LPIC-1 GNU and Unix Commands Practice Question
Which THREE of the following are valid symbolic mode expressions for the chmod command?
⚠ Common exam trap
Candidates often confuse numeric (octal) modes with symbolic modes, or assume that any single-letter class like 'c' is valid, when only u, g, o, and a are recognized by the chmod command.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
u+x
Option A (u+x) is a valid symbolic mode because it specifies the user class 'u' and adds (+) the execute permission, which is standard chmod symbolic syntax. Option C (a+rwx) is valid because 'a' represents all three permission classes (user, group, and other) and +rwx grants read, write, and execute to each. Option E (g-w) is valid because it targets the group class 'g' and removes (-) the write permission. Option B (755) is not a symbolic mode but an octal (numeric) mode, which uses digits rather than letters and operators. Option D (c+r) is invalid because 'c' is not a recognized permission class in chmod; the valid classes are u, g, o, and a.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
u+x
Why this is correct
`u+x` grants execute permission to the file's owner, using the symbolic form `[ugoa][+-=][rwxXst]`. It satisfies the stem's requirement for a valid symbolic mode expression, since `u` selects user class, `+` adds permission, and `x` specifies execute.
- ✗
755
Why it's wrong here
755 is octal notation, not a symbolic mode expression. Symbolic modes combine who (u, g, o, a), an operator (+, -, =) and permissions (r, w, x). Octal would be the correct form when setting absolute permission bits directly rather than modifying existing ones.
- ✓
a+rwx
Why this is correct
The `a` class covers user, group and other, so `a+rwx` grants read, write and execute to every permission class in one expression. Symbolic mode accepts `a` as a valid who-value alongside `u`, `g` and `o`, satisfying the stem's requirement for a valid symbolic mode expression.
- ✗
c+r
Why it's wrong here
c+r uses an invalid who value; symbolic modes accept only u, g, o and a before the operator. It would be correct if the intended class were others, written as o+r. The letter c denotes no permission class in chmod syntax, so the expression is rejected.
- ✓
g-w
Why this is correct
The `g-w` expression removes write permission from the group class, using the symbolic operator `-` to revoke a specific bit. It satisfies the stem's requirement for a valid symbolic mode expression, as chmod accepts `[ugoa][+-=][rwx]` syntax, and `g-w` conforms exactly to that grammar.
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