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Finding files or content
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apt-file
Tested apt-file examples: searching Debian's file index for a command you have not installed, and the substring trap in a plain pattern.
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find
Tested find examples: name and type filters, size and age, permissions, -exec vs -delete, and safe scripting patterns.
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grep
Tested grep examples: recursive search, regex flavours, context lines, counting, scripting, and more.
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man
Tested man and apropos examples: what the section numbers mean, searching by description, finding which package shipped a page, and reading a synopsis.
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Find and replace across many files
Replace a string across a directory tree with grep -rl and sed -i, safely for filenames containing spaces, with a dry run and a backup first.
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Find files without the permission denied noise
Why 2>/dev/null is the wrong fix for find's permission denied errors, and what -readable -prune does instead, including what happens to the exit status.
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Which package provides a file
Trace a file to the package that ships it: dpkg -S for what is installed, apt-file for what is not, and the files that belong to no package at all.