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Linux evasion techniques were previously included as part of persistence, but the number of techniques are varied enough where it likely should be its own article.
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Linux - Evasion
Summary
File Names
An Unicode zero-width space can be inserted into filenames which makes the names visually indistinguishable:
# A decoy file with no special characters
touch 'index.php'
# An imposter file with visually identical name
touch $'index\u200D.php'
Command History
Most shells save their command history so a user can recall them again later. The command history can be viewed with the history command or by manually inspecting the contents of the file pointed to by $HISTFILE (e.g. ~/.bash_history).
This can be prevented in a number of ways.
# Prevent writing to the history file at all
unset HISTFILE
# Don't save this session's command history in memory
export HISTSIZE=0
Individual commands that match a pattern in HISTIGNORE will be excluded from the command history, regardless of HISTFILE or HISTSIZE settings.
By default, HISTIGNORE will ignore all commands that begin with whitespace:
# Note the leading space character:
my-sneaky-command
If commands are accidentally added to the command history, individual command entries can be removed with history -d:
# Removes the most recently logged command.
# Note that we actually have to delete two history entries at once,
# otherwise the `history -d` command itself will be logged as well.
history -d -2 && history -d -1
The entire command history can be purged as well, although this approach is much less subtle and very likely to be noticed:
# Clears the in-memory history and writes the empty history to disk.
history -c && history -w
Hiding Text
ANSI escape sequences can be abused to hide text under certain circumstances.
If the file's contents are printed to the terminal (e.g. cat, head, tail) then the text will be hidden.
If the file is viewed with an editor (e.g. vim, nano, emacs), then the escape sequences will be visible.
echo "sneaky-payload-command" > script.sh
echo "# $(clear)" >> script.sh
echo "# Do not remove. Generated from /etc/issue.conf by configure." >> script.sh
# When printed, the terminal will be cleared and only the last line will be visible:
cat script.sh