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--- bash:command_substitution [2021/01/09 14:58] – [Portability] peter
+++ bash:command_substitution [2021/01/26 16:57] (current) – removed peter
@@ Line 1: / Line 1: @@
-====== BASH - Command Substitution ======
-Command substitution is used to insert the output of one command into a second command.
-E.g. with an assignment:
-<code bash>
-today=$(date)        # Starts the "date" command, captures its output.
-echo "$today"
-</code>
-returns:
-<code bash>
-Mon Jul 26 13:16:02 MEST 2004
-</code>
-----
-This can also be used with other commands besides assignments:
-<code bash>
-echo "Today is $(date +%A), it's $(date +%H:%M)"
-</code>
-returns:
-<code bash>
-Today is Monday, it's 13:21
-</code>
-<WRAP info>
-**NOTE:**  This calls the date command two times.
-  * The first time to print the week-day
-  * The second time for the current time.
-</WRAP>
-----
-Of course, this could just be done with:
-<code bash>
-date "+Today is %A, it's %H:%M"
-</code>
-<WRAP info>
-**NOTE:**  As with all substitutions, the results of a command substitution will undergo WordSplitting, unless the whole thing is inside double quotes.
-</WRAP>
-----
-===== Nested Command Substitutions =====
-Command substitutions may be nested within each other:
-<code bash>
-IPs=($(awk /"$(</etc/myname)"/'{print $1}' /etc/hosts))
-</code>
-Notably, once inside a command substitution, the shell begins an entirely new quoting context.
-That is, double quotes inside the substitution do not match up with double quotes outside the substitution.
-So, things like this may be done:
-<code bash>
-echo "The IPs are $(awk /"$(</etc/myname)"/'{print $1}' /etc/hosts | tr '\n' ' ')"
-</code>
-<WRAP info>
-**NOTE:**
-  * The outermost quotes delimit a single argument that will be passed to echo.
-  * The inner double quotes prevent word splitting or glob expansion on the results of the inner command substitution.
-  * The two sets of double quotes are independent of each other.
-</WRAP>
-----
-===== Command Substitutions Subshells =====
-Command substitutions create subshells, so any changes to variables, current directory, etc. inside the command substitution affect only the rest of the substitution, and not the parent shell.
-<code bash>
-var=$(cd ../../usr/bin; pwd)
-echo "$var"
-</code>
-returns:
-<code bash>
-/usr/bin
-</code>
-But now run:
-<code bash>
-pwd
-</code>
-returns:
-<code bash>
-/home/peter
-</code>
-<WRAP info>
-**NOTE:**  The exit status of the last command that was executed in the subshell is used as the exit status for the command substitution.
-</WRAP>
-<code bash>
-var=$("non existent command")
-bash: non existent command: command not found
-echo $?
-</code>
-<WRAP info>
-**NOTE:**  Command substitutions strip all trailing newlines from the output of the command inside them.
-This allows common cases such as **foo=$(grep foo bar)** to populate variables without needing a second step to remove the newline.
-Sometimes, you may want the newlines; for example, when attempting to read an entire file into a variable without data loss (except NUL bytes):
-<code bash>
-var=$(<file)   # strips trailing newlines
-# Workaround:
-var=$(cat file; printf x) var=${var%x}
-</code>
-</WRAP>
-----
-===== Portability =====
-The **$(command)** syntax is supported by KornShell, BASH, and PosixShell.
-Older shells (e.g. BourneShell) use the following syntax:
-<code bash>
-`command`
-</code>
-<WRAP important>
-**NOTE:**  These are not the apostrophe characters '...', but small ticks going from the upper left to the lower right: `...`.
-These are often called "backticks" or "back quotes".
-The use of **$(<file)** instead of **$(cat file)** is a Bashism that is slightly more efficient (doesn't require forking a cat(1) process) but obviously less portable.
-</WRAP>
-----
-===== Nesting Command Substitutions =====
-Nesting of command substitutions using the `...` syntax is more difficult.
-One must use backslashes:
-<code bash>
-IPs_inna_string=`awk "/\`cat /etc/myname\`/"'{print $1}' /etc/hosts`
-# Very Bourne-ish: use the positional params as a pseudo array
-set -- `awk "/\`cat /etc/myname\`/"'{print $1}' /etc/hosts`
-</code>
-<WRAP info>
-**NOTE:**  As one may imagine, this becomes rather unwieldy after two levels.
-</WRAP>