tt(cat)' will not display the full +contents of tt(file1) and tt(file2) (i.e. the original contents of +tt(file) repeated twice). + +The reason for this is that the multios are spawned after the tt(cat) +process is forked from the parent shell, so the parent shell does not +wait for the multios to finish writing data. This means the command as +shown can exit before tt(file1) and tt(file2) are completely written. +As a workaround, it is possible to run the tt(cat) process as part of a +job in the current shell: + +example({ cat file } >file >file2) + +Here, the tt({)var(...)tt(}) job will pause to wait for both files to be +written. + sect(Redirections with no command) -vindex(NULLCMD, use of) -vindex(READNULLCMD, use of) -pindex(CSH_NULLCMD, use of) -pindex(SH_NULLCMD, use of) When a simple command consists of one or more redirection operators and zero or more parameter assignments, but no command name, zsh can behave in several ways. +vindex(NULLCMD, use of) +pindex(CSH_NULLCMD, use of) If the parameter tt(NULLCMD) is not set or the option tt(CSH_NULLCMD) is set, an error is caused. This is the bf(csh) behavior and tt(CSH_NULLCMD) is set by default when emulating bf(csh). -If the option (SH_NULLCMD) is set, the builtin tt(`:') is inserted as a +pindex(SH_NULLCMD, use of) +If the option tt(SH_NULLCMD) is set, the builtin `tt(:)' is inserted as a command with the given redirections. This is the default when emulating bf(sh) or bf(ksh). +vindex(READNULLCMD, use of) Otherwise, if the parameter tt(NULLCMD) is set, its value will be used as a command with the given redirections. If both tt(NULLCMD) and tt(READNULLCMD) are set, then the value of the latter will be used instead |