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155 lines
4.4 KiB
Markdown
155 lines
4.4 KiB
Markdown
---
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tags:
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- bash
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- shell
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- scripting
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- expansion
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- substitution
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- text
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- variable
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- output
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- execute
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- stdout
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- save
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- result
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- return
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- value
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---
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# Command substitution
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$( <COMMANDS> )
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` <COMMANDS> `
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The command substitution expands to the output of commands. These
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commands are executed in a subshell, and their `stdout` data is what the
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substitution syntax expands to.
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All **trailing** newlines are removed (below is an example for a
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workaround).
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In later steps, **if not quoted**, the results undergo [word
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splitting](../../syntax/expansion/wordsplit.md) and [pathname
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expansion](../../syntax/expansion/globs.md). You have to remember that, because
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the word splitting will also remove embedded newlines and other `IFS`
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characters and break the results up into several words. Also you'll
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probably get unexpected pathname matches. **If you need the literal
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results, quote the command substitution!**
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The second form `` `COMMAND` `` is more or less obsolete for Bash, since
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it has some trouble with nesting ("inner" backticks need to be
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escaped) and escaping characters. Use `$(COMMAND)`, it's also POSIX!
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When you [call an explicit subshell](../../syntax/ccmd/grouping_subshell.md)
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`(COMMAND)` inside the command substitution `$()`, then take care, this
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way is **wrong**:
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$((COMMAND))
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Why? because it collides with the syntax for [arithmetic
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expansion](../../syntax/expansion/arith.md). You need to separate the command
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substitution from the inner `(COMMAND)`:
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$( (COMMAND) )
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## Specialities
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When the inner command is only an input redirection, and nothing else,
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for example
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$( <FILE )
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# or
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` <FILE `
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then Bash attempts to read the given file and act just if the given
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command was `cat FILE`.
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## A closer look at the two forms
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In general you really should only use the form `$()`, it's
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escaping-neutral, it's nestable, it's also POSIX. But take a look at
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the following code snips to decide yourself which form you need under
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specific circumstances:
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**<u>Nesting</u>**
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Backtick form `` `...` `` is not directly nestable. You will have to
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escape the "inner" backticks. Also, the deeper you go, the more escape
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characters you need. Ugly.
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echo `echo `ls`` # INCORRECT
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echo `echo \`ls\`` # CORRECT
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echo $(echo $(ls)) # CORRECT
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**<u>Parsing</u>**
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All is based on the fact that the backquote-form is simple character
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substitution, while every `$()`-construct opens an own, subsequent
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parsing step. Everything inside `$()` is interpreted as if written
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normal on a commandline. No special escaping of **nothing** is needed:
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echo "$(echo "$(ls)")" # nested double-quotes - no problem
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**<u>Constructs you should avoid</u>**
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It's not all shiny with `$()`, at least for my current Bash
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(`3.1.17(1)-release`. :!: <u>**Update:** Fixed since `3.2-beta` together
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with a misinterpretion of '))' being recognized as arithmetic
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expansion by redduck666</u>). This command seems to
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incorrectly close the substitution step and echo prints "ls" and
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")":
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echo $(
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# some comment ending with a )
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ls
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)
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It seems that every closing ")" confuses this construct. Also a (very
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uncommon ;-)) construct like:
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echo $(read VAR; case "$var" in foo) blah ;; esac) # spits out some error, when it sees the ";;"
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# fixes it:
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echo $(read VAR; case "$var" in (foo) blah ;; esac) # will work, but just let it be, please ;-)
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**<u>Conclusion:</u>**
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In general, the `$()` should be the preferred method:
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- it's clean syntax
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- it's intuitive syntax
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- it's more readable
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- it's nestable
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- its inner parsing is separate
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## Examples
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**To get the date:**
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DATE="$(date)"
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**To copy a file and get `cp` error output:**
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COPY_OUTPUT="$(cp file.txt /some/where 2>&1)"
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Attention: Here, you need to redirect `cp` `STDERR` to its `STDOUT`
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target, because command substitution only catches `STDOUT`!
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**Catch stdout and preserve trailing newlines:**
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var=$(echo -n $'\n'); echo -n "$var"; # $var == ""
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var=$(echo -n $'\n'; echo -n x); var="${var%x}"; echo -n "$var" # $var == "\n"
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This adds "x" to the output, which prevents the trailing newlines of
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the previous commands' output from being deleted by `$()`.
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By removing this "x" later on, we are left with the previous
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commands' output with its trailing newlines.
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## See also
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- Internal: [Introduction to expansion and
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substitution](../../syntax/expansion/intro.md)
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- Internal: [Obsolete and deprecated syntax](../../scripting/obsolete.md)
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