<, >, |, $: add page

pages/common/$.md

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+# Dollar sign
+
+> Expand a bash variable.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#Shell-Variables>.
+
+- Print a variable:
+
+`echo ${{VARIABLE}}`
+
+- Print the exit status of the previous command:
+
+`echo $?`
+
+- Print a random number between 0 and 32767:
+
+`echo $RANDOM`
+
+- Print one of the prompt strings:
+
+`echo ${{PS1|PS2|PS3|PS4}}`
+
+- Expand with the output of `command` and run it. Same as enclosing `command` in backtics:
+
+`$({{command}})`

pages/common/greater-than.md

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+# Greater than
+
+> Redirect output to a file.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#Redirecting-Output>.
+
+- Redirect `stdout` to a file:
+
+`{{command}} > {{path/to/file}}`
+
+- Append a file:
+
+`{{command}} >> {{path/to/file}}`
+
+- Redirect both `stdout` and `stderr` to a file:
+
+`{{command}} &> {{path/to/file}}`
+
+- Redirect both `stdout` and `stderr` to `/dev/null` to keep the terminal output clean:
+
+`{{command}} &> /dev/null`

pages/common/less-than.md

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+# Less than
+
+> Redirect a file to `stdin`.
+> Achieves the same effect as `cat file.txt |`.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#Redirecting-Input>.
+
+- Redirect a file to `stdin`:
+
+`{{command}} < {{path/to/file.txt}}`

pages/common/vertical-bar.md

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+# Vertical bar
+
+> Pipe data between programs.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#Pipelines>.
+
+- Pipe `stdout` to `stdin`:
+
+`{{command}} | {{command}}`
+
+- Pipe both `stdout` and `stderr` to `stdin`:
+
+`{{command}} |& {{command}}`