Use Command History to Recall and Re-execute Commands

The Bash history mechanism is one of the highest-value efficiency tools on the RHCSA exam. Under time pressure, being able to recall, edit, and re-execute a previous command in seconds — instead of retyping it — makes a measurable difference. This lab walks you through every technique you need, including how to keep sensitive commands out of the history file.

Step 1 — Display the history list with line numbers

The history builtin prints every command in the current session's in-memory list, each prefixed by a sequential number. Those numbers are your handles for direct re-execution.

history

To limit output to the most recent entries, pass a count:

history 20

Scroll through the output. Notice that commands are numbered from 1 at the top. Every history expansion operator you will use in the next steps references these numbers.

Step 2 — Re-execute a command by history number

Once you know a command's number from the list, you can run it instantly with !<number>:

!5

Replace 5 with any number from your own history list. The shell expands the token, prints the resolved command, and executes it.

Step 3 — Re-execute the last command with !!

!! expands to the entire previous command line. It is most useful when you forgot to prefix a command with sudo:

!!

Or:

sudo !!

When you run sudo !!, the shell first expands !! to the text of the previous command, then sudo runs that expanded string. The expanded command is echoed to the terminal before execution so you can see exactly what ran.

Step 4 — Re-execute the most recent command matching a string prefix

!string finds the most recent history entry that began with string and re-executes it:

!ls

This is useful for quickly replaying a long command (e.g., a complex find or grep) without scrolling through the list — as long as you remember how it started.

Step 5 — Search history interactively with Ctrl-R

For longer or less predictable commands, interactive reverse search is faster than scrolling:

1. Press Ctrl-R at an empty prompt.
2. Type a substring — the most recent matching command appears.
3. Press Ctrl-R again to cycle to the next older match.
4. Press Enter to execute, or Ctrl-G to cancel without running anything.

(reverse-i-search)`host': cat /etc/hostname

Step 6 — Recall and edit a command before executing

Use the Up and Down arrow keys to navigate through history. When you reach the command you want, use readline shortcuts to edit it before pressing Enter:

This technique is ideal when you want to run the same command against a different file or with a slightly different flag. There is nothing to verify here — practice navigating to a previous command, modifying one token, and executing it.

Step 7 — Delete a specific entry from history

Remove a single entry by its line number. This is the standard way to prevent a sensitive command (containing a password or token) from persisting in the history file:

First, identify the line number:

history

Then delete it:

history -d 42

Replace 42 with the actual line number from your list.

Step 8 — Clear the entire in-memory history list

To wipe all history entries from the current session's memory in one operation:

history -c

This does not immediately update ~/.bash_history on disk. The file is overwritten (with an empty list) only when the session writes history — either on logout or via history -w. To clear both memory and file together:

history -c && history -w

Step 9 — Check and set HISTCONTROL

HISTCONTROL governs which commands Bash records. The values relevant to the exam:

Check the current setting:

echo $HISTCONTROL

If the output does not include ignorespace or ignoreboth, set it now:

export HISTCONTROL=ignoreboth

To make this permanent, add the line to ~/.bashrc.

Step 10 — Prevent a command from being saved to history

With HISTCONTROL set to ignoreboth (or ignorespace), any command prefixed by one or more spaces is silently omitted from history:

 echo "my-secret-token"

Note the leading space before echo — that is the critical part.

Run the command above, then check that it does not appear at the bottom of the history list:

history | tail -5

Step 11 — Write the current session history to disk immediately

By default, Bash appends in-memory history to ~/.bash_history only when the session closes. Force an immediate write with:

history -w

This is useful before switching to another terminal where you want to access commands from the current session via Ctrl-R or grep.

Verification

Confirm the on-disk history file reflects recent work:

tail -10 ~/.bash_history

The output should show the most recently written commands in order, with no gaps unless you deliberately deleted entries.

Troubleshooting

echo $HISTCONTROL
export HISTCONTROL=ignoreboth

history -c && history -w

grep 'pattern' ~/.bash_history

What you practised — exam checklist

• history / history N — display numbered list
• !N — re-execute by number
• !! — repeat last command (and sudo !!)
• !string — re-execute by prefix
• Ctrl-R — interactive reverse search
• Arrow keys + readline shortcuts — recall and edit before executing
• ^old^new — quick inline substitution on the previous command
• history -d N — delete a single entry
• history -c — clear in-memory list
• history -w — flush to disk immediately
• Leading space + HISTCONTROL=ignoreboth — suppress a command from history

These operations appear directly in RHCSA objectives under "Use input/output redirection" and the broader command-line proficiency requirement. Mastering them reduces keystrokes across every other objective.

Related

• man bash — HISTORY section (search /^HISTORY inside the pager)
• help history — builtin reference
• How-to 3 of 6: Navigate Shell Documentation with man, --help, and info