shell history

(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/)

The shell makes it easy for us to repeat commands, this chapter explains how.

repeating the last command

To repeat the last command in bash, type !!. This is pronounced as bang bang.

student@linux:~/test42$ echo this will be repeated > file42.txt
student@linux:~/test42$ !!
echo this will be repeated > file42.txt
student@linux:~/test42$

repeating other commands

You can repeat other commands using one bang followed by one or more characters. The shell will repeat the last command that started with those characters.

student@linux:~/test42$ touch file42
student@linux:~/test42$ cat file42
student@linux:~/test42$ !to
touch file42
student@linux:~/test42$

history

To see older commands, use history to display the shell command history (or use history n to see the last n commands).

student@linux:~/test$ history 10
38  mkdir test
39  cd test
40  touch file1
41  echo hello > file2
42  echo It is very cold today > winter.txt
43  ls
44  ls -l
45  cp winter.txt summer.txt
46  ls -l
47  history 10

!n

When typing ! followed by the number preceding the command you want repeated, then the shell will echo the command and execute it.

student@linux:~/test$ !43
ls
file1  file2  summer.txt  winter.txt

Ctrl-r

Another option is to use ctrl-r to search in the history. In the screenshot below i only typed ctrl-r followed by four characters apti and it finds the last command containing these four consecutive characters.

student@linux:~$ 
(reverse-i-search)`apti': sudo aptitude install screen

$HISTSIZE

The $HISTSIZE variable determines the number of commands that will be remembered in your current environment. Most distributions default this variable to 500 or 1000.

student@linux:~$ echo $HISTSIZE
500

You can change it to any value you like.

student@linux:~$ HISTSIZE=15000
student@linux:~$ echo $HISTSIZE
15000

$HISTFILE

The $HISTFILE variable points to the file that contains your history. The bash shell defaults this value to ~/.bash_history.

student@linux:~$ echo $HISTFILE
/home/paul/.bash_history

A session history is saved to this file when you exit the session!

Closing a gnome-terminal with the mouse, or typing reboot as root will NOT save your terminal's history.

$HISTFILESIZE

The number of commands kept in your history file can be set using $HISTFILESIZE.

student@linux:~$ echo $HISTFILESIZE
15000

prevent recording a command

You can prevent a command from being recorded in history using a space prefix.

student@linux:~/github$ echo abc
abc
student@linux:~/github$  echo def
def
student@linux:~/github$ echo ghi
ghi
student@linux:~/github$ history 3
 9501  echo abc
 9502  echo ghi
 9503  history 3

(optional)regular expressions

It is possible to use regular expressions when using the bang to repeat commands. The screenshot below switches 1 into 2.

student@linux:~/test$ cat file1
student@linux:~/test$ !c:s/1/2
cat file2
hello
student@linux:~/test$

(optional) Korn shell history

Repeating a command in the Korn shell is very similar. The Korn shell also has the history command, but uses the letter r to recall lines from history.

This screenshot shows the history command. Note the different meaning of the parameter.

$ history 17
17  clear
18  echo hoi
19  history 12
20  echo world
21  history 17

Repeating with r can be combined with the line numbers given by the history command, or with the first few letters of the command.

$ r e
echo world
world
$ cd /etc
$ r
cd /etc
$

practice: shell history

Issue the command echo The answer to the meaning of life, the universe and everything is 42.
Repeat the previous command using only two characters (there are two solutions!)
Display the last 5 commands you typed.
Issue the long echo from question 1 again, using the line numbers you received from the command in question 3.
How many commands can be kept in memory for your current shell session ?
Where are these commands stored when exiting the shell ?
How many commands can be written to the history file when exiting your current shell session ?
Make sure your current bash shell remembers the next 5000 commands you type.
Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an extra putty.exe in MS Windows) with the same user account. When is command history written to the history file ?

solution: shell history

Issue the command echo The answer to the meaning of life, the universe and everything is 42.
```
echo The answer to the meaning of life, the universe and everything is 42
```
Repeat the previous command using only two characters (there are two solutions!)
```
!!
OR
!e
```

Display the last 5 commands you typed.

student@linux:~$ history 5
 52  ls -l
 53  ls
 54  df -h | grep sda
 55  echo The answer to the meaning of life, the universe and everything is 42
 56  history 5

You will receive different line numbers.

Issue the long echo from question 1 again, using the line numbers you received from the command in question 3.

student@linux:~$ !55
echo The answer to the meaning of life, the universe and everything is 42
The answer to the meaning of life, the universe and everything is 42

How many commands can be kept in memory for your current shell session ?
```
echo $HISTSIZE
```
Where are these commands stored when exiting the shell ?
```
echo $HISTFILE
```
How many commands can be written to the history file when exiting your current shell session ?
```
echo $HISTFILESIZE
```
Make sure your current bash shell remembers the next 5000 commands you type.
```
HISTSIZE=5000
```
Open more than one console (by press Ctrl-shift-t in gnome-terminal, or by opening an extra putty.exe in MS Windows) with the same user account. When is command history written to the history file ?

when you type exit