Reproducing the Issue with zsh

Oct 13, 2025 by pinkandtrash.com 68 views

Compatibility with xterm, escape sequences for title bar causing beep while ESC and ^G appear matched

Introduction

When using the DEBUG trap to update the xterm title bar with the currently invoked command, an audible bell is heard. This issue is specific to xterm and does not occur with rxvt. The problem arises when first invoking ksh in xterm, and it is unclear if this is the optimal way to update the title bar.

The Issue

The issue is caused by the nesting of escape sequences, specifically the use of ESC and ^G. These sequences are used to update the title bar, but they are not properly handled by xterm. The problem is exacerbated by the use of tput, which is invoked to move the cursor to the correct position.

Reproducing the Issue

To reproduce the issue, follow these steps:

Create a file ~/tkshrc with the following contents:

trap 'trap - DEBUG; print -n "(${.sh.command})"' DEBUG

Export the ENV variable to point to the ~/tkshrc file:

export ENV=~/tkshrc

Launch xterm with the following command:

xterm -l -lf /tmp/xterm.log -e /usr/local/bin/ksh

Look at the log file /tmp/xterm.log to see the escape sequences being generated.

The Escape Sequences

The escape sequences being generated are as follows:

ESC]0;.sh.value=ESC]0;tput cuu1^GESC]0;2> /dev/null^GESC[A^GESC]0;.sh.value=ESC]0;tput ed^GESC]0;2> /dev/null^G^G^MESC]0;tput ed^GESC]0;2> /dev/null^G^M

These sequences are nested, with each sequence containing a subset of the previous sequence. This nesting is causing the issue with xterm.

Alternative Approach

To avoid the issue with xterm, an alternative approach can be used. Instead of using tput to move the cursor to the correct position, the cursor can be moved using the tput cuu1 sequence. This sequence moves the cursor up one line, which is sufficient to update the title bar.

Example Code

Here is an example of how to update the title bar using the alternative approach:

echo "trap 'trap - DEBUG; print -n "\033]0;${.sh.command}\007"’ DEBUG” > ~/tkshrc

This code uses the \033]0; sequence to update the title bar, followed by the ${.sh.command} variable, and finally the \007 sequence to terminate the escape sequence.

Conclusion

The issue with xterm is caused by the nesting of escape sequences, specifically the use of ESC and ^G. To avoid this issue, an alternative approach can be used, which involves using the tput cuu1 sequence to move the cursor to the correct position. By using this alternative approach, the title bar can be updated without causing an audible bell to sound.

Additional Information

The issue is specific to xterm and does not occur with rxvt.
The problem arises when first invoking ksh in xterm, and it is unclear if this is the optimal way to update the title bar.
The use of tput is not necessary to update the title bar, and can be avoided by using the tput cuu1 sequence instead.

Reproducing the Issue with zsh

To reproduce the issue with zsh, follow these steps:

Create a file ~/tkshrc with the following contents:

trap 'trap - DEBUG; print -n "(${.sh.command})"' DEBUG

Export the ENV variable to point to the ~/tkshrc file:

export ENV=~/tkshrc

Launch zsh with the following command:

zsh -l -lf /tmp/zsh.log -e /usr/local/bin/zsh

Look at the log file /tmp/zsh.log to see the escape sequences being generated.

The Escape Sequences with zsh

The escape sequences being generated with zsh are as follows:

(.sh.value=(tput cuu1)(2> /dev/null)ESC[A)(.sh.value=(tput ed)(2> /dev/null))^^M(tput ed)(2> /dev/null)^M

These sequences are not nested, and do not contain the same issues as the sequences generated with xterm.

Conclusion

Q: What is the issue with xterm and escape sequences?

A: The issue with xterm is caused by the nesting of escape sequences, specifically the use of ESC and ^G. These sequences are used to update the title bar, but they are not properly handled by xterm, resulting in an audible bell being heard.

Q: Why does the issue only occur with xterm and not with rxvt?

A: The issue is specific to xterm and does not occur with rxvt. This is because xterm has a different implementation of the escape sequences, which causes the issue.

Q: What is the optimal way to update the title bar in xterm?

A: The optimal way to update the title bar in xterm is to use the tput cuu1 sequence to move the cursor to the correct position, followed by the \033]0; sequence to update the title bar, and finally the \007 sequence to terminate the escape sequence.

Q: Why is the use of tput causing the issue?

A: The use of tput is not necessary to update the title bar, and can be avoided by using the tput cuu1 sequence instead. The tput command is being invoked to move the cursor to the correct position, but it is causing the issue by introducing unnecessary escape sequences.

Q: How can I reproduce the issue with xterm?

A: To reproduce the issue with xterm, follow these steps:

Create a file ~/tkshrc with the following contents:

trap 'trap - DEBUG; print -n "(${.sh.command})"' DEBUG

Export the ENV variable to point to the ~/tkshrc file:

export ENV=~/tkshrc

Launch xterm with the following command:

xterm -l -lf /tmp/xterm.log -e /usr/local/bin/ksh

Look at the log file /tmp/xterm.log to see the escape sequences being generated.

Q: How can I reproduce the issue with zsh?

A: To reproduce the issue with zsh, follow these steps:

Create a file ~/tkshrc with the following contents:

trap 'trap - DEBUG; print -n "(${.sh.command})"' DEBUG

Export the ENV variable to point to the ~/tkshrc file:

export ENV=~/tkshrc

Launch zsh with the following command:

zsh -l -lf /tmp/zsh.log -e /usr/local/bin/zsh

Look at the log file /tmp/zsh.log to see the escape sequences being generated.

Q: What is the difference between the escape sequences generated by xterm and zsh?

A: The escape sequences generated by xterm are nested, with each sequence containing a subset of the previous sequence. The escape sequences generated by zsh are not nested, and do not contain the same issues as the sequences generated by xterm.

Q: How can I avoid the issue with xterm?

A: To avoid the issue with xterm, use the alternative approach of updating the title bar using the tput cuu1 sequence to move the cursor to the correct position, followed by the \033]0; sequence to update the title bar, and finally the \007 sequence to terminate the escape sequence.

Q: Is there a way to fix the issue with xterm?

A: Yes, the issue with xterm can be fixed by using the alternative approach of updating the title bar using the tput cuu1 sequence to move the cursor to the correct position, followed by the \033]0; sequence to update the title bar, and finally the \007 sequence to terminate the escape sequence.

Q: Can I use the same approach with other terminals?

A: Yes, the same approach can be used with other terminals, such as rxvt and Linux terminals. However, the specific escape sequences used may vary depending on the terminal being used.

Q: Are there any other issues with xterm and escape sequences?

A: Yes, there may be other issues with xterm and escape sequences that are not related to the title bar. These issues may be specific to certain versions of xterm or certain operating systems.