Unsoreted Shell Tips

Shell Tips

Normalizing whitespace

You may have a file with inconsistent whitespace, such as multiple spaces or tabs. This can make it difficult to read or process the file, for example using cut -d' ' -f1 will not work as expected.

To work around this, you can use awk to normalize the whitespace in the file. Consider the following example:

cat /etc/hosts
127.0.0.1   localhost localhost.localdomain localhost4 localhost4.localdomain4
  10.11.12.13    kerberos.mydomain.com kerberos

You can normalize the whitespace using awk like this:

cat /etc/hosts | awk '{$1=$1};1'
127.0.0.1 localhost localhost.localdomain localhost4 localhost4.localdomain4
10.11.12.13 kerberos.mydomain.com kerberos kerberos

Grepping config files without showing comments and whitespace

This command strips away the noisy parts of config files. Sometimes you may just want to know the actually non-default values and that information is hard to find when 90% of the file is comments.

cat <FILE> | egrep -v "^\s*(#|$)"

Container/VM Shells with weird cursor behavior

Ever had the situation where you are in a container or VM and the cursor is not behaving as expected? For instance, deleting a character with backspace results in the cursor moving to the right.

To solve this, set the TERM environment variable to xterm:

export TERM=xterm
# or
export TERM=xterm-256color

References

https://github.com/kubernetes/dashboard/issues/2346

Debug Shell Scripts

To debug shell scripts, you can use the -x option to print each command before executing it:

bash -x script.sh

Alternatively, you can add set -x to the script itself:

#!/bin/bash
set -x
echo "Hello World"

Optionally, you can also add the -v option to print the shell input lines as they are read:

bash -xv script.sh

The above can be a lot of help when debugging shell scripts. For example, you can see the exact command that is being executed and the output of each command. You can also see the values of variables and how they are being expanded.