Shell aliases don’t always evaluate commands at runtime

[luantranminh]

I learned something cool about shell aliases today. Sometimes they don't work the way you expect, especially when you're trying to use live command results inside them.

I had this Git alias:

alias gitpul="git pull origin $(git branch --show-current)"

I thought it would always pull my current branch. But it kept trying to pull the same old branch, no matter which branch I was on.
This happened because aliases in the shell evaluate any command substitutions ($()) when the shell starts, not when the alias is executed.

The fix: Use single quotes for the alias definition and escape the inner double quotes:

alias gitpul='git pull origin "$(git branch --show-current)"'

This delays the command substitution until the alias is actually executed, ensuring it always pulls the current branch.

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Tip

Single quotes in shell scripting delay evaluation because of how the shell interprets them. Let's break this down:

1. Double quotes (" "):

• Allow variable and command substitution.
• The shell expands variables and executes commands inside double quotes immediately.

2. Single quotes (' '):

• Treat everything inside as literal text.
• No expansions or substitutions occur inside single quotes.