8

How can I modify the contents of my bash_history file? What values or variables control how long the history lasts? Are there any other things I can change to provide finer control of my BASH history??

11

There are two variables that control the history size:

HISTFILESIZE The maximum number of lines contained in the history file. When this variable is assigned a value, the history file is truncated, if necessary, to contain no more than that number of lines by removing the oldest entries. The history file is also truncated to this size after writing it when a shell exits. If the value is 0, the history file is truncated to zero size. Non-numeric values and numeric values less than zero inhibit truncation. The shell sets the default value to the value of HISTSIZE after reading any startup files.

and

HISTSIZE The number of commands to remember in the command history (see HISTORY below). If the value is 0, commands are not saved in the history list. Numeric values less than zero result in every command being saved on the history list (there is no limit). The shell sets the default value to 500 after reading any startup files.

These two variables allow you to control the behavior of your history. Basically, HISTSIZE is the number of commands saved during your current session and HISTFILESIZE is the number of commands that will be remembered across sessions. So, for example:

$ echo $HISTSIZE 
10
$ echo $HISTFILESIZE 
5
$ history | wc
     10      29     173

In the example above, because HISTSIZE is set to 10, history returns a list of 10 commands. However, if you log out and then log back in, history will only return 5 commands because HISTFILESIZE is set to 5. This is because, once you exit your session, your HISTFILESIZE lines of your history are saved to your history file (~/.bash_history by default but controlled by HISTFILE). In other words, commands are added to HISTFILE until that reaches $HISTFILESIZE lines at which point, each subsequent line added means the first command of the file will be removed.

You can set the values of these variables in your ~/.profile (or ~/.bash_profile if that file exists). Do not set them in your ~/.bashrc first because they have no business being set there and secondly because that would cause you to have different behavior in login vs non-login shells which can lead to other problems.

Other useful variables that allow you to fine tune the behavior of your history are:

  • HISTIGNORE : This allows you to ignore certain common commands that are rarely of interest. For example, you could set:

    export HISTIGNORE="pwd:df:du"
    

    That would cause any command starting with pwd, df, or du to be ignored and not saved in your history.

  • HISTCONTROL : This one lets you choose how the history works. Personally, I set it to HISTCONTROL=ignoredups which causes it to save duplicated commands only once. Other options are ignorespace to ignore commands starting with whitespace, and erasedups which causes all previous lines matching the current line to be removed from the history list before that line is saved. ignoreboth is shorthand for ignorespace and ignoredups.

  • HISTTIMEFORMAT : This allows you to set the time format of the history file. See Pandya's answer or read man bash for details.


For further fine tuning, you have:

  • The histappend bash option. This can be set by running shopt -s histappend or adding that command to your ~/.bashrc. If this option is set

    the history list is appended to the file named by the value of the HISTFILE variable when the shell exits, rather than overwriting the file.

    This is very useful as it allows you to combine the histories of different sessions (think different terminals for example).

  • The history command has two useful options:

    • history -a : causes the last command to be written to the history file automatically

    • history -r : imports the history file into the current session.

    You could, for example, add these two commands to your PROMPT_COMMAND (which is executed each time your shell shows the prompt, so whenever you start a new shell and after each command you run in it):

    export PROMPT_COMMAND='history -a;history -r;'
    

    Combined, they ensure that any new terminal you open will immediately import the history of any other shell sessions. The result is a common history across all terminals/shell sessions.

2

Default size of the history file is 500 lines. Once the .bash_history file reaches 500 lines, the early entries get eliminated to make room for the newer lines, as in FIFO. You can change this by changing the value of the variable HISTFILESIZE which by default has the value 500.

Putting a HISTFILESIZE=10000 in your .bashrc will increase the number of lines the history file can hold to 10000, thereby increasing the life of its contents.

  • 2
    note: if you do raise HISTSIZE's limit, consider changing HISTFILE too. If you run bash --norc HISTSIZE will go back to the default value, truncating your HISTFILE on exit. – llua Oct 21 '14 at 14:58
  • Thanks, @llua. Similarly, if your system's /etc/bash.bashrc sets a HISTFILESIZE (or maybe HISTSIZE; I haven't checked which), then Bash appears to truncate .bash_history at the point it reads /etc/bash.bashrc (or at least at some point before your new limit is set in your own Bash init files). To avoid this, again, set your own HISTFILE so that your real history goes there and not into the .bash_history file that Bash is truncating. – Chris Povirk Nov 3 '17 at 12:18
  • (But don't export it, as doing so would make it visible to any child shells, which could then look to truncate it instead of .bash_history!) – Chris Povirk Nov 3 '17 at 12:36
0

Read man bash for more details covered about bash history like:

HISTCONTROL
       A  colon-separated  list of values controlling how commands are saved on the history list.
       If the list of values includes ignorespace, lines which begin with a space  character  are
       not  saved  in the history list.  A value of ignoredups causes lines matching the previous
       history entry to not be saved.  A value of ignoreboth is  shorthand  for  ignorespace  and
       ignoredups.   A  value of erasedups causes all previous lines matching the current line to
       be removed from the history list before that line is saved.  Any value not  in  the  above
       list  is  ignored.   If HISTCONTROL is unset, or does not include a valid value, all lines
       read by the shell parser are saved on the history list, subject to the  value  of  HISTIG‐
       NORE.   The  second  and subsequent lines of a multi-line compound command are not tested,
       and are added to the history regardless of the value of HISTCONTROL.
HISTFILE
       The name of the file in which command history is saved (see HISTORY below).   The  default
       value is ~/.bash_history.  If unset, the command history is not saved when a shell exits.
HISTFILESIZE
       The maximum number of lines contained in the history file.  When this variable is assigned
       a value, the history file is truncated, if necessary, to contain no more than that  number
       of  lines by removing the oldest entries.  The history file is also truncated to this size
       after writing it when a shell exits.  If the value is 0, the history file is truncated  to
       zero  size.  Non-numeric values and numeric values less than zero inhibit truncation.  The
       shell sets the default value to the value of HISTSIZE after reading any startup files.
HISTIGNORE
       A colon-separated list of patterns used to decide which command lines should be  saved  on
       the  history  list.   Each pattern is anchored at the beginning of the line and must match
       the complete line (no implicit `*' is appended).  Each pattern is tested against the  line
       after  the  checks  specified by HISTCONTROL are applied.  In addition to the normal shell
       pattern matching characters, `&' matches the previous history line.  `&'  may  be  escaped
       using  a  backslash;  the  backslash is removed before attempting a match.  The second and
       subsequent lines of a multi-line compound command are not tested, and  are  added  to  the
       history regardless of the value of HISTIGNORE.
HISTSIZE
       The  number  of  commands  to remember in the command history (see HISTORY below).  If the
       value is 0, commands are not saved in the history list.  Numeric  values  less  than  zero
       result  in  every  command being saved on the history list (there is no limit).  The shell
       sets the default value to 500 after reading any startup files.
HISTTIMEFORMAT
       If this variable is set and not null, its value is used as a format string for strftime(3)
       to  print  the  time  stamp  associated  with  each history entry displayed by the history
       builtin.  If this variable is set, time stamps are written to the history file so they may
       be  preserved  across  shell sessions.  This uses the history comment character to distin‐
       guish timestamps from other history lines.

Particularly HISTFILESIZE and HISTSIZE may you are looking for and helps you.

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