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Modify bash history from inside subshell in an interactive session

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1

I executed

history > before; (history -d $n); history > after

where $n is the number corresponding to the last command I typed in the same interactive session before executing this line,

The result is that the line marked $n is not removed from history. If I remove the parentheses so that the history -d runs in the current shell, it works as documented.

How to understand this behavior? Is it true that all scripts that manipulate history need to be sourced?

bash history

share|improve this question

edited Apr 21 at 13:15

Weijun Zhou

asked Apr 21 at 11:18

Weijun ZhouWeijun Zhou

1,705427

add a comment | 

1

I executed

history > before; (history -d $n); history > after

where $n is the number corresponding to the last command I typed in the same interactive session before executing this line,

The result is that the line marked $n is not removed from history. If I remove the parentheses so that the history -d runs in the current shell, it works as documented.

How to understand this behavior? Is it true that all scripts that manipulate history need to be sourced?

bash history

share|improve this question

edited Apr 21 at 13:15

Weijun Zhou

asked Apr 21 at 11:18

Weijun ZhouWeijun Zhou

1,705427

add a comment | 

I executed

history > before; (history -d $n); history > after

where $n is the number corresponding to the last command I typed in the same interactive session before executing this line,

The result is that the line marked $n is not removed from history. If I remove the parentheses so that the history -d runs in the current shell, it works as documented.

How to understand this behavior? Is it true that all scripts that manipulate history need to be sourced?

bash history

share|improve this question

edited Apr 21 at 13:15

Weijun Zhou

asked Apr 21 at 11:18

Weijun ZhouWeijun Zhou

1,705427

history > before; (history -d $n); history > after

share|improve this question

edited Apr 21 at 13:15

Weijun Zhou

asked Apr 21 at 11:18

Weijun ZhouWeijun Zhou

1,705427

share|improve this question

edited Apr 21 at 13:15

Weijun Zhou

asked Apr 21 at 11:18

Weijun ZhouWeijun Zhou

1,705427

asked Apr 21 at 11:18

Weijun ZhouWeijun Zhou

1,705427

asked Apr 21 at 11:18

Weijun ZhouWeijun Zhou

1,705427

1 Answer
1

active

oldest

votes

3

Each shell process has its own idea of what the command-line history is. When an interactive shell exits it will write its remembered history to ~/.bash_history for the next shell to pick up, but that's the extent of cooperation between shell processes.

In your command, the () makes the shell fork a copy of itself to run history -d command. The child process starts out with a copy of the parent's internal state, so it knows the history, and is able to make changes to its copy of it.

However, when the subshell exits, its copy of the history (which was just rewritten) is discarded together with the rest of its internal state). The subshell knows it is a subshell, so it doesn't even bother to write ~/.bash_history.

---

A script that is not sourced usually cannot manipulate history at all, because it is interpreted by a fresh non-interactive shell that doesn't even read ~/.bash_history at startup.

You can get the shell to behave like an interactive shell by specifying it on the command line:

#!/bin/bash -i
echo something

The shell that runs this script will append its commands (which includes both the shebang line and the echo something) to the ~/.bash_history it finds on disk. But of course that doesn't affect the in-memory history copy of the shell process you invoke the script from, and when it exits the changes the script made to ~/.bash_history will be lost anyway.

share|improve this answer

answered Apr 21 at 14:47

Henning MakholmHenning Makholm

48626

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Great answer. It would be better if some reference to manual pages or other documents are included.
  
  – Weijun Zhou
  Apr 21 at 16:08
  

  
  
  
  

add a comment | 

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3

Each shell process has its own idea of what the command-line history is. When an interactive shell exits it will write its remembered history to ~/.bash_history for the next shell to pick up, but that's the extent of cooperation between shell processes.

In your command, the () makes the shell fork a copy of itself to run history -d command. The child process starts out with a copy of the parent's internal state, so it knows the history, and is able to make changes to its copy of it.

However, when the subshell exits, its copy of the history (which was just rewritten) is discarded together with the rest of its internal state). The subshell knows it is a subshell, so it doesn't even bother to write ~/.bash_history.

---

A script that is not sourced usually cannot manipulate history at all, because it is interpreted by a fresh non-interactive shell that doesn't even read ~/.bash_history at startup.

You can get the shell to behave like an interactive shell by specifying it on the command line:

#!/bin/bash -i
echo something

The shell that runs this script will append its commands (which includes both the shebang line and the echo something) to the ~/.bash_history it finds on disk. But of course that doesn't affect the in-memory history copy of the shell process you invoke the script from, and when it exits the changes the script made to ~/.bash_history will be lost anyway.

share|improve this answer

answered Apr 21 at 14:47

Henning MakholmHenning Makholm

48626

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Great answer. It would be better if some reference to manual pages or other documents are included.
  
  – Weijun Zhou
  Apr 21 at 16:08
  

  
  
  
  

add a comment | 

3

Each shell process has its own idea of what the command-line history is. When an interactive shell exits it will write its remembered history to ~/.bash_history for the next shell to pick up, but that's the extent of cooperation between shell processes.

In your command, the () makes the shell fork a copy of itself to run history -d command. The child process starts out with a copy of the parent's internal state, so it knows the history, and is able to make changes to its copy of it.

However, when the subshell exits, its copy of the history (which was just rewritten) is discarded together with the rest of its internal state). The subshell knows it is a subshell, so it doesn't even bother to write ~/.bash_history.

---

A script that is not sourced usually cannot manipulate history at all, because it is interpreted by a fresh non-interactive shell that doesn't even read ~/.bash_history at startup.

You can get the shell to behave like an interactive shell by specifying it on the command line:

#!/bin/bash -i
echo something

The shell that runs this script will append its commands (which includes both the shebang line and the echo something) to the ~/.bash_history it finds on disk. But of course that doesn't affect the in-memory history copy of the shell process you invoke the script from, and when it exits the changes the script made to ~/.bash_history will be lost anyway.

share|improve this answer

answered Apr 21 at 14:47

Henning MakholmHenning Makholm

48626

• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  Great answer. It would be better if some reference to manual pages or other documents are included.
  
  – Weijun Zhou
  Apr 21 at 16:08
  

  
  
  
  

add a comment | 

Each shell process has its own idea of what the command-line history is. When an interactive shell exits it will write its remembered history to ~/.bash_history for the next shell to pick up, but that's the extent of cooperation between shell processes.

In your command, the () makes the shell fork a copy of itself to run history -d command. The child process starts out with a copy of the parent's internal state, so it knows the history, and is able to make changes to its copy of it.

However, when the subshell exits, its copy of the history (which was just rewritten) is discarded together with the rest of its internal state). The subshell knows it is a subshell, so it doesn't even bother to write ~/.bash_history.

---

A script that is not sourced usually cannot manipulate history at all, because it is interpreted by a fresh non-interactive shell that doesn't even read ~/.bash_history at startup.

You can get the shell to behave like an interactive shell by specifying it on the command line:

#!/bin/bash -i
echo something

The shell that runs this script will append its commands (which includes both the shebang line and the echo something) to the ~/.bash_history it finds on disk. But of course that doesn't affect the in-memory history copy of the shell process you invoke the script from, and when it exits the changes the script made to ~/.bash_history will be lost anyway.

share|improve this answer

answered Apr 21 at 14:47

Henning MakholmHenning Makholm

48626

#!/bin/bash -i
echo something

share|improve this answer

answered Apr 21 at 14:47

Henning MakholmHenning Makholm

48626

answered Apr 21 at 14:47

Henning MakholmHenning Makholm

48626

answered Apr 21 at 14:47

Henning MakholmHenning Makholm

48626