Update cheatsheets

tldr/pamexec

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# pamexec
+
+> Execute a shell command on each image in a Netpbm file.
+> See also: `pamfile`, `pampick`, `pamsplit`.
+> More information: <https://netpbm.sourceforge.net/doc/pamexec.html>.
+
+- Execute a shell command on each image in a Netpbm file:
+
+`pamexec {{command}} {{path/to/image.pam}}`
+
+- Stop processing if a command terminates with a nonzero exit status:
+
+`pamexec {{command}} {{path/to/image.pam}} -check`