Update cheatsheets

tac

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# tac
+
+> Display and concatenate files with lines in reversed order.
+> See also: `cat`.
+> More information: <https://www.gnu.org/software/coreutils/tac>.
+
+- Concatenate specific files in reversed order:
+
+`tac {{path/to/file1 path/to/file2 ...}}`
+
+- Display `stdin` in reversed order:
+
+`{{cat path/to/file}} | tac`
+
+- Use a specific [s]eparator:
+
+`tac -s {{separator}} {{path/to/file1 path/to/file2 ...}}`
+
+- Use a specific [r]egex as a [s]eparator:
+
+`tac -r -s {{separator}} {{path/to/file1 path/to/file2 ...}}`
+
+- Use a separator [b]efore each file:
+
+`tac -b {{path/to/file1 path/to/file2 ...}}`