# Command History & Substitution ## Command History * To show previously executed commands use the `history` command ### history ```bash history 865 find / -name 'file1.txt' 2> errors.txt 866 cat errors.txt 867 ll 868 ls errors.txt 869 ll errors.txt 870 source .bashrc 871 ll errors.txt 872 ls -lah errors.txt # It includes the executed commands and the history number associated to them history | less # To execute one of the previous commands use !HISTORY_NUMBER !868 # To execute the last command use these shortcuts !-1 !! # Up & Down arrow keys can be used to scroll through history # To execute the last "command" command - cat for example !cat ``` * The configuration options of the history are located in the `.bashrc` file ```bash cat ~/.bashrc # for setting history length see HISTSIZE and HISTFILESIZE in bash(1) HISTSIZE=1000 HISTFILESIZE=2000 ``` ## Command Substitution * Redirecting doesn't always work. * With **command substitution** a command can be replaced with its output before the entire command is executed by the shell. * Backticks **\`\`** or **$()** are used: ```bash ls -l `cat file.txt` ls -l $(cat file.txt) # Example with a file list cat file-list.txt file1.txt file2.txt file3.txt ls -l `cat file-list.txt` -rw-rw-r-- 1 root root 6 set 3 00:16 file1.txt -rw-rw-r-- 1 user user 7 set 3 00:16 file2.txt -rw-rw-r-- 1 user user 8 set 3 00:16 file3.txt ``` *** --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://blog.syselement.com/tcm/courses/linux-101/6-shells/history-sub.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.